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DEVELOPMENT AND CHARACTERIZATION OF A MICROWAVE INDUCED NITROGEN DISCHARGE AT ATMOSPHERIC PRESSURE (MINDAP) FOR ELEMENTAL ANALYSIS (PLASMA GAS)

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D e u tsc h , R obert David
DEVELOPMENT AND CHARACTERIZATION OF A MICROWAVE INDUCED
NITROGEN DISCHARGE AT ATMOSPHERIC PRESSURE (MINDAP) FOR
ELEMENTAL ANALYSIS
Ph.D.
Indiana University
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DEVELOPMENT AND CHARACTERIZATION OF A MICROWAVE
INDUCED NITROGEN DISCHARGE AT ATMOSPHERIC
PRESSURE (MINDAP) FOR ELEMENTAL ANALYSIS
Robert David Deutsch
Submitted to the faculty of the Graduate School
in partial fulfillment of the requirements for
the degree of Doctor of Philosophy in the
Department of Chemistry,
Indiana University
November, 1983
ii
T h i s is t o c e r t i f y t h a t t h e t h e s i s s u b m i t t e d by
R o b e r t David D e u t s c h has been a c c e p t e d by the Ph.D. A d v i s o r y
C o m m i t t e e as s a t i s f a c t o r y in p a r t i a l f u l f i l l m e n t o f t h e
r e q u i r e m e n t s f o r the Ph.D. d e g r e e
.
rt-jLu)
_
iii
To the Dean and F a c u l t y of the G r a d u a t e S c h o o l :
We, t h e u n d e r s i g n e d m e m b e r s o f t h e F a c u l t y o f t h e
G r a d u a t e S c h o o l and m e m b e r s o f t h e P h . D . C o m m i t t e e a p p o i n t e d
for the e x a m i n a t i o n of Ro b e r t D a v i d D e u t s c h e x a m i n e d
h i m on N o v e m b e r 22, 1 9 8 3 .
It is h e r e b y c e r t i f i e d t h a t he h a s s u c c e s s f u l l y p a s s e d
t h e e x a m i n a t i o n s in m a j o r a n d m i n o r s u b j e c t s .
W e r e c o m m e n d Mr. D e u t s c h f o r t h e d e g r e e D o c t o r o f
Philosophy.
Chai rman
^Research A d v i s e ^ ^ '
l£. ruA
—
ACKNOWLEDGMENTS
To Margaret A. Flanagan, my wife and professional associate, 1 wish
to express my love and appreciation for your encouragement,
understanding and patience throughout our graduate careers at Indiana
University.
To my son Eric Thomas, whom I love very much, I want to thank you
for your assistance and cooperation in helping with experiments.
To my mother Diane, brother Miles, and dear departed father, thank
you for instilling the perseverance needed to complete this degree.
Your love and encouragement throughout all of my educational endeavors
has been invaluable.
I sincerely wish to thank Professor Gary M. Hieftje for his
patience, advice and financial support throughout my graduate studies.
His enthusiasm in the pursuit of different problems has broadened my
awareness not only in Analytical Chemistry but in other disciplines of
science.
I would like to thank all the members of Dr. Hieftje's research
group for their scientific assistance and friendship.
Especially
helpful and important has been the friendship of Jerry Keilsohn.
Many thanks go to Dr. R. Mark Wightman and the members of his
research group for letting me use their computers to type this thesis.
A special word of appreciation goes to all the staff members of the
department for their help and assistance. Special thanks is in order
for the talented glass blowing of Don Fowler, and all the members of
the Instrument and Machine shops.
I am grateful to Dr. Ken Force for his interest and encouragement
in my pursuit of this degree.
DEVELOPMENT AND CHARACTERIZATION OF A MICROWAVE INDUCED
NITROGEN DISCHARGE AT ATMOSPHERIC PRESSURE (MINDAP)
FOR ELEMENTAL ANALYSIS
ROBERT DAVID DEUTSCH
INDIANA UNIVERSITY
BLOOMINGTON, INDIANA
ABSTRACT
Emission spectroscopy is an increasingly attractive tool for the
determination of trace chemical elements because of recent developments
in plasma excitation sources.
These sources have always had their
place in analytical chemistry but with the exponential growth and
interest in trace elemental analysis, plasmas are becoming the tool of
choice.
The two plasmas most commonly used for spectrochemical
analysis are the radiofrequency inductively coupled plasma (ICP) and
the microwave-induced plasma (MIP).
The ICP is employed primarily for
the qualitative and quantitative determination of metals in solution
but has also been successfully applied to the analysis of gaseous and
solid samples.
Unlike the ICP, the MIP has generally been used as a
detector for gas chromatography or for metals in other volatile forms.
Its inability to efficiently desolvate, atomize and excite liquid
samples has limited its analytical utility.
This dissertation focuses
on the development of a new microwave-supported plasma which overcomes
these operational difficulties.
The full potential of this new
emission source is critically evaluated.
Development of -the new MIP system was centered on using molecular
nitrogen
as the plasma support gas.
Nitrogen is not only more
economical than inert He or Ar gases used formerly, but is also more
efficient at nebulizing and atomizing solution samples.
To sustain a
stable microwave-induced nitrogen discharge at atmospheric pressure
(MINDAP), modifications were made to a microwave resonant cavity and
plasma torch; the resulting plasma can easily be ignited and extends
several centimeters beyond the cavity.
This plasma "tail flame" was
examined critically as a region for viewing atomic emission from
samples introduced into the discharge.
Background and analyte emission
in the ultraviolet (UV) and visible spectral regions was mapped to
provide information about the energy distribution in the plasma.
Also,
signal intensities were evaluated at different powers, flow rates, and
optical viewing geometries in order to ascertain optimal analyte
excitation conditions.
Analytical figures-of-merit of the MINDAP system were evaluated
using a pneumatic nebulizer and desolvation apparatus for sample
introduction.
With the plasma oriented vertically and viewed in a
side-on manner, detection limits were generally in the part-per-billion
(ng/mL) range and calibration curves (emission intensity vs. solution
concentration) extended over a range of three-to-five orders of
magnitude.
Interelement (matrix) interferences, similar to those found
in flame spectroscopy, were observed but could be overcome through the
use of "releasing agents".
The temporal stability of the plasma was
investigated with the aid of noise amplitude spectra and a correlation
was noted between emission intensity and microwave power fluctuations.
Noise amplitude-spectrum analysis was performed using a Fast Fourier
vii
Transform algorithm to identify the limiting noise source of the
system.
These results are discussed and compared with those obtained
from other emission sources.
Development and characterization of the MINDAP system included
fundamental studies involving the physical characteristics of the
plasma.
Excitation, ionization and rotational temperatures were
determined and found to be approximately equal, suggesting that local
thermodynamic equilibrium (LTE) might be established in the discharge.
Spatially resolved excitation and ionization temperatures were
comparable.
Similarly, spatial emission profiles for elements of
different excitation potentials closely followed those expected from a
Boltzmann distribution. These results support the suggestion that
excitation of analyte species in the tail flame occurs by a thermal
mechanism.
In the final study, a microarc atomizer was used to introduce
nanogram-to-microgram quantities of atomic vapor reproducibly into the
plasma.
The technique of microsampling and preatomization is well
suited for sample introduction into any emission source.
The
microarc-MINDAP combination was evaluated for the analytical
figures-of-merit noted earlier.
Compared to results obtained with the
pneumatic nebulizer, the microarc-MINDAP system produced lower
detection limits and greater freedom from classical interferences.
This combination provides the economy and sensitivity needed for
routine analysis.
viii
TABLE OF CONTENTS
LIST OF TABLES
xi
LIST OF FIGURES
xii
CHAPTER 1
INTRODUCTION
REFERENCES
CHAPTER 2
DEVELOPMENT OF A MICROWAVE INDUCED NITROGEN
DISCHARGE AT ATMOSPHERIC PRESSURE (MINDAP) _
1
5
6
\
EXPERIMENTAL
Microwave Cavity
Torch
Operating the Plasma
Plasma Features
Reagents
9
19
25
26
31
32
RESULTS
Viewing Geometry
Applied Power and Nitrogen Flow
Viewing Position in the Tail-Flame
33
33
41
54
CONCLUSION
57
REFERENCES
61
CHAPTER 3
ANALYTICAL CHARACTERIZATION OF A MICROWAVE
INDUCED NITROGEN DISCHARGE AT ATMOSPHERIC
PRESSURE (MINDAP)
64
EXPERIMENTAL
65
RESULTS AND DISCUSSION
Detection Limits
Working Curves
Precision
Interferences
66
66
68
68
71
CONCLUSION
85
REFERENCES
86
ix
CHAPTER 4
PHYSICAL MEASUREMENTS OF A MICROWAVE INDUCED
NITROGEN DISCHARGE AT ATMOSPHERIC PRESSURE
TEMPERATURES AND THERMODYNAMIC EQUILIBRIUM
88
89
EXPERIMENTAL
Temperature measurements and calculations
Electronic Excitation Temperature
Ionization Temperature
Electron Number Density
Rotational Temperature
93
95
95
96
98
100
RESULTS
Electronic Excitation Temperature
Ion/Atom Ratios and Ionization Temperature
Effect of Central-Channel Nitrogen Flow
Rotational Temperature
Spatial Profiles of Elemental Emission
Spatially Resolved Temperatures
104
104
107
114
117
120
120
DISCUSSION
125
CONCLUSION
128
REFERENCES
130
CHAPTER 5
IDENTIFICATION OF LIMITING NOISE
SOURCES IN THE MINDAP
132
EXPERIMENTAL
Wavelength-Scan Flicker-Ratio Measurement
Time Constant Effects on Precision
Time Trace Correlation
Noise Amplitude Spectral Determination
133
134
135
136
137
RESULTS
Precision and Low-Frequency Noise
High-Frequency Noise
141
141
147
DISCUSSION
155
CONCLUSION
168
REFERENCES
170
CHAPTER 6
MICROSAMPLE INTRODUCTION INTO THE MINDAP
USING A MICROARC ATOMIZER
171
EXPERIMENTAL
173
RESULTS AND DISCUSSION
Analytical Figures-of-Merit
Detection Limits
Working Curves
Precision
Interferences
181
182
182
186
186
190
X
CONCLUSION
190
REFERENCES
197
CHAPTER 7
VITA
CONCLUSION AND FUTURE WORK
199
CONCLUSION
199
FUTURE WORK
200
203
xi
List of Tables
Table
Page
2-1
Specific components of the MINDAP system.
12
2-2
Interfering background band emission.
42
3-1
Detection limits of the MINDAP.
67
4-1
Temperatures for several Atmospheric Pressure MIPs.
92
4-2
Temperatures for several Atmospheric Pressure ICPs.
94
4-3
Spectroscopic data for electronic excitation
temperature measurements using Fe (I) emission.
97
4-4
4-5
5-1
6-1
6-2
Ionization temperatures calculated for a range
of ion/atom ratios for different electron number
densities.
Ill
The power required to heat nitrogen to a specified
temperature.
126
Initial and statistical values for each signal
collected from the time tracings for 400 min.
161
Detection limits for the MINDAP microarc and
nebulizer sample introduction systems.
183
Comparison of detection limits for discretesample-introduction systems.
185
xii
List of Figures
Figure
Page
2-1
Schematic diagram of MINDAP measurement system.
11
2-2
Schematic diagram of the desolvation system.
16
2-3
Schematic diagram of uni - bipolar converter.
18
2-4
Different viewing geometries for the MINDAP system.
21
2-5
Side-on view of modified TM qjq resonant cavity.
24
2-6
Cross-sectional view of cavity, torch and plasma.
29
2-7
Comparison of viewing geometries for 190 - 360 nm
spectral region.
35
Comparison of viewing geometries for 390 - 465 nm
spectral region.
37
Comparison of viewing geometries for 650 - 800 nm
spectral region.
39
2-10
Effect
of power on plasma length.
44
2-11
Effect
of powerand flow rate on Fe (I) emission.
46
2-12
Effect
of power on several atom transitions.
49
2-13
Effect
of power on several ion transitions.
51
2-14
Effect
of power on signal-to-background noise ratio.
53
2-8
2-9
2-15
Spectral scans of plasma background and analyte
emission for different vertical positions in the
tai1-flame.
56
2-16
Emission spectrum for 10 ng/mL Li and 100 ng/mL K.
60
3-1
Analytical working curves for MINDAP.
70
3-2
Sodium interference on 10 ppm calcium emission.
73
3-3
Sodium interference on 10 ppm calcium emission with
cesium added as an ionization suppressant.
76
3-4
Aluminum interference on 10 ppm calcium emission.
78
3-5
Phosphate interference on 10 ppm calcium emission.
80
xiii
Figure
3-6
3-7
4-1
Page
Phosphate interference on 10 ppm calcium emission
with 0.01M EDTA added as a releasing agent.
82
Aluminum interference on 10 ppm calcium emission
with 0.01M EDTA added as a releasing agent.
84
Emission spectrum of the N£+ First
Negative system.
103
Spatially averaged excitation temperatures
in the MINDAP tail-flame.
106
4-3
Magnesium ion/atom ratios in the MINDAP tail-flame.
109
4-4
Ionization temperatures in the MINDAP tail-flame.
113
4-5
Effect of central channel nitrogen flow rate on
ionization temperatures.
116
Rotational temperatures from the discharge region
of the MINDAP.
119
Spatial emission profiles of several elements
in the tail-flame of the MINDAP.
122
Spatially resolved electronic excitation and
ionization temperatures in the MINDAP tail-flame.
124
An illustration of the signals used for
obtaining a noise - amplitude spectrum.
139
5-2
Effect of time constant on precision.
143
5-3
Noise spectrum of OH band emission over
the frequency range of 0 - 5 Hz.
146
Effect of calcium concentration on emission
signal and precision.
149
Noise spectrum of OH band emission over
the frequency range of 0 - 500 Hz.
151
Noise spectrum of OH emission when large
amounts of water were intentionally introduced
into the plasma.
154
Effect of microwave power on two different
impedance settings.
158
Time tracings of the emission signal, forward
and reflected power for a 400-min time period.
160
4-2
4-6
4-7
4-8
5-1
5-4
5-5
5-6
5-7
5-8
xiv
Figure
5-9
Page
Correlation plot of forward power and emission
signal.
163
Correlation plot of forward power and
reflected power.
165
Correlation plot of reflected power and
emission signal.
167
6-1
Microarc-MINDAP operational configuration.
175
6-2
Block diagram of the microarc-MINDAP
instrumentation.
177
Microarc-MINDAP emission-time profile for
1 ng copper.
180
Calibration curves for the microarc-MINDAP
system.
188
6-5
Sodium interference on 10 ng of calcium.
192
6-6
Phosphate interference on 10 ng of calcium.
194
5-10
5-11
6-3
6-4
1
CHAPTER 1
Introduction
Today one of the most popular- emission techniques for the trace
and ultra-trace detection of elements uses the radiofrequency
(27.12 MHz) inductively coupled plasma (ICP).
This atmospheric-
pressure electrical discharge is used to decompose an introduced sample
' (aerosol, solid, powder, vapor) into its atomic and ionic constituents
and excite them.
The popularity of this plasma has arisen because it
is easy to use, has few interferences, operates at atmospheric pressure
and has made possible the absolute and relative detection of elements
down to the sub - pg and sub-parts-per-billion limits, respectively.
Although ICP atomic emission spectrometry exhibits a high degree
of elemental specificity, reproducibility and sensitivity, it has
drawbacks which many investigators have been diligently attempting to
overcome.
These liabilities are of three basic kinds; cost, size and
susceptibility to interferences.
Another electrical discharge is one that has been used for
analytical determinations since the 1950 's (1), is operated at
2450 MHz, and is called a microwave induced plasma (MIP).
The MIP was
originally sustained at reduced pressure in a resonant cavity.
In such
a configuration, this emission source was exploited as an elementselective detector for analysis of metals and nonmetals eluting from a
gas chromatograph (2-8).
References 6 through 8 constitute a good
review with many applications and references for the GC-MIP
combination.
The low-pressure MIP was also used as a source of intense
monochromatic emission from the actinide and lanthanide elements
2
in what is known as an electrodeless discharge lamp (EDL) (9,10).
Although practical and sensitive, the MIP was judged inconvenient
because of its operation at reduced pressure.
This fact, coupled with
the overwhelming acceptance of the ICP, precluded the further rapid
development of the low-pressure MIP.
In 1973 the development and characterization of an atmosphericpressure microwave-induced plasma (11-13) rekindled interest in this
emission source.
Since then the MIP has been applied to a variety of
determinations, not only in analytical chemistry but also in clinical
biochemistry (14,15) and geology (16,17).
The major advantages of this
type of plasma source are its low power and gas-flow requirements, the
antithesis of the ICP's failings.
Unfortunately, there are two
remaining features that hinder further acceptance of the atmosphericpressure MIP:
its inability to efficiently decompose aerosol samples
and the absence of automated systems for impedance matching (as are
available for the ICP).
This dissertation is devoted to the
development of a practical, element selective, sensitive microwaveinduced plasma atomic emission source operated at atmospheric pressure
for the detection of trace elements.
Chapter 2 describes the problems that afflict most microwave
plasmas and how recent advances in ICP instrumentation promoted the
development of a new MIP source for the analysis of aerosol samples.
This new plasma, sustained using nitrogen as the nebulizer and
plasma-supporting gas, employs a modified torch and microwave cavity to
enable the discharge to be easily ignited and maintained.
The new
plasma has a flame-like appearance and extends several centimeters
beyond the resonant cavity.
Termed a Microwave Induced Nitrogen
3
Discharge at Atmospheric Pressure (MINDAP), the device readily accepts
aerosol from a conventional nebulizer sample-introduction system.
Different viewing geometries (end-on and side-on) are assessed for
utility and signal-to-background ratios.
The dependence of analyte
emission intensity and signal-to-noise ratio on flow rate and microwave
power was determined.
In Chapter 3 the MINDAP was operated under the conditions and
the configuration determined in Chapter 2 to be optimal.
in the subsequent
Criteria
in
used
evaluation were the detection limits of sixteen
elements at thirty-three wavelengths, the susceptibility of the MINDAP
to interference from aluminum, phosphate and sodium on the emission
signal of calcium, linearity of calibration curves and the precision of
analytical measurements.
Further characterization of the MINDAP system included a study of
its fundamental characteristics.
In Chapter 4, excitation, ionization
and rotational temperatures of the plasma were determined and found to
be approximately equal, suggesting that local thermodynamic equilibrium
(LTE) is approached in the discharge.
Spatially resolved excitation
and ionization temperatures were comparable.
Similarly, spatial
emission profiles for elements of different excitation potentials
closely followed those dictated by a Boltzmann distribution.
Origins of noise (fluctuations) in the emission signal of the
MINDAP are studied in Chapter 5.
Four different methods:
time
constant effect, wavelength-scan flicker-ratio measurement, noise
amplitude spectral determination, and time-trace correlation were used
to identify noise sources in the system.
Noise amplitude spectral
analysis was performed using a Fast Fourier Transform algorithm to
4
identify the noise frequency components generated by the nebulizer
sample-introduction system and the MINDAP.
Analyte emission noise
spectra were acquired for desolvated and undesolvated aerosol, for
varying analyte concentration, and for background emission from
nitrogen species and hydroxyl radicals generated from water vapor.
The
system appears to be flicker-noise limited below about 1 Hz; only 60
and 120 Hz signals are detected in the frequency region from 0 to
500 Hz.
In Chapter 6, a microarc atomizer was used to introduce nanogram
quantities of atomic vapor reproducibly into the MINDAP.
The technique
of microsampling and preatomization is well suited for sample
introduction into any emission source.
The microarc-MINDAP combination
was evaluated for the same analytical figures-of-merit employed in
Chapter 3.
Compared to results obtained with the pneumatic nebulizer,
the microarc-MINDAP system produced lower detection limits and greater
freedom from classical interferences.
This combination provides the
economy and sensitivity needed for routine analysis.
In the final chapter of this thesis, a summary of the results are
presented along with some ideas about future work needed to fully
understand and develop* this new plasma into an accepted technique for
the determination of trace elements.
5
REFERENCES
1.
H. P. Broida and M. W. Chapman, Anal. Chem., 30, 2049 (1958).
2.
C. A. Bache and D. J. Liske, Anal. Chem., 39, 786 (1967).
3.
J. P. J. van Dalen, P. A. de Lezenne Coulander and L. de Galan,
Anal. Chim. Acta, 94, 1 (1977).
4.
B. D. Quimby, P. C. Uden and R. M. Barnes, Anal. Chem., 5Q_, 2112
(1978).
5. C. A. Bache and D. J. Liske, Anal. Chem., 43, 950 (1971).
6. I. S. Krull and S. Jordan, Amer. Lab., 12_, 21 (1980).
7. A. T. Zander and G. M. Hieftje, Appl. Spectrosc., 35, 357 (1981).
8. B. L. Sharp, Sel. Ann. Rev. Anal. Sci., 4^, 37 (1976).
9. M. J. Al. Ani, R. M. Dagnall, T. S. West, Analyst, 92, 597 (1967).
10. R. Avni, and J. D. Winefordner, Spectrochim. Acta, 30B, 281
(1975).
11.
C. I. M. Beenakker, Spectrochim. Acta, 31B, 483 (1976).
12.
C. I. M. Beenakker and P. W. J. M. Boumans, Spectrochim. Acta,
32B, 173 (1977).
13.
C. I. M. Beenakker, B. Bosman and P. W. J. M. Boumans,
Spectrochim. Acta, 33B, 53 (1978).
14.
H. Kawaguchi and B. L. Vallee, Anal. Chem., 47_, 1029 (1975).
15.
I. Atsuya, G. M. Alter, C. Veillon, and B. L. Vallee, Anal.
Biochem., 79, 202 (1977).
16. K. Govindaraju, G. Mevelle and C. Chouard, Anal. Chem., 48, 1325
. (1976).
17.
J. 0. Burman, "Applications of Plasma Emission Spectrochemistry",
ed. R. M. Barnes, Heyden Publishers, 1979, p.15.
6
CHAPTER 2
Development of a Microwave Induced Nitrogen
Discharge at Atmospheric Pressure (MINDAP)
Atomic emission spectrometry is today perhaps the most widely used
method for trace elemental analysis.
Of the alternative emission
sources being employed, rf plasma systems have provided some of the
most sensitive measurements.
The rf plasmas are comprised principally
of two groups: the inductively coupled plasma (ICP) which usually
operates at 27.12 and 40.68 MHz and the microwave-induced plasma (MIP)
which operates at 2450 MHz.
For solution analysis, the ICP is by far
the most widely employed because aerosol can be easily directed into it
from a suitable nebulizer system.
The ICP is then capable of
desolvating the aerosol sample, decomposing the dried salt particles
into their atomic and ionic constituents, and exciting the resulting
atoms and ions.
Most analytical measurements are made high in the ICP
tail flame where background emission from the plasma is minimal.
Although the ICP would appear to be a nearly ideal source for
atomic emission measurements, its initial and operating costs are
extremely high.
Moreover, intense background emission, even in the
plasma tail-flame, complicates the measurement of many analytical
lines.
As a result, high-resolution dispersion systems or exotic
modulation schemes (1) are needed to minimize spectral interferences.
In comparison, the MIP is inexpensive to purchase and operate.
It
enables the sensitive detection of both metals (2) and nonmetals (3,4)
but has been limited to use with samples introduced in a vaporized or
7
atomized form.
The most successful means of sample introduction into
the MIP has been through use of a gas chromatograph (2-6).
Other
methods that have been employed include thermal and nonthermal
atomizers (7-10), hydride generators (11,1-2), and laser vaporization
devices (13).
Recent reviews (14,15) have listed many other
alternative sample-introduction techniques for the MIP.
. Solution analysis is difficult and cumbersome with the MIP, mainly
because of its low power (20 - 200 W), low support-gas flow rate
( < 2 L/min), small physical dimensions (4 mm in diameter), low thermal
temperature (^2000°K), and an operating frequency which corresponds
to an absorption band of water.
Ironically, many of these same
features are what make the MIP such an affordable and potentially
useful emission source.
Clearly, a MIP system that could directly
analyze aerosol samples without the need for complicated sampleintroduction arrangements would be very appealing as a compact, easyto-use, and sensitive emission spectroscopic tool.
Although the MIP is practical and sensitive, the overwhelming
acceptance of the ICP has foreshadowed its development.
Current trends
are to improve the performance of the ICP and reduce its cost.
One of
the first ICP improvements was the design of a torch that controlled
and stabilized the flow pattern of the plasma gas and consequently the
plasma discharge.
Recently, it has been found possible to reduce the
instrumental and operational costs of the ICP but maintain the
sensitivity, simply by reducing the plasma size.
Numerous publications
have demonstrated that a reduction in power consumption, support-gas
flow rate, and torch size do not significantly degrade sensitivity or
analytical performance (16-19).
8
Other factors that contribute to the atom-excitation ability of
the ICP are its operating frequency and type of support gas.
Increasing the frequency from 27.12 to 40.68 MHz has a pronounced
effect on sensitivity (20).
Similarly, ICPs sustained with nitrogen
gas (because of its economy, availability, and ionization energy
comparable to Ar) have been extensively studied (21-27).
Unfortunately, the N^ ICP yielded poorer detection limits under
similar operating conditions than the Ar discharge (28-31).
Although
in other studies, Capitelli, et al. (28), and Barnes and Nikdel (29)
determined experimentally and theoretically that nitrogen ICPs are more
efficient at decomposing analyte particles than argon ICPs.
Other
information about signal-to-background ratio, ion-to-atom ratio and
applied power have suggested that the nitrogen ICP is closer to local
thermodynamic equilibrium (LIE) than a comparable argon plasma (24,26).
Nitrogen has been used also as a support gas for other electrical
discharges.
Cobine and Wilbur (30) sustained an atmospheric-pressure
microwave (1000 MHz) plasma with nitrogen and other diatomic gases.
These plasmas were found to possess an unusually high thermal
temperature which the authors attributed to the heat of association of
the molecules dissociated by the discharge.
Eckert, £t^ _al_. noted that
molecular gases possess more thermal energy than do atomic gases
because of the energy available in the rotational, vibrational and
electronic levels.
In contrast, energy storage in raonatomic gases is
possible only in translational or electronic states (21).
Another advantage to using nitrogen as a plasma gas accrues from
the possible formation of "active nitrogen" (31).
Two techniques, APAN
(Atmospheric Pressure Active Nitrogen) (32) and METALS (Metastable
9
Energy Transfer for Atomic Luminescence) (33,34), have exploited the
high energy available in active nitrogen for exciting metal and
nonmetal atoms.
Interest in the microwave plasma (2450 MHz) has been revived
largely because of the recent development of a resonant cavity to
easily sustain either an argon or helium plasma at atmospheric pressure
(5,35,36).
Another contributing improvement adapted from the ICP has
been the stabilization and control of support-gas flow patterns (37)
through the design of a new microwave plasma torch.
These developments
have been utilized here to generate a stable discharge in flowing
nitrogen.
In this chapter, a microwave induced nitrogen discharge at
atmospheric pressure (MINDAP) is sustained by a new torch assembly and
evaluated with an pneumatic nebulizer and desolvated-aerosol sampleintroduction system as an analytical emission source.
Operating
parameters (power, flow rate, and viewing geometry) were optimized for
signal-to-noise and signal-to-background ratio.
A discussion of the
background spectral features, their origin, and their effect on
analytical measurements is presented.
EXPERIMENTAL
The instrumentation used in this study is shown schematically in
Figure 2-1; Table 2-1 lists all optical, electrical and gas-handling
equipment and the respective manufacturers.
Sample solutions are
introduced into the plasma with a conventional pneumatic nebulizer /
spray chamber system.
A concentric glass nebulizer (38) operating at a
10
Figure 2-1:
Schematic diagram of the MINDAP measurement system.
The
broken lines indicate that the computer was remotely
located from the experimental apparatus.
The Tektronix
4051 terminal was used only for plotting the collected data from the PDP 11/34 computer and so its connection is
designated by dashed lines.
i-to-V
- current to voltage converter.
B.P.C.
- Bipolar converter.
H.V.P.S. - High Voltage Power Supply.
PMT
- Photomultiplier tube.
11
PIa*ma
I to V
AAA/V”
Monochromator
—/ 'onvor t«r—
-5 B.P.C. — i
tmp I If I«r
»
Samp I
H.V.
P.S.
IntroductIon
Svatam
PDP 11/34
T«rmI no I
or MXNC
Computar
Graphic
Plottar
TaktronIx
4051
TarmInaI
Table 2-1:
COMPONENT
Specific componenCs of MINDAP system.
TYPE/DESCRIPTION
COMPANY/LOCATION
Gas handling equip.
Regulators/flow meters
Type 603
Matheson Corp.,
Joliet, IL.
Spherical Mirror
f = 108 mm, MgF„
overcoat, d » 108 mm
Oriel Corp.,
Stamford, Conn.
Lens
f = 50 mm, suprasil
Diameter = 75 mm
Melles Griot,
Irvine, CA
Microwave cavity
Modified TMq ^q cavity
I. U. Machine Shop,
Bloomington, IN
Microwave generator
Model 420, 500 W
@ 2450 Mhz
Micro - Now Instr. Co.
Chicago, IL
Microwave powersupply cables
RG 115 A/U
Times Wire and Cable
Corp., Conn.
Double-stub tuner
Model DS109
Weinschel Eng.,
Gaithersburg, MD
Plasma torch
Modified ICP-style
torch. See Fig. 2-6.
I. U. Glass Shop,
Bloomington, IN
NebulizerSpray chamber
concentric glass conventional spray
chamber
I. U. Glass Shop,
Bloomington, IN
Aerosol condenser
Model 9270
Ace Glass Inc.,
Vineland, NJ
Monochromator
EU-700, f.l. = 0.35 m
Heath Co.,
Benton Harbor, MI
PMT
R928
Hamamatsu Co.,
Middlesex, NJ
H. V. Power Supply
Model 244 @ -800 V
Keithley Instr.,
Cleveland, OH
Current Amplifier
Model 427
Keithley Instr.,
Cleveland, OH
Amplifier
Uni - bipolar converter
driver and multiplier
Author's design
(Fig. 2-3)
13
Table 2-1:
Specific components of MINDAP system (continued).
TYPE/DESCRIPTION
COMPANY/LOCATION
CRT
ADM 3A
Lear Siegler Inc.,
Anaheim, CA
Data Acquisition
PDP 11/34; MINC-11/23
Digital Equip. Corp.,
Maynard, MA
Graphic System
Tektronix 4051 terminal
Tektronix Co.,
Beaverton, OR
Recirculating cooler
Multicool system
FTS Systems,
Stone Ridge, NY
COMPONENT
14
flow rate of 1.75 L/min of nitrogen gas aspirates sample solution at a
rate of 1.8 mL/min.
The resulting aerosol passes through a
conventional spray chamber and desolvation system (39) en route to the
MINDAP.
The desolvation apparatus, shown in Figure 2-2, consists of a
glass tube 20 mm o.d. x 150 mm long which was wrapped with heating
tape.
A Variac was used to control the voltage necessary to maintain
the tube at approximately 140°C for drying the aerosol.
The water
vapor was then removed by two tandem spiral condensing tubes which were
maintained at -20°C by a refrigerated recirculating water-methanol
bath.
Unlike other MIPs the MINDAP is capable of decomposing and
analyzing aerosols injected directly into it; however, under these
conditions both background and noise levels were higher.
Consequently,
desolvation was employed for all studies described below.
Emission from the plasma was focussed onto the entrance slit of
the monochromator using either a lens or mirror assembly.
The
resulting PMT photocurrent was converted to a proportional voltage by a
current amplifier and then to a bipolar (-5 to +5 V) signal by a
laboratory-constructed amplifier.
This final signal was then
compatible with the input of the analog-to-digital converter of the PDP
11/34 computer.
The schematic diagram of the bipolar amplifier is shown in Figure
2-3.
This amplifier consists of four stages, each implemented with an
operational amplifier.
The first stage (A) adds an offset voltage (-5
V) to the input signal (0 - 10 V) and provides intermediate gain in
multiples of 1, 2, and 5.
The second amplifier stage (B) is adjusted
for unity gain and inverts the output of the first stage to produce
15
Figure 2-2:
Schematic diagram of the desolvation system employed for
all measurements.
To Pl a s m a
AerosoI
Dryer
Nebulizer
Chamber
Rec ircuI at ing
Coo Iant
Samp Ie
So Iut ion
Heat ing
Tape
Tandem
Condenser
Drai n
17
Figure 2-3:
Schematic diagram for the unipolar-to-bipolar converting
amplifier.
0.01
uF
0.01
uF
I n put
»-VSAAAA-
356
356
356
ba in
-1 8
-
+10
VO Its
|— W M A /*— ^ OFFset
356
Output
19
the correct signal polarity.
The third stage (C) is used to drive the
voltmeter and to avoid loading the second stage.
The final stage (D)
is a voltage follower used as a driver to send signals up to 100 ft to
the remote computer.
To test the circuit and determine the line losses
and noise, a constant-voltage source was connected to the constructed
amplifier and also directly to a MINC-11/23 computer (to monitor the
voltage fluctuations of the voltage supply) through four feet of cable.
The signals from both computers were compared over a 24-hr period; each
deviated from the actual signal by less than 1 part in 4096 (the
resolution of the A/D converter).
Two different viewing geometries (axial and radial) for the MINDAP
system have been evaluated and are diagrammed in Figure 2-4.
The axial
(end-on) method of viewing is the most common geometry for MIP
systems.
In a recent review by Zander and Hieftje (15), such plasma
systems are described.
In this configuration, the emitted radiation
from the plasma inside the cavity was collected with a lens, or series
of lenses, and focused onto the monochromator with unity magnification.
In the radial (side-on) configuration, a spherical mirror formed a 2:1
magnified image of the plasma tail-flame on the monochromator entrance
slit.
Microwave Cavity
The Beenakker cavity described by Zander and Hieftje (8) was
initially used for igniting and sustaining the nitrogen discharge.
Although that configuration enabled a nitrogen plasma often to be
sustained, the discharge wandered about the torch and usually resided
20
Figure 2-4:
Different viewing geometries for the MINDAP system.
A.
Radial (side-on) configuration.
B.
Axial (end-on) configuration.
J..M. Impedance Matcher (double-stub tuner).
TaiI-Flame
Monochromator
Cav ity
MICROWAVE
P.S.
Samp Ie
AerosoI
B
Cavi ty —
“l i
A
Monochromator
--- >
Samp Ie
Aeroso1
V
JL
I. M.
MICROWAVE
P.S.
P
M
T
22
on the inner walls of the open quartz tube.
Moreover, it was difficult
to minimize reflected power without extinguishing the plasma, and the
cavity and all electrical connections became hot with use.
These
heating difficulties have been reported by others (15,40) and recent
studies have sought to overcome them through cooling and internal
tuning of the resonant cavity (41,42).
Unfortunately, cooling the
cavity with air did not eliminate these problems in the present study.
Instead, it was necessary to modify the resonant cavity and torch design.
The cavity was altered in two ways to permit its use with
different plasma gases and torch diameters.
The first modification
permitted simplified, reproducible alignment of different diameter
torches in the center of the cavity where the electric field is
greatest.
Copper adapters and face plates were machined to fit
differently sized plasma torches.
The adapters had a standard outer
diameter which were slip-fitted into the cavity housing.
New face
plates with similarly sized holes were machined because the face plates
were not thick enough to securely support a slip-fitted adapter.
The second cavity modification extended the tuning range of the
system.
The brass tuning screws ordinarily mounted on the cavity (both
axially and radially) were replaced with a double-stub tuner and quartz
tuning rod as shown in Figure 2-5.
The rod was positioned axially into
the cavity and parallel to the plasma torch.
It has been stated that
the tuning range of such a system depends on the diameter and
composition of the dielectric tuning rod, and on its axial and radial
position from the cavity center (43).
Different diameter quartz rods
were tested and a 6-mm-diameter rod inserted axially halfway into the
cavity was optimum for igniting and tuning the nitrogen discharge.
23
Figure 2-5:
Side-on view of modified TMq j q resonant cavity.
dimensions are in mm.
All
24
U G — 58
Connector
C o u p I ing
Loop
:
I*-5
s
s
PIasma
Tube
s
Q u a r tz
T u n in g
s
s
Rod
Copper
Body
25
Torch
The nitrogen discharge was initially sustained in a standard 6-mm
o.d., 4-mm i.d. open quartz tube.
Unfortunately, even with the cavity
modification, the plasma was then not always temporally and spatially
stable, probably because of the uncertainty in the gas-flow pattern
through the tube.
This instability would occasionally cause the plasma
to reside on the inner walls of the discharge tube and create a "hot
spot".
Etching of the tube was apparent from the resulting increased
Si and Na emission; this etching eventually led to the tube's
destruction.
The plasma instability also influenced the degree to
which the analyte vapor mixed with the discharge.
These kinds of
plasma instabilities have been noted by others also (15,34).
To overcome these problems, a stable gas-flow pattern was produced
through use of a torch similar to that employed with the ICP (44).
A
similar approach was taken by Bollo-Karma and Codding (37), who used a
threaded quartz insert to create in a MIP the spiral-flow pattern
characteristic of the ICP.
The present torch is similar in concept but
simpler in design; it consists of two concentric quartz tubes, an outer
one of 6-mm o.d. (4-mm i.d.) and an inner one of 1-mm o.d. (0.5-ram
i.d.).
The inner tube is approximately 2.5 cm in length.
A stable
tangential gas-flow pattern is created by forcing the plasma gas to
flow around the central tube.
This pattern was verified using the
hydrodynamic technique described by Sexton, Savage and Hieftje (45).
The nitrogen flow introduced tangentially contains the analyte
aerosol and sustains the plasma.
The low flow through the central tube
is mainly to keep the hot plasma from contacting that tube and melting
26
it.
If the plasma does touch the central tube, the tube will soften
and deform, changing both the tuning of the cavity and the stable
spiral-flow pattern of the sustaining plasma gas.
Initially, it was
intended to introduce the analyte through the central tube (similar to
an ICP) but the back pressure created by the small orifice of the torch
made using a flow-dependent nebulizer difficult.
Although a flow-
independent sample-introduction system such as an ultrasonic nebulizer
could conceivably be used, one was not available for use in the present
study.
Operating the Plasma
In early trials the new MINDAP torch was mounted like that of
Codding (37), where the central tube (or Codding's threaded insert) was
aligned with the outside wall of the microwave cavity.
Once the dis­
charge was ignited, a suspended plasma was then formed and tuning was
accomplished through use of the axial quartz rod and double-stub
tuner.
Unfortunately, it was not always obvious which tuning element
needed to be adjusted, since the position of each element altered the
effect of the others.
employed.
Consequently, an iterative adjustment was
Later, it was determined that the quartz rod was always
located halfway into the cavity for minimum reflected power.
Fixing
the quartz rod at this optimal place then made lighting the MINDAP as
easy as with other microwave plasmas.
In the final and presently used configuration (see Figure 2-6),
the axial quartz rod has been eliminated entirely; intracavity tuning
is effected with the MINDAP torch itself.
The torch is mounted so its
27
central tube extends into the TMQ10 cavity.
The added dielectric
from the the central tube serves the same purpose as the axial quartz
rod did formerly.
As with the quartz rod, this arrangement promotes
reproducible plasma ignition because the central tube is always fixed
in position.
Importantly, the reflected power is even lower with this
configuration than with the quartz-rod tuning element, probably because
of the radially central position of the added dielectric.
Plasma
torches with different central-tube lengths were tested; it was found
that the optimal length for plasma stability and tuning was equal to
half the inside cavity thickness (approximately 1.25 cm).
Whenever the
torch was replaced, minor adjustments of the double-stub tuner were
necessary to compensate for variations in the diameter and length of
commercial quartz tubes.
At present, the major obstacle in tuning the
cavity is oxide formation on the solid copper housing; periodically,
the cavity must be disassembled and cleaned with a household coppercleaning product.
In practice, the MINDAP is ignited in the following manner.
A
flow of greater than 1 L/min of nitrogen gas (typically 1.75 L/min) is
introduced into the side-on inlet and approximately 30-100 mL/min
through the central tube of the torch.
An applied power of greater
than 200 W (typically 250 - 350 W) is applied to the cavity.
A
tungsten wire attached to a wooden match stick (for insulation) is
inserted and moved about in the quartz torch within the ™
cavity.
q ^q
The wire is inductively heated by the field and the ejected
28
Figure 2-6:
Cross-sectional close-up view of the modified torch,
nitrogen discharge, and modified
cavity.
29
AFTERGLOW
CGreen Sheath)
Blue Discharge
™oio Cavi ty
Pink Discharge
Copper
Adapter
Modified Torch
m
■n2 * „Dri®d
Samp Ie
/
N.
30
electrons seed the flowing gas to initiate the plasma.
The plasma is
then tuned with the double-stub tuner for minimum reflected power.
Interestingly, when the plasma is not tuned properly there is an
audible hiss from the discharge.
Elimination of this noise corresponds
with minimum reflected power of about 5 - 20 W with 250 W of applied
power.
The MINDAP was not studied at power levels exceeding 250 W because
of limitations of the cables, tuning stubs and electrical connections.
At higher power settings the tuning stubs had to be carefully set or
large amounts of power could be reflected from the cavity, causing it,
the cables, and tuning stubs to become hot.
Another problem at such
high power levels was the tendency of the plasma to contact the walls
of the torch and etch them when sudden changes in tuning occurred.
This situation not only led to spurious signals but also made the
plasma spatially unstable.
In the present configuration the cavity is equipped with holes for
air cooling, although they were not needed for the power levels
studied. A plasma has been operated for long continuous periods of time
(18 hours) and the cavity temperature has never exceeded 50°C.
This
increase in temperature is gradual over the entire period of
operation.
With extended use, there is no noticeable change in the
appearance of the plasma or in the emission intensity from the
background and analyte.
31
Plasma Features
After ignition, the suspended plasma uniformly fills the discharge
tube and extends outside the cavity in a flame-like manner (Figure
2-6). The resulting plasma contains four distinct regions which are
distinguishable by color.
Two of the regions arise from the electrical
breakdown of the gas within the cavity and consist of a blue zone
centered in the discharge tube and a pink glow that surrounds it (46).
Their emission spectrum is comprised of the ^
First Positive, ^
Second Positive and Nj+ First Negative band systems (46-49).
These
pink and blue regions have been described as having a high degree of
ionizational and vibrational excitation and have been thought to be
where local heating of the support gas occurs (49).
These afterglows
extend through the cavity and slightly beyond it.
The other two discharge regions exist principally in the extended
tail flame of the discharge and are characterized by green and yellow
afterglows.
The green afterglow forms a sheath around the tail flame,
largely dictates its color, and reportedly results from the excitation
of impurities such as water or oxygen (47).
Common names applied to
this afterglow are "airglow", "aurora", and "air afterglow" (47,50).
The fourth zone is the characteristic yellow emission of the
Lewis-Rayleigh bands.
These bands are not always visible in the‘tail
flame but they can be optically detected.
The yellow emission was
first thought to be sodium etched from the torch walls, although
careful inspection proved otherwise.
These bands signal the existence
of "active nitrogen", so called because of its high degree of chemical
reactivity.
The length and intensity of each of the four discharge
32
regions is affected by gas purity, flow rate, pressure, electrical
power, temperature and also the length by which the discharge tube
extends beyond the cavity (48,49).
Under the conditions described above for igniting the nitrogen
discharge, a "suspended" plasma is formed (37).
However, it is
possible also to produce an ICP-like annular plasma by changing the
flow rate through the central tube and by altering the central tube's
axial position in the cavity.
This alternative plasma operating mode
is not extraordinary for this type of torch (4).
However, the annular
MINDAP can be sustained without residing on the inner walls of the
torch as reported earlier (37).
The results presented in this chapter
are for a discharge operating in a suspended rather than annular mode
because of the flow-dependent sample-introduction system that was
used.
It should be possible also to introduce sample aerosol into the
central tube and to employ an annular plasma if either a flow
independent or low-flow nebulizer is employed.
Reagents
All stock solutions were prepared as outlined by Dean and Rains
(51) using distilled-deionized water and reagent-grade acids, metals
and salts.
solutions.
Analytical standards were made by dilution of these stock
33
RESULTS
The MINDAP system was optimized for those conditions that produced
maximum analyte signal and signal-to-noise ratio.
The operational
conditions that were evaluated are applied microwave power, gas flow
rate through the sample-introduction system (which also sustains the
plasma), optical viewing geometry and spatial position in the plasma.
Viewing Geometry
The first parameter to be optimized was the viewing geometry of
the plasma.
Spectral scans were obtained for both radial (side-on) and
axial (end-on) viewing arrangements (Fig. 2-4) for three different
wavelength regions and with 10 yg/mL of several analytes present in
each:
(Li,K).
190 - 360 nm (Zn,Mg,Cu); 390 - 465 nm (Ca,Sr); and 650 - 800 nm
In these scans (see Fig. 2-7 to 2-9) the most intense peak
represents full-scale deflection of the readout system.
In the radial
configuration, the emission from the first 6 mm of the plasma tailflame was optically detected.
Operating conditions for the recording
of these spectra were an applied microwave power of 250 W, reflected
power of 20 W, 1.75 L/min of nitrogen gas sustaining the plasma and
0.03 L/min nitrogen through the central channel of the torch.
Spectra
for the two viewing geometries are similar in shape over the region 190
- 360 nm (Fig. 2-7) but the respective intensities are different:
the
axially obtained spectrum was approximately five times more intense than
that in the radial viewing configuration.
Emission from Zn, Mg and Cu
can be seen, but only with difficulty above a background consisting
34
Figure 2-7:
Comparison of optical viewing geometries for the spectral
region 190 - 360 nm in the MINDAP plasma.
Analyte
aerosol introduced into the plasma with desolvation was
10 yg/ml of Cu, Mg, and Zn.
Applied power was 250 W with
a nebulizer flow rate of 1.75 L/min.
radially.
B. MINDAP viewed axially.
A. Plasma viewed
» V'
190
Zn I
Zn I
230
PIacma
270
Mg II
CnnO
Mgl
Ax 1a I Iy
Mg II
Wavelength
V Iowed
O)-
Mg I
310
Cu I
Cu I
350
tn —
Cu I
Cu I
LO
Ul
36
Figure 2-8:
Comparison of optical viewing geometries for the spectral
region 390 - 465 nm in the MINDAP plasma.
Analyte
aerosol introduced into the plasma with desolvation was
10 yg/ml of Ca and Sr.
Conditions as in Fig. 2-7.
Plasma viewed radially.
B. MINDAP viewed axially.
A.
>
390
U)
(0—1
G>
Ca II
Ca II
Ca II
Ca II
405
cn
Sr II
Sr I
"
Wavelength
420
S X - L L Ca I
Sr II
Ca I
435
Cnm)
450
Sr I
Sr I
465
38
Figure 2-9:
Comparison of optical viewing geometries for the spectral
region of 650 - 800 nm in the MINDAP plasma.
Analyte
aerosol introduced into the plasma with desolvation was
10 pg/ml of Li, and K.
Conditions as in Fig. 2-6.
Plasma viewed radially.
B. MINDAP viewed axially.
A.
39
•H
►J
A. Plasma Vlawad Radially
r”
650
1
680
710
740
1- 1--- 1--- 1
770
800
Wavelength CnnO
H
i-l
►4
B. PIamma V iewed Ax Ia IIy
•*<
650
680
710
740
Wavelength CnnO
770
803
40
mostly of the NO, OH, NH, and N^ Second Positive band systems.
Both
zinc and magnesium are in spectral regions containing intense bands
from NO and OH.
Although copper emission occurs within the OH band,
the background level is relatively low at those wavelengths.
In the region between 390 - 465 nm, there is a distinct difference
between optical viewing geometries (Figure 2-8).
During axial viewing,
intense band emission from the N^+ First Negative and N^ Second
Positive systems dominates the spectrum and interferes with analyte
emission.
These bands are produced during ionization of nitrogen
inside the cavity; the strongest emission is from the N^+ (0,0)
bandhead at 391.4 nm.
Interestingly, the amount of energy required to
ionize and excite this nitrogen system is 18.74 eV (52).
Only two
monatomic gases that have been used to sustain MIPs have higher
ionization energies: helium (24.48 eV) and neon (21.56 eV).
The labels
in Figure 2-8 indicate only the approximate position where each element
should emit; specific lines could not be confidently located above the
background.
In comparison, the radially viewed plasma shows no significant
contribution from the
First Negative system for on-scale
measurements of analyte lines at the same solution concentration.
Viewed in this manner, the MINDAP produces a high signal level and
signal-to-noise ratio.
In the region from 650 - 800 nm (Fig. 2-9), the axially obtained
spectrum contains relatively weak band emission from the N^ First
Positive system, which interferes with the lithium (670.8 nm) and
potassium (766.5 and 769.9 nm) lines.
In contrast, radial viewing of
the plasma shows no significant interfering bands.
41
The superiority of side-on viewing is clearly evident from the
spectral scans from 390 nm to 800 nm (Figs. 2-8, 2-9).
Table 2-2 lists
all molecular bands that spectrally interfere with analyte emission in
either viewing arrangement.
The most troublesome bands that affect the
side-on geometry will be those in the ultraviolet spectral region (190
- 360 nm).
Applied Power and Nitrogen Flow
The operating conditions under which the plasma is sustained have
a marked effect on the length of the discharge tail-flame.
An increase
in applied power had the most noticeable effect (Fig. 2-10).
This
behavior can be explained by an increase in the excited-state
population of nitrogen which occurs at higher powers.
This increased
population would excite to a greater extent atmospheric constituents
beyond the cavity and thereby extend the length of the air afterglow.
Although Figure 2-10 was obtained when no sample aerosol was being sent
into the discharge, the presence of the aerosol does not appreciably
affect the tail-flame length.
Also, varying the nitrogen flow rate
does not affect the plasma length as greatly as does applied power.
The effects of applied microwave power and nitrogen flow rate on
iron atom emission at 371.9 nm are shown in Figure 2-11.
At all flow,
rates, the background-corrected signal increases with power and reaches
a plateau around 225 W.
However, at a flow rate of 2.1 L/min of
nitrogen, the signal intensity is only half that at lower flows,
presumably because of the shorter analyte residence time in the cavity,
a decrease in efficiency of the nebulizer, a change in the
42
Table 2-2:
aRef. 53
Interfering background band emission.
Element
Wavelength (nm)
Zn (I)
213.8
NO System
Mg (II)
279.6
OH System
Mg (I)
285.2
OH System
Cu (I)
324.7
OH and NH Systems
Ca (II)
393.7
N^+ First Negative
N^ Second Positive
Ca (I)
422.7
N^+ First Negative
N^ Second Positive
Sr (II)
407.8
N£+ First Negative
N£ Second Positive
Sr (I)
460.7
N 2 + First Negative
N 2 Second Positive
Li (I)
670.8
N 2 First Positive
K
766.5
N 2 First Positive
(I)
Interfering species3
43
Figure 2-10:
Effect of applied microwave power on plasma tail-flame
length (distance from cavity) with a flow rate of
1.75
L/min and 30 ml/min of nitrogen in the support and
central-channel torch ports, respectively.
Ccm)
Length
Tail-FI am©
1 S 0
200
AppIied Power CVO
250
300
45
Figure 2-11:
Effect of applied power and nitrogen flow rate on iron atom
emission at 371.9 nm.
All measurements are background-
corrected.
* - 1.35 L/min nitrogen gas.
A - 1.75 L/min nitrogen gas.
o-2.1
L/min nitrogen gas.
20
00
150
200
A p p 1ied Power CVO
250
300
47
concentration of exciting nitrogen species and a cooling of the
plasma.
These results indicate that the maximum signal occurs at a
flow rate of 1.75 L/min for this type of sample-introduction system.
Other elements that were studied showed the same trends apparent in
Figure 2-11.
Intensity measurements were made for several analyte atom and ion
transitions at applied power levels between 120 - 250 W and a nitrogen
flow rate of 1.75 L/min.
For the atom lines (Fig. 2-12) as with Fe (I)
(above), signals increase to a plateau with increasing power.
The
power level at which each element reaches a plateau appears to be a
function of the element's excitation potential:
sodium (excitation
energy = 2.08 eV) reaches a plateau near 170 W, whereas calcium (2.92
eV) levels off at 220 W.
Magnesium (4.33 eV) does not reach a plateau
within the power range of the present system, although a plateau power
of 300 W would be expected if the change with excitation energy is
linear.
These results are similar to those observed with a 7 MHz ICP
with nitrogen as the coolant gas (26).
In contrast, ion emission (Fig 2-13) does not appear to level off
at applied powers within the tested range.
Moreover, ion-line
intensity increases rapidly only at input powers above approximately
170 W.
This behavior indicates that the MINDAP should be operated at
power levels greater than 170 W for analytical measurements, especially
if ion lines are to be determined.
Signal-to-background-noise ratios were measured for power levels
of 120 - 250 W and are shown for atom and ion lines in Figure 2-14.
With the largest observable signal-to-background-noise ratio and signal
intensity occurring at 250 W for both atom and ion lines, the optimum
48
Figure 2-12:
Effect of applied microwave power on emission intensity
of several atom transitions in the MINDAP.
not drawn to the same scale.
1.75
Curves are
Nitrogen flow rates were
L/min and 30 ml/min as the support and
central-channel gases respectively,
o -
10pg/ml
Ca (I)
422.67 nm
* -
10yg/ml
Mg (I)
285.21 nm
+ -
10yg/ml
Na (I)
588.99 nm
Intensity
0
Line
0.
Atom
0.
0.
1 0 0
200
Applied Power CUD
250
300
50
Figure 2-13:
Effect of applied microwave power on the intensity of two
ion transitions in the MINDAP.
the same scale.
Curves are not drawn to
The nitrogen flow rates were 1.75 L/min
and 30 ml/min as the support and central-channel gases
respectively.
o - 10 yg/ml
Ca (II)
393.36 nm
* - 10 yg/ml
Mg (II)
280.27 nm
Ion Line I n t e n s i t y
— ©
©
©
cn
AppIi ed
©
Power
i\)
©
©
CVO
I\)
01
©
15
<S>
©
<S>
<s>
l\)
-k
0)
00
52
Figure 2-14:
Effect of applied microwave power oh the emission
signal-to-background noise ratio in the MINDAP.
o -
10yg/ml
Ca (I)
422.67 nm
* -
10yg/ml
Mg (II)
279.55 nm
+ -
10yg/ml
Na (I)
588.99 nm
5000
Background
N oise
6000
ignaI
/
2000
1000
GO
1 0 0
150
A p p I
250
200
i e d
P o w e r
( W )
300
54
operating conditions for elemental determinations using this system
are:
a suspended plasma viewed in the side-on configuration with an
applied power of 250 W and a nitrogen flow rate of 1.75 L/min.
Viewing Position in Plasma
Spectral scans from 190 - 350 nm were acquired at the optimum
power and flow rate at three different vertical viewing positions in
the plasma tail flame:
(Figure 2-15).
0 mm (top of the cavity), 5 mm, and 15 mm
At 0 mm, there is strong background emission from the
NO, NH, Ng, and OH band systems, as well as from the analyte
introduced.
At 5 mm, no change in spectral features is apparent except
that the overall intensity has diminished by a factor of five.
At
15 mm, the OH band becomes the dominant spectral feature and interferes
with the emission of both magnesium and copper.
There are basically four changes that occur as the viewing region
moves higher in the plasma tail flame:
overall emission intensity
decreases; NO, NH, N^ band intensity decreases; analyte emission
intensity decreases more rapidly than the background band emission; and
the OH intensity increases relative to the other components of the
spectrum.
These changes can be understood by recognizing that all
observed species but OH, and therefore their emission bands, originate
in the primary discharge region.
Moving away from this area and into
the afterglow (tail-flame) then reduces the magnitude of each band.
In
contrast, OH bands arise principally from the interaction of the plasma
with atmospheric water vapor (the sample aerosol has been desolvated).
Consequently, OH emission is maximal in the upper region of the tail
55
Figure 2-15:
Spectral scans of plasma background and analyte emission
for different vertical positions in the plasma tail
flame.
Analyte aerosol introduced into the plasma with
desolvation was 10 pg/mL of Zn, Cu, and Mg.
viewed in a side-on fashion (Fig. 2-4).
C are not on the same scale.
A.
0.0 mm
B.
5.0 mm
C.
15.0 mm
(top of the cavity)
Plasma
Scans A, B, and
190
Haight
5 .0
230
270
Uav«i«nflbh CnnO
<3
Mg II
Mg II
Mg I
310
s
Cu I
Cu I
Cu I
Cu I
Cu I
350
Ln
O'
57
flame. In the studies described later in this thesis, the integrated
emission was collected from the first 6 mm of the tail-flame in order
to maximize signal-to-background ratio and to smooth out any
fluctuations caused by tail-flame waver.
CONCLUSION
The MINDAP has many characteristics that are attractive in an
emission source for elemental analysis.
The gas-flow pattern is stable
and centered by a torch similar in design to the ICP concentric-tube
construction.
The new plasma is easily lit and aerosol sample
introduction is conveniently accomplished with a pneumatic nebulizer.
The optimized microwave power and gas-flow rates used to sustain the
plasma are slightly higher than those generally employed in argon and
helium MIPs, but are far lower than required for the ICP.
Impedance
matching of the power supply to the cavity for these high powers was
easily attained with the added dielectric from the modified torch and
tuning stubs.
The appearance of this new plasma is unusual compared to other
atmospheric-pressure MIPs.
The nitrogen discharge extends outside the
cavity to lengths determined by the operational conditions. For
example, under the recommended applied power of 250 W and a nitrogen
flow rate of 1.75 L/min, the discharge extends 10 cm beyond the
cavity.
This added discharge length provides longer analyte residence
times in the plasma and the consequent possibility of more complete
decomposition of the introduced analyte.
This long tail-flame also
offers an alternative viewing configuration for analyte emission.
The
58
plasma can be viewed either axially (as is common with most MIP
systems) or radially (as with an ICP system).
Based on background
emission spectra, the side-on arrangement was judged superior.
The potential of this system for chemical analysis is readily seen
in Figure 2-16.
Under the optimum operating and optical viewing
conditions, 10 ng/ml of lithium and 100 ng/ml of potassium are easily
detected.
The zero (0) level marker indicates the magnitude of the
dark-current contribution from the PMT.
This spectrum shows also that
there is essentially no background continuum emission to affect
measurements at these wavelengths.
In the following chapter the MINDAP
system will be critically evaluated from its analytical
figure-of-merits as a new, inexpensive, compact, sensitive and
versatile emission source for elemental analysis.
59
Figure 2-16:
Emission spectrum from 650 - 785 nm of the MINDAP tail
flame viewed radially.
Analyte aerosol introduced into
the plasma with desolvation was 10 ng/ml of Li and
100 ng/ml of potassium.
Applied power was 250 W with a
nebulizer flow rate of 1.75 L/min.
discussion of zero (0) marker.
See text for
0
WwUiyA
h i T r r f i i i i i ii
"f
~
i
660
680
700
720
i r i~
ii
~
~
t~
riri
740
760
780
i i
W a velength Cnm)
61
References
1.
S. W. Downey and G. M. Hieftje, Anal. Chim. Acta, 141, 193 (1982).
2.
C. A. Bache and D. J. Lisk, Anal. Chem., 43, 950 (1971).
3.
A. J. McCormack, S. C. Tong, and W. D. Cooke, Anal. Chem.,
1470 (1965).
4.
W. R. McLean, D. L. Stanton and G. E. Penketh, Analyst, 98, 432
(1973).
5.
C. I. M. Beenakker, Spectrochim. Acta, 32B, 173 (1977).
6.
H. A. Dingjan and H. J. Dejong, Spectrochim. Acta, 36B, 325
(1981).
7.
L. R. Layman
and G. M. Hieftje, Anal. Chem., 47, 194 (1975).
8.
A. T. Zander
and G. M. Hieftje, Anal. Chem., 50, 1257 (1978).
9.
F. L. Fricke, 0. Rose, Jr., and J. A. Caruso, Talanta, 23, 317
(1975).
37,
10. J. H. Runnels and J. H. Gibson, Anal. Chem., 39, 1399 (1967).
11. W. B. Robbins, J. A. Caruso and F. L. Fricke, Analyst, 104, 35
(1979).
12.
P. Barett and T. R. Copeland, "Applications of Plasma Emission
Spectroscopy", p.139, R. M. Barnes ed., Heyden Publishers, 1979.
13. T. Ishizuka and Y. Uwamino, Anal. Chem., 51_t 125 (1980).
14. I. S. Krull and S. Jordan, Amer. Lab., 12^, 21 (1980).
15.
A. T. Zander
and G. M.
Hieftje, Appl. Spectrosc., 35, 357 (1981).
16.
R. N. Savage
and G. M.
Hieftje, Anal. Chem., 51, 408 (1979).
17.
A. D. Weiss, R. N. Savage and G. M. Hieftje, Anal. Chim. Acta,
124, 245 (1981).
18.
R. Rezaaiyaan, G. M. Hieftje, H. Anderson, H. Kaiser and B.
Meddings, Appl. Spectrosc., 36, 627 (1982).
19.
L. Ebdon, D. J. Mowthorpe and M. R. Cave, Anal. Chim. Acta, 115,
171 (1980).
20.
B. Capelle, J. M. Mermet and J. Robin, submitted to Appl.
Spectrosc., 1982.
62
21.
H. U. Eckert, F. L. Kelly and H. N. Olsen, J. Appl. Phys., 39,
1846 (1968).
22.
S. Greenfield and P. B. Smith, Anal. Chim. Acta, 59, 341 (1972).
23.
H. P.
Freeman
andJ.D.Chase, J. Appl. Phys., 39^, 180 (1968).
24.
A. Montaser and J. Mortazavi, Anal. Chem., 52, 255 (1980).
25.
A. Montaser,
292 (1981).
26.
S. Greenfield and D. T. Burns, Anal. Chim. Acta, 113, 205 (1980).
27.
R. M. Barnes and G. A. Meyers, Anal. Chem., 521, 1523 (1980).
28.
M. Capitelli, F. Cramarossa, L. Triolo, and E. Molinari, Combust.
Flame, 15* 23 (1970).
V. A. Fassel and J. Zalewski, Appl. Spectrosc., 35,
29.
R. M. Barnes and S. Nikdel, Appl. Spectrosc., _30_, 310 (1976).
30.
J. D. Cobine and D. A. Wilbur, J. Appl. Phys., 22^, 835 (1951).
31.
A. N. Wright and C. A. Winkler, "Active Nitrogen", Academic Press,
N. Y., 1968.
32.
A. P. D'Silva, G. W. Rice and V. A. Fassel, Appl. Spectrosc., 34,
578 (1980).
33.
W. B.
Dodge III, andR.0. Allen, Anal. Chem., 53, 1279 (1981).
34.
G. A. Capelle
(1978).
andD.G. Sutton, Rev. Sci. Instrum., 49, 1124
35.
C. I. M. Beenakker, B. Bosman, and P. W. J. M. Boumans,
Spectrochim. Acta, 33B, 373 (1978).
36.
C. I. M. Beenakker, Spectrochim. Acta, 3IB, 483 (1976).
37.
A. Bollo-Kamara and E. G. Codding, Spectrochim. Acta, 36B, 973
(1981).
38.
J. E. Meinhard, ICP Inf. Newsl. 2., 163 (1976).
39.
C. Veillon and M. Margoshes, Spectrochim. Acta, 23B, 553 (1968).
40.
B. L. Sharp, Sel. Ann. Rev. Anal. Sci., j4, 37 (1976).
41.
C. B. Boss, personal communication, 1983.
42.
D. L. Haas, J. W. Carnahan, and J. A. Caruso, Appl. Spectrosc.,
37., 82 (1983).
43.
J. P. J. van Dalen, P. A. DeLezenne Coulander, and L. de Galan,
Spectrochim. Acta, 33B, 545 (1978).
63
44. T. B. Reed, J. Appl. Phys., 32, 2534 (1961).
45. E. Sexton, R. N. Savage and G. M. Hieftje, Appl. Spectrosc., 33,
643 (1979).
46. U. H. Kurzweg and H. P. Broida, J. Mol. Spec., _3, 388 (1959).
47. U. H. Kurzweg, A. M. Bass and H. P. Broida, J. Mol. Spec., _1, 184
(1957).
48.
G. E. Beale Jr. and H. P. Broida, J. Chem. Phys., 31, 1030 (1959).
49.
J. Kishman, E. Barish and R. Allen, Preprint 1982.
50.
J. W. Chamberlain, "Physics of the Aurora and Airglows", Academic
Press, New York, 1961.
51.
J. A. Dean and T. C. Rains, "Flame Emission and Atomic Absorption
Spectrometry", Vol. 2, M. Decker, New York, 1971, Ch. 13.
52.
J. P. Robin, Proc. Analyt. Atom. Spectrosc., _5, 79 (1982).
53.
R. W. B. Pearse and A. G. Gaydon, "The Identification of Molecular
Spectra", John Wiley and Sons, Inc., New York, 1976.
64
CHAPTER 3
Analytical Characterization of the Microwave Induced
Nitrogen Discharge at Atmospheric Pressure (MINDAP)
In Chapter 2 the microwave induced nitrogen discharge at
atmospheric pressure (MINDAP) was shown to possess many attractive
features for atomic emission spectrometry.
The initial expense of a
microwave power supply, impedance matching devices (tuning stubs),
cavity and cables are quite low (ca. $5000).
Also, because the plasma
is sustained using nitrogen at less than 2 L/min, the operating costs
are less than with either helium or argon which are respectively 2 and
3 times more expensive.
The instrumentation is compact because of the
low microwave power required ( 500 W) and the small size of a resonant
cavity operated at a frequency of 2450 MHz.
Importantly, the MINDAP
extends beyond the cavity to a length of 10 cm when the input microwave
power is 250 W.
This length enables side-on viewing (similar to the
inductively coupled plasma) and increases the analyte residence time
within the plasma.
Side-on viewing reduces strongly the background
emission from the discharge whereas a long analyte residence time
provides more complete decomposition of an introduced aerosol sample,
and enables more efficient signal collection.
Sample introduction is
straightforward in the MINDAP system, unlike that in most other
microwave plasmas.
Nitrogen has good nebulization qualities and the
flow rate used to sustain the plasma is compatible with most common
nebulizers.
Moreover, the applied power is slightly higher than
ordinarily used with other MIPs, making the MINDAP more tolerant of
water and large sample volumes.
'65
In this chapter the MINDAP is critically evaluated for use as an
atomic emission source for analytical measurements.
Results show that
the plasma exhibits detection limits (ng/mL) comparable to those of
other more elaborate and costly emission sources.
Calibration curves
are linear over a range of 3 to 5 orders of magnitude, typical of most
plasma sources.
In contrast,, concomitant interferences are similar to
those observed in flame spectrometry, and can be overcome in much the
same way with ionization suppressants 'and releasing agents.
A
comparison with other emission sources is made and the analytical
potential of the MINDAP assessed.
EXPERIMENTAL
The data collection and operating conditions for the MINDAP systemare identical to those described in Chapter 2.
The detection limit is
defined here as the concentration of analyte that produces a
background-corrected signal that is 1.96 (95% confidence interval)
times the standard deviation of the blank signal.
The mean signal and
background levels were obtained from fifteen consecutive
integrations.
1
-s
The limits of detection were obtained by extrapolation
from analyte solution concentrations of at least an order of magnitude
greater than those calculated.
66
RESULTS AND DISCUSSION
Detection Limits
Detection limits for sixteen elements were determined at
thirty-three different wavelengths and are compared in Table 3-1 with
those obtained from other microwave and inductively coupled plasmas.
Overall, the MINDAP provides sensitivity comparable to that of
competitive sources.
The lowest MINDAP detection limits arise from
atomic transitions; in fact, only five atom lines are more sensitive in
the argon MIP and ICP:
Cd, Cu, Mg, Pb, and Zn.
In contrast, ion-line
detection limits in the MINDAP are ordinarily worse than those of the
other techniques listed.
Ion lines such as those from Cd(II) and
Fe(II) were superimposed on strong background band emission from the NO
system, making their detection even more difficult.
Detection limits
for the easily ionizable elements were among the best found, perhaps
because the MINDAP is a more nearly thermal source than other plasmas.
From Table 3-1, it is apparent that spectral lines of lower
excitation potential exhibit better detection limits than those of
higher potential.
This feature suggests that analyte excitation in the
MINDAP occurs partially by a thermal mechanism, similar to that in a
chemical flame.
If so, and if the MINDAP is in local thermodynamic
equilibrium (LTE), the excited-state population would follow a
Boltzmann distribution.
No other microwave plasma yet reported has
been shown to be in LTE, although Busch and Vickers (1) postulated that
plasmas at atmospheric pressure should attain LTE because of their high
particle density.
67
Table 3-1:
Detection Limit Comparison for Atomic Emission
Atmospheric-Pressure Plasma Sources (ng/mL)
Element
Wavelength (nm)
MINDAP
Ar-ICPa
A1 (I)
Ba (II)
(II)
(I)
(II)
Ca (II)
(II)
(I)
Cd (II)
(I)
Cu (I)
(I)
Fe (II)
(I)
(I)
K (I)
(I)
Li (I)
Mg (II)
(II)
(I)
Na (I)
Ni (I)
(I)
Pb (I)
(I)
Pd (I)
(I)
Sr (II)
(II)
(I)
V (I)
Zn (I)
396.15
455.40
493.40
553.55
614.17
393.36
396.85
422.67
214.44
228.80
324.75
327.39
238.20
371.99
373.48
766.49
769.89
670.78
279.55
280.27
285.21
588.99
341.47
352.45
368.35
405.78
340.45
363.47
407.77
421.55
460.73
437.92
213.85
13.
15.
40.
19.
99.
2.3
7.5
19.
0.87
1.5
61. f
a
b
c
d
e
f
g
h
i
j
Ref.
Ref.
Ref.
Ref.
Ref.
Ref.
Ref.
Ref.
Ref.
Ref.
2, unless otherwise indicated
3
4
5
6
7
8
9
10
11
0.13
0.33
6.7
1.7
1.9
3.6
6.5
3.1
73. f
32.
30. b
1 . 2
410.
28.
4.4
2.3
280.
36.
98.
5.4
6.5
0 . 2 2
13.
1 1 .
1 2 .
0.29
1 0 .
5.3
84.
91.
45.
24.
16.
2 1 .
3.1
47.
1 2 0 .
.
Ar-MIP
60. c
1 0
. i
e
0 . 6
.
1 .
9.
1 0 .
400.
2 0
1.9
1
0 . 1 0
0 . 8
h
e
j
e
j
- j
e
0 . 2 0
1 . 1
19.
32.
30.
230.
180.
29.
36.
0.28
0.51
45.
77. f
1 . 2
5. j
80. j
. j
0 . 8
e
1 . 0
g
2 0 0
5. j
1 0 . j
80. d
0.5 g
68
Working Curves
Calibration curves for four elements, shown in Figure 3-1, are
linear over three-to-five orders of magnitude and are representative of
those obtained for other elements. Working curves were extended to
100 ng/mL, which is between two and three orders of magnitude above the
detection limits.
The dynamic range is expected to extend at least
another two orders of magnitude to the detection limit.
Similarly,
working curves have been extended to 5000 yg/mL, above which they begin
to bend downward.
This rolloff at higher concentrations is presumably
caused by a decrease in nebulizer efficiency.
At such high salt
concentrations, memory effects also were noted from analyte residue on
the inside of the quartz torch and the desolvation apparatus.
For
concentrations of 1000 yg/mL or less, no such effects were encountered.
Precision
Relative standard deviations were determined for a series of ten
consecutive measurements of analyte signal and background at concen­
trations two orders of magnitude and greater above the detection
limit.
The %RSD values remained constant for all concentrations and
were 0.3% for the blank and between 0.5% - 1.8% for the analyte; this
precision is among the best obtained for MIP systems (11) used for the
analysis of aerosol samples.
69
Figure 3-1:
Analytical working curves for the MINDAP.
o - Cu (I)
324.75 nm
* - K
(I)
766.49 nm
+ - Sr (I)
460.73 nm
□ - Ca (II)
393.36 nm
Re Iat ive
Intens
ity
IQ
10*
0 .1
10
100
1000
10000
**sj
o
Concentration
Cjug/m I)
71
Interferences
Classical interferents are those such as phosphate or aluminum
that depress analyte emission or those such as the alkali metals that
enhance analyte signals.
These types of interferences are well
documented in flame spectrometry literature (12-14).
Unfortunately, interference effects in the microwave plasma are
not as consistent or well understood as those in flame work.
Skogerboe
and Coleman (6 ) and Lichte and Skogerboe (5) concluded that interfer­
ences caused by the presence of easily ionizable species (e.g., sodium)
were not present in their plasma.
In contrast, many other workers have
documented strong interferences caused by sodium (10,11,15-19).
There
seems to be a general agreement, however, on the depressing effect of
phosphate on calcium emission from a MIF.
In assessing the analytical potential of the MINDAP, interelement
interferences were examined that involve vaporization (Al, P0^) and
ionization (Na).
Figure 3-2 shows that both calcium atom and ion
emission are enhanced as sodium concentration increases to 1%.
This
behavior is quite different from that observed in flames and is more
like that associated with an ICP or MIP.
In flame studies, the
increased concentration of electrons caused by sodium addition is known
to shift the calcium ionization equilibrium, enhancing the atom line
and decreasing the ion line.
£n contrast, the effect of sodium in
other plasmas is to change excitation behavior and thereby enhance both
atom and ion emission.
Although a non-thermal mechanism would therfore
seem to be operating in the MINDAP tail-flame, the behavior might also
be caused by a change in the plasma dimensions.
When sodium is added
72
Figure 3-2:
Sodium interference on 10 yg/ml calcium atom emission at
422.67 nm.
0.5
Emission
0.4
0.3
Ca
Intensity
0.6
0. 1
0.2
0
0
10 0 0
2000
3000
4000
5000
6000
lo
Sodium
Concentration
Cjug/m L)
74
to the MINDAP, it appears wider, much as has been observed in other
discharges (20,21).
A wider plasma would fill the discharge tube more
completely and thereby increase the interaction of the analyte with the
plasma.
This situation increases atomization efficiency and produces a
net increase in the emission signal.
In practice the effect of sodium or another easily ionized element
can be reduced or eliminated by the addition to both standard and blank
solutions of an excess amount of another easily ionizable species.
ability of
1 0 0 0
The
yg/ml cesium to reduce the ionization interference is
shown in Figure 3-3.
Even at this high cesium concentration, the
effect of sodium is still apparent, although less than when cesium is
absent.
At a concentration of 2500 yg/mL cesium, the enhancement was
able to be eliminated (see Fig. 3-3).
Aluminum and phosphate both depress calcium emission signals as
shown in Figures 3-4 and 3-5.
Although not presented in Figure 3-5,
the effect of phosphate on calcium emission intensity is constant to a
phosphate/calcium molar ratio of 60.
The behavior in Figures 3-4 and
3-5 is similar to that often observed in flames and other MIPs.
The
depression is probably caused by the formation of refractory phosphatecalcium and aluminum-calcium species, which reduce the rate at which
free calcium can vaporize (12-14).
Suppression of the formation of
those refractory materials is ordinarily accomplished by the addition
of a "releasing agent" such as EDTA or lanthanum (14,22,23).
plotted in Figures 3-6 and 3-7, the interference can be
the addition of EDTA as the ammonium salt.
As
eliminated by
75
Figure 3-3:
Sodium interference on 10 yg/ml calcium emission with
1 0 0 0
yg/ml of cesium added as an ionization suppressant.
Dashed line represents the addition of 2500 yg/mL of
cesium.
* - Ca (I)
422.67 nm
o - Ca(II)
393.36 nm
>
-4->
X U
(0
C
<D
C
H
0.15
c
0
»
0.1
(0
E
LU
200
Sodium
400
600
Concentration
800
Cjug/mLD
1000
77
Figure 3-4
Aluminum interference on 10 yg/ml calcium emission,
o - Ca (I)
422.67 nm
* - Ca(II)
393.36 nm
ity
.2
Re Iat ive
.6
Intens
2
.
8
.4
0
0
200
400
600
800
1000
Co
A I Concentration
(jug/mL)
79
Figure 3-5
Phosphate interference on 10 yg/ml calcium emission.
* - Ca (I)
422.67 nm
o - Ca(ll)
393.36 nm
Re Iat ive
Intens ity
2
5
5
0
oo
o
P04 / Ca Molar
Ratio
81
Figure 3-6:
Overcoming phosphate interference on 10 pg/ml calcium
emission by the addition of 0.01M EDTA as a releasing
agent.
Phosphate added as H 3 PO4 and EDTA as the
NH^ salt.
* - Ca (I)
422.7 nm
o - C a (II)
393.36 nm
Relative
P04
/
Ca
Molar
Ratio
Z8
Intensity
83
Figure 3-7:
Use of 0.01M EDTA to overcome the interference of
aluminum on
1 0
yg/ml calcium emission.
* - Ca (I)
422.67 nm
o - Ca (II)
393.36 nm
2
Calcium
Emission
Intensity
3
0
250
500
750
Aluminum Concentration C^ig/mL!)
1000
85
CONCLUSION
The MINDAP is a promising tool for
atomic-emission analysis of solutions.
low cost, sensitive
The plasma is easy to operate,
adapts conveniently to common nebulizer systems and uses inexpensive
nitrogen to operate.
Detection limits are low for most elements and
comparable to those offered by other plasma systems.
Working curves
are linear over three-to-five'orders of magnitude and the available
precision is excellent (0.5% - 1.8%).
Interelement interferences exist
but can be eliminated with the aid of ionization suppressants and
releasing agents as in flame spectrometry.
The MINDAP has many features that are more characteristic of
flames than plasmas.
Atom line emission is more intense than that of
ions, elements of lower excitation potential have better detection
limits, and similar effects from concomitants are observed.
results are interesting because the current
MIP do not include a thermal mechanism.
These
theories of excitation in a
In the following chapter
information concerning the plasma's physical characteristics are
described and used to explain the MINDAP's successful operation with an
aerosol sample-introduction system.
86
REFERENCES
1.
K. W. Busch and T. J. Vickers, Spectrochim. Acta, 28B, 85 (1973).
2.
P. W. J. M. Boumans,
3.
P. W. J. M. Boumans and R. M. Barnes, ICP Inf. Newsl., 3^ 445
(1978).
4.
R. K. Skogerboe and G. N. Coleman, Anal. Chem., 48, 611A (1976).
5.
F. E. Lichte and R. K. Skogerboe, Anal. Chem., 45, 399 (1973).
6
.
Spectrochim. Acta, 36B, 169 (1981).
R. K. Skogerboe and G. N. Coleman, Appl. Spectrosc., 30, 504
(1976).
7. N. Furuta, C. W. McLeod, H. Haraguchi, and K. Fuwa, Appl.
Spectrosc., 34, 211 (1980).
8
. R. K. Skogerboe, D. L. Dick, D. A. Pavlica, and F. E. Lichte,
Anal. Chem., 47, 568 (1975).
9. K. Fallgatter, V. Svoboda, and J. D. Winefordner, Appl.
Spectrosc., 25, 347 (1971).
10. H. Kawaguchi, M. Hasegawa, and A. Mizuike, Spectrochim. Acta, 27B,
205 (1972).
11. C. I. M. Beenakker, B. Bosman and P. W. J. M. Boumans,
Spectrochim. Acta, 33B, 373 (1978).
12.
C. Th. J. Alkemade and R. Herrmann, "Fundamentals of Analytical
Flame Spectroscopy", Ch. 9, John Wiley and Sons, New York, 1979.
13.
R. Mavrodineanu and H. Boiteux, "Flame Spectroscopy", Ch. 12, John
Wiley and Sons, New York, 1965.
14.
D. G. Peters, J. M. Hayes and G. M. Hieftje, "Chemical Separations
and Measurements", Ch. 20, W. B. Saunders Co., Philadelphia, 1965.
15.
C. Veillon and M. Margoshes, Spectrochim. Acta, 23B, 503 (1969).
16.
G. Pforr and K. Langner, Z. Chem., 5_, 115 (1965).
17.
G. Pforr and V. Kapicha, Collect. Czech. Chem. Coramun., 31,4710
(1969).
18.
S. Murayama, Spectrochim. Acta, 25B, 553 (1969).
19.
I. Kleinmann and V. Svoboda, Anal. Chem., 41^, 1029 (1969).
20.
W. E. Rippetoe, E. R. Johnson and T. J. Vickers, Anal. Chem.,
436 (1975).
47,
87
21.
P. W. J. M. Boumans, "Theory of Spectrochemical Excitation",
Hilger and Watts, London, 1966.
22.
A. C. West and W. D. Cooke, Anal. Chem., 32, 1471 (1960).
23.
J. I. Dinnin, Anal. Chem., 32, 1475 (1960).
88
CHAPTER 4
Physical Measurements of the Microwave Induced
Nitrogen Discharge at Atmospheric Pressure
In the previous two chapters several interesting observations were
made concerning the MINDAP's excitation ability.
In Chapter 2, a
direct correlation was observed between the excitation potential of an
element and the applied microwave power at which emission intensity
peaked.
Similarly, detection limits are found to be better for
elements of low excitation potential and Chapter 3 indicated that
atomic lines are more sensitive than their respective ion lines.
These
conditions suggest that a "thermal" mechanism is possibly responsible
for analyte excitation in the MINDAP.
In this chapter physical
measurements were made in an attempt to explain the phenomena by which
analyte is decomposed and excited in the MINDAP.
As stated previously, one of the problems encountered during the
analysis of solutions in a MIP is its low thermal energy.
From results
obtained with the nitrogen ICP, it would be anticipated that the MINDAP
should be closer to LTE than a conventional MIP.
Evidence to support
this supposition can be found in analyte excitation behavior discussed
in previous chapters.
In the present chapter, a quantitative
determination of several temperatures in the MINDAP is undertaken.
There are two regions of the plasma that are of particular
importance for spectroscopic measurements:
the primary discharge zone
where decomposition of the sample occurs and the tail-flame where
analyte excitation and emission are observed.
In the discharge region,
thermal (rotational) temperatures were measured for various operating
89
conditions and found to be between 4500 and 5000°K.
The tail-flame
also was extensively studied for its excitation ability using
spectroscopic temperatures, ion/atom ratios and spatially resolved
emission profiles as figures-of-merit. A tail-flame excitation
temperature of approximately 6000°K was measured, whereas ion-to-atom
ratios for magnesium lines were typically less than 0.5.
The spatial
profiles reveal that temperatures decrease as the distance from the
discharge region increases.
Similarly, analyte emission profiles
appear to depend on an element's excitation potential.
A comparison
with temperatures of other microwave and inductively coupled plasmas is
also presented.
TEMPERATURES AND THERMODYNAMIC EQUILIBRIUM IN LABORATORY SOURCES
For a system to be in thermodynamic equilibrium, it must meet
several criteria:
the velocity distribution of all particles in the
system must follow that dictated by Maxwell's equations; the population
of excited states must conform to a Boltzmann distribution; the
atom-ion equilibria must behave according to the Saha-Eggert equation;
and the distribution of the electromagnetic radiation must be in
agreement with Planck's law (1).
Simply stated, a system is in
thermodynamic equilibrium if it can be described by a single
temperature.
"Temperature" is in this sense a descriptive term that
relates to the magnitude of each type of energy associated with the
source.
Various temperatures describe the distribution of kinetic
energy of the electrons (electron temperature), of atoms and molecules
(translational, gas, or kinetic temperature), and the population of
90
atoms in different energy states (excitation or spectroscopic
temperature).
Ordinarily, it is assumed that lower-energy excitation
temperatures (rotational,vibrational) are the same as that which
governs the translation of large particles such as atoms and ions.
Most laboratory flames and plasmas are not in complete
thermodynamic equilibrium for a number of reasons.
One is that
radiation emitted by a source cannot be adequately described by
Planck's law because the source is transparent over large spectral
regions and, at atmospheric pressure, collisionally induced transitions
are far more frequent than radiative ones (1,2).
Another reason is
that thermal gradients are established as the gases leave the primary
reaction zone and mix with cooler atmospheric gases.
The situation
that exists when thermal equilibrium is established in particular
spatial locations in the source is termed "local thermodynamic
equilibrium" (LTE).
Laboratory flames and plasmas are generally
described qualitatively and quantitatively in terms of their deviation
from LTE.
At atmospheric pressure, LTE would be expected to prevail in
flames and plasmas because of their high particle density (1,3-5).
This suggestion appears to be valid for most chemical flames, but
appears not to hold for high-frequency electrical plasmas (MIP and
ICP).
In flame systems, two different situations exist; in the inner
cone (primary reaction zone) LTE seems not to be established but in the
secondary combustion zone (hot flame gases), which is a few centimeters
above the reaction zone,
thermal equilibrium is achieved (6,7).
conclusions are derived from localized temperature measurements.
the downstream flame gases, temperatures describing excitation,
These
In
91
ionization, and translation are equal and range between 2600°K to
3300°K for different types of flames and operating conditions.
In
contrast, no single temperature can be defined in the primary reaction
zone, although a rotational temperature of 5600°K for an
acetylene-air and acetylene-oxygen flame contrast sharply with the
spectroscopic temperature of only 2500°K (6).
This disparity
suggests that the exchange of rotational energy through collisions is
more effective than the exchange between electronic and rotational
energies (6).
Clearly, deviation from LTE in this region is
considerable.
The degree to which a particular plasma approaches LTE depends
strongly on its physical nature and operating conditions.
Microwave
induced plasmas (MIP) operating at 2450 MHz on low power and flow rates
of either argon or helium gas tend to be compact discharges that remain
inside the resonant cavity that supports them.
Because the MIP is
small, spatially averaged temperatures are usually reported, some of
which are listed in Table 4-1-.
The inequality of the various tempera­
tures indicates a substantial departure from LTE.
The low thermal
temperature explains why analysis of aerosols directly introduced into
the MIP is difficult and also why volatilization and atomization
problems occur (8).
Compared to a MIP, the inductively coupled plasma (ICP) operates
at lower frequencies (27.12 and 40.68 MHz), at higher powers and flow
rates, and yields a much larger discharge.
Although this larger
discharge ('vlOO mm in length) enables it to be mapped spatially, few
local regions appear to approach LTE.
Kalnicky (9), Mermet (10) and
Kornblum (11) all agree that under their experimental operating
92
Table 4-1:
Temperatures Reported for Several AtmosphericPressure Microwave-Induced Plasmas.
__________________ Temperatures (°K)__________________
Support
Gas
Electronic
Excitation
Ionization
Rotational
Ar
5000 (a,b,c)
5000 (a)
2000 (a)
He
7250 (d), 8000 (c)
--
2100 (e)
a Ref. 12
b Ref. 13
c Ref. 14
d Ref. 15
e Ref. 8
93
conditions the Ar-ICP is not in LTE.
In contrast, Weiss (16) has shown
through extensive spatial mapping that there are regions in the Ar-ICP
where LTE is approached as well as regions that depart from LTE, as
claimed by others (9-11).
A helium ICP has been sustained and
temperature measurements indicate that it too is not in LTE (8 ).
A
comparison of these temperatures is presented in Table 4-2 for ICPs
supported in several gases.
Recently, ICPs have been described that use nitrogen as an
inexpensive diluent or replacement for the argon support gas (17-19).
Temperatures measured for such argon-nitrogen and pure nitrogen plasmas
are included in Table 4-2. Argon plasmas were found to have higher
excitation temperatures (2
0
,2 1 ) but nitrogen-doped plasmas decomposed
particles more rapidly and efficiently (18,21,22).
Similarly, lower
ion-to-atom ratios and lower total continuum radiation is reported for
the Ar-N^ and ^
plasmas (18).
These results suggest that nitrogen
introduced into the ICP increases its thermal temperature and results
in a plasma that is closer to LTE.
EXPERIMENTAL
The MINDAP was sustained as a suspended plasma and viewed radially
for all measurements reported in this chapter.
Except for those
involving rotational temperatures, all measurements were made with the
spectrometric system discussed in Chapter 2.
Rotational temperatures
were determined using a high-resolution monochromator (Model HR1000,
I.S.A. Inc., Metuchen, N. J.) with a slit width of 10 pm (0.04 A°
resolution).
94
Table 4-2:
Temperatures Measured in Several AtmosphericPressure Inductively Coupled Plasmas.
Temperatures_(°K)________________
Support
Gas
Ionization
Rotational
Ar
6000-9000
He
4100 (a)
2400 (a)
Ar-N2
4500-5100
5000-7000 (b,c)
n2
a Ref.
Electronic
Excitation
8
b Ref. 20
c Ref. 23
d Ref. 24
e Ref. 11
f Ref. 25
7000-8000 (f)
2100, 4000 (e)
6000-8000 (d)
95
Spatial emission profiles for several elements of different
excitation potential were collected in the tail flame of the MINDAP.
1
A
-nnn circular aperture was positioned in front of the entrance slit of
the Heath monochromator upon which radiation from the plasma was
focussed.
The spatial resolution for these profiles was 0.5 mm
vertically and 50 ym horizontally of the central portion of the
tail-flame.
The microwave cavity and torch assembly were translated
vertically by means of a programmable stepper motor (Denco Inc.,
Tucson, Ar.); a distance of 5 cm for this travel was sufficient to
acquire the entire profile for each element.
Data for the temperature
measurements were not Abel inverted for radial spatial information.
Similarly, all intensity measurements were corrected for the photo­
metric response of the measurement system.
Temperature Measurements and Calculations
Equations used to describe the various temperatures of the plasma
necessarily assume that energy distribution is thermal and that an
observed emission intensity can be predicted from the Boltzmann
relation.
However, complete LTE is not required for the definition of
a particular kind of temperature (e.g., ionization) or for its
measurement; it is necessary to assume only that equilibrium is
established among the levels described by that temperature.
Electronic Excitation Temperature.
The spatially integrated
excitation temperature of a 50 ym-wide section from the central portion
of the first
6
mm of the MINDAP tail flame was determined using iron as
96
a thermometric species.
An iron solution of 1000 yg/raL was introduced
into the plasma and the relative radiances of six atom lines in the
spectral region of 355 - 385 nm were measured.
The excitation
(spectroscopic) temperatures at different applied microwave powers and
nitrogen flow rates through the side-on inlet of the torch were
determined from a logarithmic form of the Boltzmann equation (26,27)
(eq. 4-1).
in
where
I
-
V (kIE>
(4-1)
= relative intensity of specific iron lines
= wavelength of the observed iron transition
g^
= statistical weight of the upper state
A ^ = transition probability
k
= Boltzmann constant
E^
= excitation energy of the excited state
TE
= electronic excitation temperature
A plot of the left hand side of equation 1 against the excitation
energy of the excited state (E^) yields a straight line with a slope
inversely proportional to the excitation temperature [-l/(kT_)].
fe
Table 4-3 summarizes the transitions, statistical weights, transition
probabilities and excitation energies used for these measurements.
Ionization Temperature.
Spatially integrated intensity
measurements of magnesium atom (285.4 nm) and ion (279.55 nm) lines
were collected with the same spatial position and resolution as used
97
Table 4-3:
Spectroscopic data for Electronic Excitation
Temperature measurements using Fe (I) emission
lines3 .
Wavelength (nm)
Eki(em_1)
358.120
34844
13
1.03
360.886
35856
5
0.797
371.99
26875
11
0.163
373.487
33695
5
0.886
381.584
38175
7
0.948
382.043
33096
7
0.638
aRef. 2
with excitation temperatures.
The ion-atom ratios were calculated from
these values and substituted into the Saha-Eggert equation (27)
(eq. 4-2),
I_ I
4.83 x 1015
n,
e
A
where
hi
( 9k Ak1 \
/
\
\ 9k Ak1 / A
Xk1
/I
5040(V, + V,
\
(4-2)
” i°n/atom intensity ratio
ng
a electron number density
* ionization temperature
Vj
= ionization
energy (7.65 eV for Mg)
V^j.
= excitation
energy for the ion line (4.43 eV)
a excitation
energy for the atom line (4.35 eV)
from which ionization temperatures were calculated by an iterative
process.
Because of the form of the equation, no explict expression
for temperature is obtained.
Consequently, temperature values are
substituted into the Saha-Eggert equation and a resultant ion/atom
ratio is obtained.
determined one.
This ratio is compared to the experimentally
The temperature is varied until both experimental and
calculated ion/atom ratios are equal.
These temperatures were obtained
for several applied powers and nitrogen flow rates through both inlet
ports of the plasma torch.
Electron Number Density.
Two methods were employed in an attempt
to determine electron number densities in the MINDAP:
the series-limit
99
line-merging technique described by Montaser, £ £
(28) and the
method involving Stark-broadening of the hydrogen Balmer line described
by Griem (29).
The line-merging technique used by Montaser and described earlier
by Pannekock (30) and then by Inglis and Teller (31) takes advantage of
the fact that the principal quantum number of the last discernible line
in a series depends on the electron number density according to
equation (4-3):
log (n) = 23.26 - 7.51og ( n j
(4-3)
In eq. 4-3 n^ is the principal quantum number at which the merging of
the series lines occurs and n is the sum of the electron and ion
densities.
In an electrically neutral plasma, such as those used for
elemental analysis, the electron and ion densities are equivalent, so
the electron density is just half the value (n) obtained from eq. 4-3.
Experimentally, a solution containing a single element, either Al,
Li, or Ca, at a concentration between 2500 ppm and 5000 ppm is
introduced into the plasma.
The emission spectrum from the
transitions (see ref. 28) is then scanned;
2
P -
2
D
The principal quantum
number from the last discernible transition, where the lines merge into
a continuum, is then used for the calculation of electron number
density (eq. 4-3).
This technique
is easy to employ but is criticized
because of the difficulty involved
inidentifying the last
"discernible" line in the series.
In addition, transitions must be
chosen in spectral regions where plasma background emission will not
interfere with them.
100
Electron densities can be obtained also by accurately measuring
the full-width at half maximum (FWHM) of the Stark-broadened H
p
emission line at 481.6 nm.
After deconvolution of the line profile, to
account for instrumental, collisional and Doppler broadening, the
electron density can be related to the FWHM by eq.(4-4):
«e - C < v V
where
AAg
<*-*>
S )3/2
= FWHM of the Hg line
C(ne>Te) = coefficient that depends on temperature and
electron density
These C values are compiled in Griem's book (29) for different values
of electron temperature and number density.
In practice, a spectral scan of the H
line is all that is
P
necessary to determine the FWHM of the line because of its considerable
Stark broadening ( 1 - 5 A0) (4,9).
Broadening from the effects
mentioned above are ordinarily small and therefore can be neglected.
Introduction of a solution, water, or even hydrogen gas enhances the H^
emission signal and makes the determination less sensitive to
electronic and plasma background noise.
Rotational Temperatures.
Rotational temperatures were calculated
from the relative intensities of lines in the R and P branches of the
N£+ First Negative band system.
These measurements were made in
the discharge region of the plasma because the intensity of the lines
was much stronger than in the tail-flame.
Calculations of the
101
rotational temperatures from the OH band in the tail-flame were
attempted, but assignment of each line was nearly impossible.
A spectral scan of the
2
£ -
2
£ transition, taken in the
primary discharge region, is shown in Figure 4-1.
Individual lines
were assigned on the basis of the data of Childs (32).
The intensity
of each line in the R and F branch can be described from eqs. (4-3) and
(4-6) (33).
Ij = 4
(4-5)
4
where
A
a C
em
V
/Q = constant because the wavelength range is
r
small.
Cgm ® emission constant depending on the dipole
moment,
V
is the optical frequency of the transition and
Qr is the number of molecules in the rotational states.
K1
= quantum number of the upper state
K"
= quantum number of the lower state
2
B'
= rotational constant of the upper
Ip
= line intensity of the R branch
Ip
= line intensity of the P branch
h
= Planck's constant
c
= speed of light
k
= Boltzmann constant
£ state
= rotational temperature
Combining equations (4-5) and (4-6) and assuming a Boltzmann
102
Figure 4-1:
Vibrational-rotational emission spectrum of the
2
E transition of the
+
9
E -
First Negative band system
viewed axially in the MINDAP with a high-resolution
monochromator.
Operating conditions were an applied
microwave power of 250 W, reflected power 20 W, nitrogen
flow rate of 1.75 L/min and 0.1 L/min through the side-on
and central-channel of the plasma torch, respectively.
eoi
R E L A T IV E IN T E N S IT Y
in
o
G>
(0 -0 )
O)
to
oo
-G>
ocd-------
•s
-j
ad
O)
co
■>
in
9C4 -------
m —
1
00 i
-<S ?
O) c
CO ®
®
>
o
(O
-CJ)
00
CO
j
-O )
oo
CO
104
distribution among rotational levels, permits the rotational
temperature to be computed [equation (4-7)].
In CaI/(K'+K"+l))
where
3
A - B'K'(K"+l)hc/(kTr)
(4-7)
B'hc/k = 2.983
a = 1 for even values of K' of the R branch
a = 2 for odd values of K' of the R branch
A semilogarithmic plot of In (ai/(K'+K"+l)) vs. K'(K"+1) in eq. (4-7)
will yield a straight line with a slope of -B'hc/(kTr)) from which
the rotational temperature can be determined.
This method has the
advantage of requiring only relative intensities.
RESULTS
Electronic Excitation Temperatures
Spectroscopic temperatures were determined for a range of applied
microwave power levels and nitrogen gas-flow rates (through the side-on
inlet of the torch).
Interestingly, a linear relationship was observed
between applied microwave power and temperature (Fig. 4-2).
For the
various nitrogen flow rates, the temperature varies from an average
value of 4100°K for an applied power of 150 W to 5800°K at 250 W.
Not surprisingly, excitation temperatures were least for the highest
nitrogen flow rate which was employed (2.1 L/min).
As more gas is
introduced into the plasma, more of the gas molecules have to be heated
105
Figure 4-2:
Excitation temperature integrated over the first
the MINDAP tail flame.
6
mm of
An iron solution of 1000 yg/mL
was introduced as a thermometric species.
* - 1.35 L/min
nitrogen flow in the side-on port of the
MINDAP torch.
+ - 1.75 L/min
nitrogen flow in the side-on port of the
MINDAP torch.
o-2.1
L/min
nitrogen flow in the side-on port of the
MINDAP torch.
Exc itat ion
Temperature
CIO
6000
5500
5000
4500
4000
3500
150
250
200
AppIied Power
CW)
300
107
and the lower the temperature will be for a given power input.
Strangely enough, temperatures for 1.35 and 1.75 L/min are quite
similar.
The same dependence on flow rate was noticed for the emission
intensity of iron atoms in Chapter 2 (Fig. 2-11).
These experimentally
obtained temperatures also support the optimum operating conditions
selected in Chapter 2.
It is not surprising that at 250 W of applied
power the MINDAP compares favorably with the argon MIP and ICP.
A
comparison of excitation temperatures for these different plasmas
(Table 4-1, 4-2) indicates that the MINDAP, Ar-MIP and ICP should all
have similar excitation abilities.
The detection limit comparison of
Chapter 3 (Table 3-1) also supports this conclusion.
Ion/Atom Ratios and Ionization Temperatures
Figure 4-3 shows the magnesium ion-to-atom line intensity ratio as
a function of applied power and nitrogen flow rate.
As the power
increased so did the ratio in a 1/(A+Bx) fashion where A and B are the
intercept and slope from the best-fit curve to the data.
This
increasing ion/atom ratio is consistent with the results obtained in
Chapter 2, in which atom-line intensities reached a plateau whereas ion
signals increased steadily with applied microwave power.
The effect of
increasing the nitrogen flow rate from 1.35 L/min to 2.1 L/min was less
than 5% at any power level.
The magnesium ion/atom ratio obtained here is much smaller than
that commonly reported in the Ar-ICP, which for most elements is
typically greater than one and for magnesium approximately 11 (34,35).
This "ion line advantage" in the ICP is one which does not exist in
108
Figure 4-3:
Ion-atom line-intensity ratios for 10 yg/mL magnesium
introduced into the plasma in three different flow rates
of nitrogen:
2.1 L/min, 1.75 L/min, and 1.35 L/min.
There is less than a 5% deviation among the curves
obtained at different flow rates.
-
Ratio
Ion
/ Atom
0.3
1 0 0
150
200
250
300
109
AppIied Microwave
Power
CIO
110
either flames or MIPs.
When nitrogen was introduced into the Ar-ICP a
decrease in ion-to-atom ratio was observed, suggesting that in the
presence of nitrogen the ICP is closer to LTE (18,19).
Ionization temperatures can be calculated from the ion/atom line
ratios of Fig. 4-3 if electron densities are known.
Unfortunately,
Stark broadening of the hydrogen Balmer line could not be used because
it was not visible in the emission spectrum.
Addition of molecular
hydrogen or water vapor into the MINDAP served only to increase the
intensity of the molecular (NH,N0,0H) bands; no atomic emission from
either hydrogen or nitrogen (which can also be used for electrondensity measurements) was observed.
The line-merging technique for electron-density determination was
also tried but to no avail.
The weak transitions could not be observed
for aluminum, lithium, calcium or potassium even though their
convergent series lay in different spectral regions.
Therefore, in
order to calculate the ionization temperatures, an electron density was
tentatively assumed.
For atmospheric-pressure plasmas this number is
often between 1 0 ^ - 1 0 ^ cm ^ (4).
In Table 4-4 ionization
temperatures for the MINDAP are calculated for several different
assumed electron number densities.
Depending on the ion/atom ratio, a
temperature difference of 1300 - 1600°K is obtained for a 100-fold
increase in electron density.
For the calculation of ionization
temperatures, shown in Figure 4-4, an electron density of
5 x 10
cm
3
was selected; Zander (36) and Fallgatter, et al. (12)
determined electron number densities to be between
10
15
1 0
^
and
-3
cm
for He and Ar MIPs operating at atmospheric pressure.
Ill
Table 4-4:
Ionization temperatures calculated for a range
of ion/atom ratios observed in the MINDAP (see
Fig. 4-3) and for different assumed electron
number densities (n ).
e
Ion/Atom Ratio
n
e
0 . 1
0 . 2
0.3
0.4
1.0E14
4683
4849
4951
5026
2.5E14
4904
5086
5198
5280
5.0E14
5085
5280
5401
5490
7.5E14
5198
5401
5527
5620
1.0E15
5280
5490
5620
5716
2.5E15
5561
5793
5937
6043
5.0E15
5793
6043
6200
6316
7.5E15
5937
6200
6365
6486
1.0E16
6043
6316
6486
6613
112
Figure 4-4:
Ionization temperatures calculated using the ion/atom
ratios o£ Fig. 4-3 and an assumed electron density of
5x10
14
-3
cm .
The central-channel nitrogen flow rate
was 0.1 L/min.
* - 1.35 L/min
nitrogen flow in the side-on port of the
MINDAP torch.
o - 1.75 L/min
nitrogen flow in the side-on port of the
MINDAP torch.
+ - 2.1
L/min
nitrogen flow in the side-on port of the
MINDAP torch.
CIO
Temperature
Ionization
6000
5500
5000
4500
1 0 0
150
250
200
300
113
AppIied
Power
CIO
114
Four basic conclusions can be drawn from a comparison of
calculated ionization and excitation temperatures in the MINDAP
tail-flame.
First, ionization temperatures increase linearly with
applied power as do excitation temperatures (Fig. 4-2).
Secondly and
most practically, high temperatures (5000 - 6000°K) suggest the
ability of the tail-flame to excite and ionize analyte. Third, although
excitation and ionization temperatures are similar, they differ by as
much as 20% at some applied powers.
This difference is not surprising,
since only a single electron density was used for calculating
ionization temperatures at all operating conditions.
In MIPs (40) the
electron number density appears to be slightly dependent on the applied
power; at low powers the electron density is lower than for higher
powers.
This effect would decrease the calculated ionization
temperature at the lower powers and increase them at the higher applied
powers, making them closer to measured excitation temperatures.
Fourth, excitation temperature is dependent on flow rate whereas
ionization temperature is not.
These latter two differences strongly
suggest a departure from LTE in the MINDAP tail-flame, although this
deviation appears to be less dramatic than in most other plasma
sources.
Effect of Central-Channel Nitrogen Flow
Central-channel nitrogen flow rates have ranged over 0.03 0.1 L/min during various stages in the MINDAP's development.
Ionization temperatures were calculated as above for central-channel
flow rates between 20 - 800 mL/min and at the optimum operating
115
Figure 4-5:
The effect of central-channel nitrogen flow on the
ionization temperature calculated from measured ion/atom
line ratios and an assummed electron density of
5x10
14
-3
cm .
Operating conditions were an applied
microwave power of 250 W, reflected power of 20 W and a
flow rate of 1.75 L/min through the side-on inlet of the
MINDAP torch.
CIO
5600
Ionization
5700
Temperature
e»
5500
5400
5300
5200
100 200 300 400 500 600 700 800 900 1000
Central
Channel
Flo w Ra t e
CmL/min)
116
0
117
conditions; values are shown in Figure 4-5.
At a central-channel flow
rate of 20 mL/min, the plasma is positioned immediately beyond the
central tube.
Consequently, some energy is lost to the torch that
would ordinarily be available to ionize and excite the introduced
aerosol.
As the flow rate increases the temperature also increases
because the plasma moves away from the tube and is cooled less by it.
At higher flow rates ( > 500 mL/min), a decrease in temperature occurs
because a greater volume of support gas must be heated.
Also, such a
high gas flow moves the discharge off center and at the same time
begins to punch a hole in the base of the discharge (similar to an
ICP).
For later studies, the central-channel flow rate was maintained
at 0.1 L/min to conserve gas, promote plasma stability and provide high
temperatures.
•Rotational Temperatures
Rotational temperatures measured axially in the primary discharge
region are plotted in Figure 4-6 for a range of operating powers and
nitrogen flow rates.
These temperatures, measured with no aerosol
introduced into the MINDAP, correspond to the thermal energy available
in the discharge region and are therefore an indication of the plasma1s
ability to decompose aerosol samples into their atomic constituents.
As with the other measured temperatures (ionization, excitation), a
linear increase with applied power was noted.
In contrast, higher flow
rates of nitrogen gas produced higher temperatures.
Importantly,
rotational temperatures were somewhat lower (^500°K) under a given
set of conditions than either excitation or ionization temperatures
118
Figure 4-6:
Rotational temperatures calculated from R and P branches
of the
2
E -
2
E
+
First Negative band system.
The plasma was viewed axially and no aerosol was
introduced into it.
The central channel flow ratewas
0.1 L/min.
0
-
2 . 1
L/min
nitrogen flow in the side-on port of the
MINDAP torch.
* - 1.75 L/min
nitrogen flow in the side-on port of the
MINDAP torch.
+ - 1.35 L/min
nitrogen flow in the side-on port of the
MINDAP torch.
r\
5500
5200
<D
C
L
£
4900
<D
+
I—
_
4600
+
0
c
0
-+J
4300
0
-4->
0
q
:
200
Microwave
250
Power
CWD
120
even though they were obtained in the absence of aerosol introduction
and in the center of the MINDAP discharge, presumably its hottest
zone.
Of course, it would not be surprising if this hot zone were well
removed from LTE and that the excitation and ionization temperatures in
it were much higher than the rotational temperatures.
Nonetheless, the
discharge center is clearly a thermally hot (5000°K or greater)
environment and one well suited for atomization of aerosol samples.
Spatial Profiles of Elemental Emission
Spatial emission profiles for four elements shown in Figure 4-7
reveal that the greatest signal is generated just above the tip of the
MINDAP torch.
Interestingly, the vertical position where analyte
emission becomes relatively constant depends at least partially on the
element's excitation potential.
Sodium, with an excitation energy of
2.08 eV, levels off near 38 mm above the cavity whereas zinc (5.75 eV)
emission approaches a constant value much lower in the tail flame
( 1 2
mm).
Spatially Resolved Temperatures
Spatially resolved excitation and ionization temperatures were
measured over the first 18 mm of the MINDAP tail-flame and are plotted
in Figure 4-8.
The substantial difference between the temperatures
close to the cavity ( 0 - 3 mm) argues that LTE is not approached there
presumably because the main discharge (which originates inside the
cavity and is assumed to be well removed from LTE) extends outside the
121
Figure 4-7:
Spatial emission profiles of several elements in the tail
flame of the MINDAP.
Operating conditions were an
applied power of 250 W, reflected power 20 W, flow rate
of 1.75 L/min and 0.1 L/min through the side-on and
central channel of the plasma torch.
A -
10yg/mL A1
(I) 396.2 nm
B -
10yg/mLZn
(I) 213.8 nm
C -
10yg/mL Sr
(I) 460.7 nm
D -
10yg/mL Na
(I) 588.9 nm
122
B
HZI
- 0.9
•.0
-
0.8
- 9.7
9.7
• 9.6
8.8
- 9.8
- 9.4
- 9.3
- 9.2
- 9 .1
- 9
4
3
2
I
8
4
3
M * . © M in
9
Plo
R-lol
t
123
Figure 4-8:
Spatially resolved electronic excitation and ionization
temperatures in the tail flame of the MINDAP.
conditions are 250 W applied power,
2 0
Operating
W reflected power,
with flow rates of 1.75 L/min and 0.1 L/min through the
side-on and central channel inlets of the plasma torch.
* - excitation temperature
o - ionization temperature
Temperature
CIO
7500
6500
5500
4500
3500
0
3
6
9
12
15
Height Above Cavity (mm)
18
21
125
cavity.
Higher in the tail-flame the two temperatures are much closer,
indicating a different, possibly thermal excitation mechanism.
DISCUSSION
The MINDAP discharge region and tail-flame have been described in
this chapter in terms of the energy available for decomposing aerosols
and exciting the resulting atoms and ions.
Applied power has been the
parameter that affects most greatly each of the temperatures measured.
It is interesting to calculate the gas temperature which would be
expected as a result of simple Joule heating at each applied power.
This temperature can be calculated from eq. 4-8:
W = cs_Mfr (T - 273)
G G__________
(4-8)
fSTP
where
W
= power required to heat the gas to a given temperature
(watts).
c
«*conversion factor (4.19 joules/cal).
Sg
= specific heat of the gas (cal/g).
M
= molecular weight of the gas (g/mol).
£„
= flow rate of the gas (L/sec).
^STP = ^ *414 L/mol of gas at standard temperature and
pressure.
T
= temperature (°K).
Table 4-5 summarizes the results from this computation for nitrogen gas
at flows used in this study.
126
Table 4-5:
Applied power required to heat nitrogen to
specified temperatures assuming constant
heat capacity.
8
M 0.2477 cal/g
M =* 28 g/mol
Flow Rate (L/min)
Temperature
1.35
1.75
2.1
6000
167
217
260
5500
160
198
237
5000
138
180
215
4500
123
160
192
4000
109
141
169
127
At an applied power of 250 W, with a reflected power of 20 W and a
flow rate of 1.75 L/min, the amount of power calculated to heat
nitrogen up to 5500°K is almost 200 W.
This requirement leaves 50 W
unaccounted, of which 20 W can be ascribed to reflected power.
The
remaining 30 W is probably dissipated as heat in the cavity, cables,
tuning stubs, electrical connections, and also as radiation losses and
from absorption of the microwave power by water.
Unfortunately, the calculated values in Table 4-5 and the
experimentally determined temperatures contradict each other.
The
rotational temperatures increase with flow rate at any given power,
excitation temperatures are highest for 1.75 L/min and ionization
temperatures are found to be independent of flow rate.
This disparity
is not too surprising since the values in Table 4-5 are based on the
assumption of constant heat capacity and thermal conductivity.
In
fact, these parameters change strongly with temperature, especially for
a diatomic gas like nitrogen.
Barnes and Nikdel (37) have described
mathematically nitrogen's heat capacity and its electrical and thermal
conductivities for several temperature ranges below
2 0
,0
0 0
°K.
Between 6500 and 9000°K the thermal conductivity behaves anomolously,
dropping to a minimum and then increasing with temperature.
Even with
corrections for these changes, it is unlikely that the calculated gas
temperatures would agree with those that describe ionization and
electronic excitation temperatures.
Clearly, the deviation between
rotational and the other temperatures indicates that the plasma is not
entirely in LTE.
A useful comparison can be made between a flame system and the
MINDAP.
Both sources emit more intense atom, rather than ion, lines,
128
both exhibit interelement interferences which can be overcome through
use of releasing agents, and both possess distinct spatial regions
where non-thermal and thermal mechanisms appear to prevail.
Each
source is best viewed from its side and is limited in sensitivity by
background spectral interferences in the UV.
However, the MINDAP has
more energy available for dissociating and exciting analyte.
In the helium and argon microwave plasmas, Busch and Vickers (5)
noted that the ratio of the excitation temperatures was similar to the
ratio of ionization potentials of the support gases.
Ar-MIP and the MINDAP, similar results are found.
approximately equal at 1.02.
In comparing the
Here the ratios are
This evidence supports the theory that
the mean electron energy in a MIP depends on the ionization potential
of the support gas (5).
CONCLUSION
Temperature measurements in the MINDAP indicate that the plasma
core is thermally hot, more so than flames and other MIPs, and that the
MINDAP is comparable to argon MIPs and ICPs in its excitation ability.
The spatially averaged temperatures calculated from the Boltzmann and
Saha-Eggert equations indicate that excitation and ionization in the
tail-flame are more closely related to a thermal mechanism than one
involving Penning ionization or any of the others proposed for MIPs and
ICPs.
Additional evidence that supports a thermal mechanism includes
the relationship between position of maximum analyte emission and an
element's excitation potential, and the similarity of spatially
resolved ionization and excitation temperatures high in the tail-
129
flame.
These measurements support the hypothesis in ICP literature
that it is possible to achieve LTE with the use of nitrogen as a
supporting plasma gas.
Unfortunately, without knowledge of tail-flame
rotational temperatures, accurate electron number densities and the
role that other excitation processes play in the tail-flame, the
existence of LTE cannot be conclusively confirmed or rejected.
The previous two chapters, combined with the findings here, reveal
the potential importance of this new microwave emission source for
analytical and physical measurements.
The plasma has two well defined
regions, one in the primary discharge zone which is highly energetic
(at least 18.74 eV) and which sustains the discharge and the other
downstream where background bands and continuum radiation are much
less. The thermal temperature in the primary discharge is much higher
than in other MIPs, making the MINDAP more tolerant of aerosol samples
introduced into it and more efficient at decomposing them into their
atomic constituents.
Because the MINDAP is more thermal in character
than most plasma sources, the emission behavior of elements is
predictable, atom lines are more intense than ion lines, and lines of
lower excitation energy are more sensitive.
130
REFERENCES
1.
I. Reif, V. A. Fassel and R. N. Kniseley, Spectrochim. Acta, 28B,
105 (1973).
2.
I. Reif, V. A. Fassel and R. N. Kniseley, Spectrochim. Acta, 33B,
807 (1978).
3.
S. Greenfield, H. McD. McGeachin and P. B. Smith, Talanta, 22, 1
(1975).
4. A. T. Zander and G. M. Hieftje, Appl. Spectrosc., 35, 357 (1981).
5. K. W. Busch and T. J. Vickers, Spectrochim. Acta, 28B, 85 (1973).
6
. R. Mavrodineanu and H. Boiteux, "Flame Spectroscopy", John Wiley
and Sons, New York, 1965.
7. C. Th. J. Alkemade and R. Herrmann, "Fundamentals of Analytical
Flame Spectroscopy", John Wiley and Sons, New York, 1979.
8
. M. H. Abdullah and J. M. Mermet, Spectrochim. Acta, 37B, 391
(1982).
9.
D. J. Kalnicky, V. A. Fassel and R. N. Kniseley, Appl. Spectrosc.,
31_, 137 (1977).
10. J. M. Mermet, Spectrochim. Acta, 30B, 383 (1975).
11. G. R. Kornblum and L. deGalan, Spectrochim. Acta, 32B, 71 (1977).
12. K. Fallgatter, V. Svoboda and J. D. Winefordner, Appl. Spectrosc.,
25, 347 (1971).
13.
A. T. Zander, R. K. Williams and G. M. Hieftje, Anal. Chem., 49,
2372 (1977).
14. R. D. Deutsch, unpublished results.
15. A. T. Zander and G. M. Hieftje, Anal. Chem., 50, 1257 (1978).
16. A. D. Weiss, M. S. Thesis, Bloomington, In., 1980.
17.
A. Montaser and J. Mortazavi, Anal. Chem., 52, 255 (1980).
18. R. M. Barnes and G. A. Meyer, Anal. Chem., 52, 1523 (1980).
19.
A. Montaser, V. A. Fassel and J. Zalewski, Appl. Spectrosc., 35,
292 (1981).
20.
M. H. Abdullah and J. M. Mermet, J. Quant. Spectrosc. Radiat.
Transf., 19, 83 (1978).
21.
R. M. Barnes and S. Nikdel, Appl. Spectrosc.,
310 (1976).
131
22.
M. Capitelli, F. Cramarossa, L. Triolo and E. Molinari, Combust.
Flame, 15, 23 (1970).
23.
F. Cramarossa and G. Ferraro, J. Quant. Spectrosc. Radiat.
Transf., 14, 159 (1974).
24.
P. B. Zeeman, S. P. Terblanche, K. Visser and F. H. Hamm, Appl.
Spectrosc., 32, 572 (1978).
25.
J. F. Alder, R. M. Bombelka and G. F. Kirkbright, Spectrochim.
Acta, 35B, 163 (1980).
26.
R. H. Tourin, "Spectroscopic Gas Temperature Measurements",
Elsevier, New York, 1962, p.47.
27.
P. W. J. M. Boumans, "The Theory of Spectrochemical Excitation",
Hilger and Watts, London, 1966.
28.
A. Montaser, V. A. Fassel and G. Larsen, Appl. Spectrosc., 35, 385
(1981).
29.
H. Griem, "Plasma Spectroscopy", McGraw-Hill, New York, 1964.
30.
A. Pannekock, Mon. Not. R. Astr. Soc., J98, 694 (1938).
31.
D. Inglis and E.
Teller, Astrophys. J., j)0, 439 (1939).
32.
W. H. J. Childs,
Proc. Roy. Soc., A137, 641 (1932).
33.
G. Herzberg, "Spectra of Diatomic Molecules", Van Nostrand, New
Jersey, 1950.
34.
P. W. J. M. Boumans and F. J. De Boer, Spectrochim. Acta, 32B, 365
(1977).
35.
N. Furuta and G. Horlick, Spectrochim. Acta, 37B, 53 (1982).
36.
A. T. Zander, personal communication, 1981.
37.
RJ M. Barnes and
S. Nikdel, J. Appl. Phys., 47, 3929 (1976).
132
CHAPTER 5
Identification of Limiting Noise Sources in the MINDAP
Accuracy, precision and the limits of detection of any analytical
method depend on the magnitude of extraneous fluctuations imposed on
the measured signal.
These fluctuations, also called noise, are
apparent as a variation about the mean signal value.
In order to
improve the performance of a measurement system, each noise component
and its origin must be identified and, ideally, reduced or eliminated.
The noise features of several spectrochemical sources have been
studied in the past, particularly the inductively coupled plasma (1-3),
microwave-induced plasma (4) and various analytical flames (5-10).
The
general conclusion is that, at concentrations near the detection limit,
background noise is the precision-limiting factor, whereas above the
detection limit analyte flicker is dominant.
Similarly, noise power
spectra reveal that the ICP is flicker-limited at frequencies below
about 10 Hz (1), indicating that integration times greater than 0.1 s
will not provide any improvement in signal-to-noise ratio (3).
Talmi, £t _al. (4) examined the frequency composition of the noise
from an argon microwave-induced plasma and found the noise spectrum to
be essentially white (i.e., flat).
This behavior is important because
the S/N ratio then improves proportionately with the square root of the
signal measurement period.
In this chapter the noise associated with the emission signal from
the MINDAP is critically evaluated.
Precision measurements obtained
for various signal integration periods and noise spectra both indicate
133
that the MINDAP is flicker-noise limited at frequencies below 0.2 0.5 Hz.
This behavior is independent of desolvation, sample
introduction method and analyte concentration, suggesting it to be
characteristic of the MINDAP itself.
Importantly, a dominant noise
source was found to be the power supply, showing that MINDAP signals
could be improved through better filtering and stabilization of the
microwave source.
A discussion of various noise sources and their
effects is presented.
EXPERIMENTAL
The MINDAP was operated as a suspended plasma (cf. Ch. 2) at an
applied
power of 250 W and reflected power 20 W.
The plasma support
and central-channel nitrogen flow rates were 1.75 L/min and 0.1 L/min,
respectively.
The detection electronics were the same as in Chapter 2, except
that the MINC 11/23 computer was exclusively used for data collection
and computation.
All programs were written using Ver. 2.0 of MINC
BASIC.
Four different experimental methods were used to identify the
noise sources which were associated with the signal.
were:
These techniques
wavelength-scan flicker-ratio measurement, time-constant effect,
time-trace correlation and noise amplitude spectral determination.
Below is a description of each technique, how it was used and the
information that it provides.
134
Wavelength-Scan Flicker-Ratio Measurement
It has been shown by Winefordner (6 ) that the relative amounts of
white (shot) and flicker noise produced by an emission source can be
determined by means of a particular form of wavelength scanning.
In
this method, a monochromator scans a spectral region containing either
background or analyte'emission; at each wavelength, the emission signal
is collected at a known sampling rate.
The mean signal and standard
deviation at each wavelength are then computed and used for determining
the magnitude of each kind of noise.
The total amount of noise on the signal at any wavelength is, by
definition, equal to the standard deviation of the signal.
In turn,
the total noise (N^) is just the quadratic sum of the shot (Ng) and
flicker (N^) components (equation 5-1).
N
t
- t(N
s
) 2
+ (N, ) 2
r
Because shot noise follows
] 1 / 2
(5-1)
Poission statistics, its value is equal to the square root of the
average signal (S) (eq. 5-2).
From the measured total noise and
(5-2)
calculated shot-noise magnitudes, the flicker contribution can be
calculated from eq. 5-3.
At each wavelength in the spectrum,
Nf = [(Nt
) 2
- S]
1 / 2
(5-3)
135
a ratio is calculated from the flicker- and shot-noise values.
If this
ratio is greater than two, flicker (either signal or source) is
considered to be the dominant noise affecting the measurement at that
particular sampling frequency.
For this application 30 data points were collected for each
wavelength over a spectral region of
frequency of either 42 or 1 Hz.
1 0
to
2 0
nm, at a sampling
The average, standard deviation and
flicker ratio were calculated at each wavelength.
From each flicker
ratio, the dominant noise (shot or flicker) was identified.
Time Constant Effect on Precision
Precision indicates the reproducibility of a signal over a
particular measurement time period.
The MINDAP emission signal was
monitored using time constants that ranged from 1 ms to 10 s.
The
average, standard deviation and relative standard deviation were
computed for an observation period equal to
1 0 0
time constants.
The limiting noise from these results can be identified from a
logarithmic plot of precision (%RSD) vs. time constant.
In a region
where the dominant noise is white, a square-root relationship exists
between the noise amplitude and bandwidth of the measurement system
(11,12).
Therefore, increasing the time constant will yield a
proportionately smaller bandwidth, and thereby reduce the noise but not
the signal.
This reduced noise will result in an increase in precision
and appear as a straight line with a slope of -0.5 on the logarithmic
plot.
However, if the noise is flicker-limited, its amplitude is
proportional to the reciprocal of the frequency (1 /f) and no
136
improvement in precision is gained by increasing the time constant
(zero slope on logarithmic plot).
where n >
1
Finally, if the noise is' l/fn ,
, increasing the time constant will decrease the precision
of the measurement.
This effect will produce an upward shift of the
logarithmic curve.
Time-Trace Correlation
Time tracings of emission can be used efficiently to identify
common noise sources in an instrument or technique.
If there is a
common source of noise (i.e., plasma tail-flame waver) that affects
analyte and background emission, a similarity will be observed between
the simultaneous time tracings of the two signals.
The degree of this
coherence can be quantified from a correlation plot of the two
parameters.
Time correlation plots were constructed by monitoring the
emission, forward-power and reflected-power signals from the MINDAP for
a 400-minute time period.
In order to record the power signals, a
reflected power meter and BNC connections for each meter were installed
on the power supply.
All signals were collected simultaneously using
three A/D converter channels on the MINC 11/23 computer.
This correlation experiment was performed over a two-day period.
On the first day, the power supply was turned off and remained off for
24 hours.
This "off time" was necessary in order to determine the
short-term fluctuations that arise from the warmup of the power
supply.
Data were collected only after the power supply had been on
for three minutes.
Within this three-minute time period,
90 s were
137
needed for a thermal relay inside the power supply to engage and
transfer the high voltage to the magnetron tube, after which the plasma
was lit and quickly tuned for minimum reflected power.
For the
remainder of the experiment, neither the tuning nor the power was
adjusted.
Noise Amplitude Spectral Determination
Noise amplitude spectral measurements reveal the frequency
composition of the noise present on a signal.
In the present study,
noise amplitude spectra were acquired for desolvated and undesolvated
aerosol, for varying analyte concentration, and for both analyte and
plasma background emission features of the MINDAP.
The procedure used for the acquisition of noise amplitude spectra
is illustrated in Figure 5-1.
For selected emission features, both a
background (A) and signal (B) trace were digitized.
Initially,
fluctuations in the plasma background (A) are detected, after which a
sample is introduced into the MINDAP and an increase in the
occurs (B).
d.c. level
The fluctuations associated with either A or B levels are
collected at a sampling rate at least twice that of the highest desired
frequency in the noise spectrum, thereby satisfying the Nyquist
theorem.
In acquiring a noise spectrum, accurate control of the measurement
bandwidth was necessary to reduce the effects of aliasing (1).
For
these measurements an active low-pass filter (Krohn - Hite model 3342)
with a very sharp roll-off (-96 dB/octave) was inserted between the
fast current amplifier and the computer's A/D converter.
138
Figure 5-1.
Schematic diagram of the signals used here for obtaining
a noise-amplitude spectrum.
A.
Background emission from the plasma.
B.
Analyte emission.
C.
Real frequency component from the Fourier
Transformation.
D.
Imaginary frequency component from the Fourier
Transformation.
E.
Resulting noise-amplitude spectrum for low-frequency
flicker-limited noise.
139
ffl
<
140
Noise amplitude spectral data were acquired for the frequency
ranges of 0 - 5 and 0 - 500 Hz at sampling rates of 14 and 1400 Hz,
respectively.
The frequency cut-off
( - 6
dB point) of the low-pass
filter was set to the Nyquist frequency (7 and 700 Hz) to ensure that
the contribution from aliased signals at 5 and 500 Hz was negligible
(3).
Five sets of time-domain data, each consisting of 1024 points,
were acquired at each sampling rate.
The resulting data sets were then
transformed to the frequency domain by a Fast Fourier Transform (FFT)
algorithm supplied with MINC BASIC.
The resulting five noise spectra
were averaged to yield the spectrum which is reported (1 ).
In the process of implementing the FFT, both real (C) and
imaginary (D) spectral components are produced; each contains positive
and negative frequencies over the sampled ranges (i.e., -7 to +7 Hz).
The peak in the middle of the real frequency spectrum (0 Hz)
corresponds to the d.c. contribution.
The amplitude spectrum (E) of the positive noise frequencies is
calculated as the square root of the sum of the squares of the real (C)
and imaginary (D) frequency components.
The resulting spectrum was
plotted on a Tektronix 4662 plotter as frequency (Hz) vs. root-meansquare current (nA).
The bandwidth or resolution (R) of the noise-spectrum measurement
system is calculated from equation 5-4.
E - 2^
In eq. 5-4, N is the total
(5-4)
number of data points (1024) and fmflx is the maximum frequency which
is properly sampled (7 or 700 Hz).
The factor 2 indicates that only
141
half the number of data points collected is unique (i.e., -7 to 0 Hz
frequency range contains the same information as 0 to 7 Hz). The
measurement bandwidths for 7 and 700 Hz frequency ranges were 0.0137
and 1.37 Hz, respectively.
Noise spectra are often displayed in power rather than amplitude
terms.
Noise amplitude and power can be related to each other by the
square of the noise amplitude per unit bandwidth of the measurement
system, as shown mathematically in equation 5-5.
Np = (Na )2/R
(5-5)
Here, Np is the noise power, N^ is the noise amplitude and R is the
bandwidth of the measurement (defined above).
RESULTS
Precision and Low-Frequency Noise
The effect of the measurement time constant on relative standard
deviation is often determined as a first step in improving the
precision of any analytical measurement.
The relationship between
these parameters for the emission of a 10 pg/mL calcium solution in the
MINDAP is shown in Figure 5-2.
As the time constant is increased from
1 ms to 1 s, the %RSD decreases steadily.
indicates that flicker (l/fn where n «
not contribute significantly.
1
The slope of the line
) noise is present but does
At time constants between 1 - 5 s, the
%RSD reaches a minimum (n = 1), indicating the best signal-to-noise
142
Figure 5-2
Effect of measurement time constant on precision from a
10 yg/mL calcium soulution introduced into the MINDAP.
Calcium measured as atomic emission at 422.7 nm.
10
RSD
8
6
4
2
0
0.001
0.01
0.1
10
1 0 0
143
Time Constant
(sec)
144
ratio that can be obtained under the present operating conditions
( 1% RSD).
Above 5 s the system is susceptible to long-term drift
(n > 1 ), as is evident from the upward shift of the curve.
The wavelength-scan flicker-ratio method described by Winefordner
(6 ) supports the conclusions derived from Fig. 5-2.
At both sampling
frequencies (42 and 1 Hz) the flicker-ratio method yielded a flickerto-shot noise ratio less than two, indicating that at these frequencies
flicker is not the dominant noise arising from either the plasma
background (OH band from 300 - 310 nm and N^+ bands from 385 - 395
nm) or analyte emission (Ca, Mg, Na) in the MINDAP.
The exact sampling
rate at which flicker noise became dominant was not established with
this method since it can be more easily found from a noise spectrum.
The different types of noise (shot, flicker and interference) can
easily be distinguished in a noise-amplitude spectrum.
Figure 5-3 is a
noise amplitude spectrum of OH emission at 306.4 nm over the frequency
range of 0 - 5 Hz.
In this spectrum two types of noise can be
positively identified:
a white-noise component, which is well removed
from 0 Hz and has a noise amplitude that is flat with frequency, and a
flicker-noise component which has a noise amplitude proportional to the
reciprocal of the frequency.
The frequency where the 1/f curve merges
with the white noise, 0.2 - 0.5 Hz, defines the sampling rate for
maximum attainable precision.
spectrum for
Interestingly, the noise amplitude
emission at 391.4 nm exhibits the same shape as
Fig. 5-3, suggesting a noise source common to the two.
These noise
amplitude spectra complement the precision and flicker-ratio
measurements but are more accurate for identifying each type of noise
145
Figure 5-3.
Noise spectrum o£ OH band emission at 306.4 nm over the
frequency range 0 - 5 Hz.
0.35
Root
Mean
Squar•
Currant
CnA3
0.7
___________ I___________ I___________ I___________ I___________
0
1
2
3
5
146
Frequency (Hz)
4
147
present, the relative magnitude of each and its frequency composition.
Analyte emission-noise spectra for desolvated solutions of calcium
and sodium (0.1 - 1000 pg/mL) were similar to that in Figure 5-3 with a
1/f behavior becoming prominent between 0.2 - 0.5 Hz.
As the analyte
concentration increased, for the 0 - 5 Hz spectrum, the mean signal and
noise amplitudes increased proportionately (see Fig. 5-4).
These
results support the hypothesis that the MINDAP is limited by
multiplicative noise and that analyte flicker is dominant for
concentrations well above the detection limit.
The effect of introducing into the plasma undesolvated aerosols
was to increase both the mean signal and its associated noise.
The
shape of the noise amplitude spectrum remained similar to that in
Fig. 5-3, although the relative standard deviation for the 0 - 5 Hz
spectrum increased from 2.5% (for desolvated aerosol) to 4%.
These
precision values were obtained independently of analyte concentration
over the range 0.1 to 1000 pg/mL.
These findings support the
observation made in Chapter 2 that the MINDAP can handle aerosols
directly, although the S/N ratio is better when desolvation is
employed.
High-Frequency Noise
A noise amplitude spectrum between 0 - 500 Hz (Fig. 5-5) from the
emission of atmospheric water vapor (OH) reveals three distinct
features:
a low-frequency flicker component (A), and peaks at
60 Hz (B) and 120 Hz (C).
The 60 and 120 Hz spikes do not arise from
interference noise associated with the detection electronics, since a
148
Figure 5-4.
Effect of calcium concentration on emission signal and
precision.
o - Calcium atom emission for various concentrations of
analyte (slope'll).
The noise associated with each
emission signal is smaller than the circle repre­
senting the emission intensity.
* - %RSD for Ca (I) emission at each concentration.
This
is drawn on a linear scale ranging from 2.5% - 3.5%.
Re Iat ive
Intens ity
10
10*
RSD
10
1 0 0
1000
10000
149
Analyte
Concentration
Gug/mL)
150
Figure 5-5.
Noise spectrum of OH band emission at 306.4 nm over the
frequency range 0 - 500 Hz.
A - low frequency flicker of rms amplitude 0.5 nA.
B - 60 Hz peak of rms amplitude 4 nA.
C - 120 Hz peak of rms amplitude 3 nA.
Frequency (Hz)
152
spectrum of similar amplitude taken with a flashlight did not exhibit
these peaks.
Instead, this interference noise probably originates in the
microwave power supply.
The power supply basically consists of a high-
voltage transformer, a full-wave rectifier, some filtering capacitors
and the magnetron tube (source of 2450 MHz).
Ripple noise from the
filtering circuit superimposed on the output high-frequency waveform is
believed to couple to the plasma and to modulate it.
The resulting
modulation is then viewed as strong peaks at the modulating frequency
(60 Hz) and its harmonics.
This hypothesis is supported by the behavior of the plasma when
even more water vapor is introduced into it.
Additional vapor was
intentionally swept into the MINDAP by passing the nitrogen support gas
through a gas dispersion tube, immersed in distilled water.
The
resulting plasma became visibly unstable and could be seen to pulsate.
Moreover, the noise spectrum of the OH 306.4 nm emission band contained
exaggerated contributions from the power supply ripple and its
harmonics (Fig. 5-6).
This peculiar behavior can be qualitatively
attributed to the strong water-vapor absorption band at the microwave
operating frequency.
Presumably, the 60 and 120 Hz ripple on power
transferred into the plasma couples with the water vapor directly,
causing it to amplify the plasma modulation.
This modulation then
becomes so strong that the plasma is nearly extinguished during each
modulation cycle.
The resulting nonlinear modulation generates the
range of 60 Hz harmonics apparent in Fig. 5-6.
153
Figure 5-6.
Noise spectrum of the OH 306.4 nm band when a large flow
of water vapor is intentionally passed into the MINDAP.
CnA3
Currant
Root
M*an
Squara
12.5
100
200
300
500
154
Frequency (Hz)
155
DISCUSSION
There are two major kinds of noise:
additive and multiplicative.
Additive noise arises independently of a measured signal (emission).
It is easier to understand and overcome in a measurement, although it
is usually not dominant at high analyte concentrations. Examples of
additive noise in a spectrometric system are stray light variations,
detector offset drift and amplifier baseline noise.
Methods often used
for minimizing additive noise are blank subtraction and signal
modulation (10,13,14).
In contrast, multiplicative noise is introduced simultaneously
with the signal and is more difficult to correct (15).
As its name
implies, this kind of noise multiplies the signal, in effect, making
the resulting fluctuations proportional to both the noise source
magnitude and the signal itself.
Multiplicative noise arises from
fluctuations in source temperature, sample introduction systems, power
transfer, impedance tuning, source flicker, gas dynamics (flow rate and
pressure) and analyte concentration.
The only way to overcome
multiplicative noise is to ratio the signal of interest to a reference
that is related to the original noise source.
A common form of such
correction involves the use of an internal standard.
It appears that the MINDAP system is most greatly affected by
multiplicative noise.
The similarity of noise spectra obtained from
analyte emission and various background spectral features argues for a
common, multiplicative source, as does the independence of precision
with analyte concentration (Fig. 5-4).
Several possible origins of
multiplicative noise exist; in addition to those mentioned above,
156
microwave power-supply fluctuations would be suspected.
Figure 5-7
shows the dependence of OH emission (306.4 nm) on applied microwave
power for two different impedance-matcher tuning settings.
At a
constant tuning setting, emission intensity varies linearly with
power.
However,
changing the tuning, but not reflected power,
establishes a different proportionality between power and emission
intensity.
From Fig. 5-7,,any fluctuations in applied power should produce
proportional variations in an emission signal.
Moreover, Figures 5-5
and 5-6 suggest the possible importance of power-supply variations as a
noise source.
To determine the importance of such fluctuations,
simultaneous time tracings were obtained of plasma background emission
(N£+ 1st Negative system at 391.4 nm) and of applied and reflected
microwave power.
These tracings are reproduced in Fig. 5-8 and are
plotted as the deviation of the signal from its normalized mean.
Table 5-1 lists the initial values and statistical features of each of
the three signals.
A careful examination of the three traces reveals a
slight similarity between those for forward power (A) and background
emission (B).
This similarity suggests that power-supply noise is a
strong, if not dominant, source of variation in the MINDAP.
However,
Table 5-1 shows that the relative standard deviation of the background .
trace is considerably greater than that of forward power, indicating
that other significant noise sources exist.
Correlation plots (Figures 5-9 to 5-11) of the three sets of data
verify the foregoing qualitative observations.
The only plot that
shows no correlation is that of the background emission signal and
reflected power.
The correlation coefficient for the forward power and
157
Figure 5-7
The effect of microwave power and two different
impedance-matcher settings on the emission intensity of
OH at 306.4 nm.
2
Re Iat ive
Intens ity
3
170
230
210
A p p 1 ied P o w e r
CW)
250
158
190
159
Figure 5-8.
Time tracings of the relative fluctuations from the mean.
A.
Forward power.
B.
N^+ First Negative System.
C.
Reflected power.
Relative Amplitude
Time
Cm in)
09T
161
Table 5-1:
Initial and statistical values for each
signal collected from time tracings
over a period of 400 min (see Fig. 5-8).
N2+
Signal
Forward Power
Reflected Power
Initial
1.618
261
25.9
Max
1.729
274
27.9
Mean
1.576
266
25.4
Min
1.457
257
21.8
Std. dev.
0.65
3.32
1.32
% RSD
4.12
1.25
4
162
Figure 5-9.
Correlation plot of the forward power with the emission
signal from the
plasma background.
Powor
Forward
163
Emission Signal
164
Figure 5-10.
Correlation plot of the forward microwave power with
reflected power.
F o r w a r d P ower
Reflected
Power
£91
166
Figure 5-11.
Correlation plot of the reflected power with emission
signal from N£+ plasma background.
Pow«r
R«fl«ct«d
167
Emission Signal
168
emission signal is
0.55.
0 . 6 8
and between forward and reflected power is
This degree of correlation supports the argument that emission
intensity is influenced by fluctuations in the power applied to the
plasma.
As suggested above, additional factors contribute to signal
instability in the MINDAP.
One such factor is air turbulence.
The
plasma's tail flame, which extends ten centimeters from the cavity and
has a diameter of approximately
surrounding air drafts.
6
mm, is easily perturbed by
In fact, in the last eleven minutes of the
time-tracings the air currents in the room markedly affected the
plasma's stability.
Accounting for this pertubation (by discarding
those data points), the correlation coefficients from Figs. 5-9 to 5-11
were recalculated.
Only the correlation coefficient for the signal
with forward power was affected, increasing from 0.68 to 0.74.
There are other factors that will multiplicatively influence
emission intensity which might not appear as a change in microwave
power.
These additional parameters are:
instability of the magnetron
tube's output frequency, changes of either the amount or type of
dielectric introduced into the cavity, changes of impedance matching
(as noted in Fig. 5-7), and the efficiency with which each electrical
connection transfers the applied power.
CONCLUSION
The MINDAP appears limited at high analyte concentration by
multiplicative noise arising at least in part from the power supply,
impedance matcher, analyte concentration, and source flicker.
A
169
flicker-limiting frequency of 0.2 to 0.5 Hz was determined for the
MINDAP and found to be independent of analyte concentration, element
excitation potential and the existence of desolvation.
At low
frequencies, noise spectra retained the same shape but increased in
amplitude when aerosol was introduced into the plasma without
desolvation.
This finding supports the observation made in Chapter 2
that desolvation of an introduced aerosol increases the signal-to-noise
ratio.
.
At present, a significant and perhaps dominant source of
multiplicative noise arises from the power supply and power transfer
systems.
Nonetheless, after seven hours of free running, the relative
standard deviation for the power and signal fluctuations was less than
5%.
Similarly, the precision for an analytical determination, lasting
typically 30 s, was also less than 5%.
The results in this chapter have provided information about
several factors that affect the precision and sensitivity of the MINDAP
system.
Other factors which need to be considered to totally
characterize this plasma source are the effects of gas flow rate and
pressure on signal stability.
In the next chapter, sample is introduced into the plasma in a
completely atomized form, and should therefore indicate the full
potential of the MINDAP as an analytical emission source.
170
REFERENCES
1.
R. M. Belchamber and G. Horlick, Spectrochim. Acta, 37B, 17
(1982).
2.
G. L. Walden, J. N. Bower, S. Nikdel, D. L. Bolton and J. D.
Winefordner, Spectrochim. Acta, 35B, 535 (1980).
3.
R. H. Belchamber and G. Horlick, Spectrochim. Acta, 37B, 71
(1982).
4.
Y. Talmi, T. Crosmun, and N. M. Larson, Anal. Chem., 48, 326
(1976).
5.
G. M. Hieftje and R. I. Bystroff, Spectrochim. Acta, 30B, 187
(1975).
.
K. Fujiwara, A. H. Ullman, J. 0. Bradshaw, B. D. Pollard and J. D.
Winefordner, Spectrochim. Acta, 34B, 137 (1979).
7.
C. Th. J. Alkemade; Tj. Hollander, K. E. J. Honings, H. A.
Koenders and R. J. J. Zijlstra, Spectrochim. Acta, 34B, 85 (1979).
.
C. Th. J. Alkemade, H. P. Hooymayers, P. L. Lijnse and T. J. M. J.
Vierbergen, Spectrochim. Acta, 27B, 149 (1972).
9.
C. Th. J. Alkemade, Tj. Hollander, H. Snipe and R. J. J. Zijlstra,
Spectrochim. Acta, 36B, 77 (1981).
6
8
10.
M. Markinkovic and T. J. Vickers, Anal. Chem.,
42^, 1613 (1970).
11.
G. M. Hieftje, Anal. Chem., 44, 81A (1972).
12.
G. M. Hieftje, Anal. Chem., 44, 69A (1972).
13.
C. Th. J. Alkemade, W. Snelleman, G. D. Boutilier, B.D. Pollard,
J. D. Winefordner, T. L. Chester and N. Omenetto, Spectrochim.
Acta, 33B, 383 (1978).
14.
G. D. Boutilier, B. D. Pollard, J. D. Winefordner, T. L. Chester
and N. Omenetto, Spectrochim. Acta, 33B, 401 (1978).
15.
C. Th. J. Alkemade, W. Snelleman, G. D. Boutilier and J. D.
Winefordner, Spectrochim. Acta, 35B, 261 (1980).
171
CHAPTER
6
Microsample Introduction into the MINDAP
Using a Microarc Atomizer
In the preceding chapters, the microwave-induced nitrogen
discharge at atmospheric pressure (MINDAP) has been used with a driedaerosol sample-introduction system.
exhibits
In this configuration, the MINDAP
high temperatures (Chap. 4), low detection limits (Chap. 3),
and matrix interferences which can be overcome in a manner similar to
that employed in flame spectrometry.
In this chapter the MINDAP system
is investigated as an analytical atomic emission source for
microsampling analysis using a microarc atomizer.
The
(1 ) as a
microarc atomizerwas developed in 1974 by Layman and Hieftje
device to convert discrete microquantities of liquid sample
into atomic vapor for emission analysis in a microwave plasma.
The
microarc is a high-voltage, low-current discharge that sequentially and
efficiently desolvates, vaporizes and atomizes sample volumes from
to 40 yL (1).
0 . 1
This concept of separate atomization and excitation was
discussed also by Falk, et al. (2) who showed how it could enhance the
sensitivity of other atomic emission measurements.
The microarc has been successfully combined with several plasma
emission sources (1,3-5).
In conjunction with the inductively coupled
plasma (ICP) (3,4), the microarc yielded lower detection limits than
other microsampling techniques applied to the ICP (6-9).
Argon and
helium microwave-induced plasmas (MIP), when coupled to the microarc
(1,5), offer the sensitivity and freedom from interferences that has
172
been associated with the ICP.
Yet their physical size, instrumentation
and operating requirements are more compact and economical.
Elemental analysis using a microwave plasma as the excitation
source has been limited to samples introduced as a vapor effluent from
gas chromatography (10,11), thermal atomizers (12,13), hydride
generators (14,15), and laser vaporization devices (16).
Nebulizer
systems have also been employed (17) but to a lesser extent and less
successfully.
Analyte is preferably introduced as a vapor because the
MIP lacks the thermal energy needed to decompose the sample.
Another
limitation of the MIP is its small physical size which restricts the
amount of sample that can be introduced before overloading occurs.
The MINDAP system overcomes many of the inconveniences ordinarily
associated with microwave plasmas:
it readily accepts aerosol samples,
possesses the high thermal energy needed for sample decomposition, and
is relatively unaffected by high analyte concentrations.
The MINDAP
system has previously been evaluated for continuous solution analysis,
and is now the subject of investigation with discrete microvolume
aliquots from the microarc sample-introduction technique.
This is the first time that the microarc has been used in a
molecular-gas atmosphere (i.e., nitrogen).
Its operating
characteristics are therefore slightly different from those in an inert
monatomic gas atmosphere and a qualitative description of its behavior
is included.
The microarc-MINDAP combination yields pg - fg detection
limits, a broad linear dynamic range, good precision (3 - 7% rsd), and
essentially no interference from either sodium or phosphate.
173
EXPERIMENTAL
Connection of the microarc atomizer to the MINDAP was straight­
forward and is detailed.in Figure 6-1.
Initially it was attempted to
introduce the microarc-atomized sample through the central channel of
the plasma torch.
However, in this configuration the suspended MINDAP
plasma was perturbed when the arc was struck.
Consequently, in this
investigation the microarc was connected to the side-on gas inlet of
the torch (Figure 6-1).
Timing of the microarc and data collection were controlled by a
laboratory computer in a manner similar to that described by Keilsohn,
Deutsch and Hieftje (3).
shown in Figure 6-2.
A block diagram of the experimental set-up is
The computer controls the duration of the
desolvation period, the ignition and duration of the arc, and the
collection and processing of the emitted-radiation signals.
The MINDAP plasma was sustained using the conditions determined in
Chapter 2 to be optimal for the nebulizer-based system.
These
conditions produced a suspended plasma configured vertically, with an
applied microwave power of 250 W and reflected power of 20 W. The gas
flow rates through the central channel of the torch and through the
microarc assembly were 0.1 L/min and 1.75 L/min, respectively.
Analyte
emission was collected in a radial viewing configuration from the first
6
m m of the plasma tail flame.
The microarc electrodes (30-gauge tungsten wire as the cathode
and 24-gauge stainless-steel syringe needle for the anode) were
positioned to support a stable and reproducible discharge in the
flowing nitrogen atmosphere; an interelectrode spacing of approximately
Figure 6-1:
Microarc-MINDAP operational configuration.
M
I
N
D
A
P
h”
^ \ aaaa >
discharge
Viewing
direction
m
MINDAP torch
TEMqio
Anode
176
Figure 6-2:
Block diagram of the instrumentation used to collect and
analyze the transient emission signal from the
microarc-MINDAP system.
PMT
=■ R928 Photomultiplier tube
i-to-V Amp
= Keithley 427 current amplifier
H.V.P.S.
= Keithley 244 High voltage power supply
asma
Monochromator
uWAVE
Power
Su p p I y
Constant
Current
Sour ce
ower
SuppIy
MINC
Arc
T im e r
1I
Computer
177
178
0.5 mm vas employed.
The current and voltage needed to sustain the
microarc discharge in the flowing nitrogen atmosphere were 28 ma and
1800 Vp_pi which were higher than those required in the flowing argon
systems
(1,3).
Analyte desolvation was accomplished by ohmically
heating the cathode from a constant current supply of 4V at 2A
(typically 30
8
for 1 pL of solution) before initiating the burn.
The
appearance of the microarc discharge in the nitrogen atmosphere was
similar to that noted in other gases.
The stable arc emits a bluish-
colored plume, characteristic of the electrical breakdown of nitrogen.
When first struck, the arc anchors to the tip of the hairpin-shaped
cathode and then uniformly surrounds it.
Because of the rather small separation between the microarc
electrodes, care was required to dispense the microvolume sample onto
only the cathode.
In trials when the anode was contaminated by sample
solution, the arc was difficult to initiate and, once struck, generated
broadband emission from the electrode material.
The duration of the
arc was empirically optimized by monitoring both signal and background
emission traces, as described previously (3).
For arc times beyond one
second there was no detectable analyte emission; consequently, in later
determinations the arc duration was one second and data were collected
for two seconds.
This additional time enabled the computer to
establish a baseline, allowed for analyte transport to the plasma and
encompassed total decay of the emission signal.
A typical
emission-time profile accumulated under these operating conditions is
displayed in Figure 6-3.
179
Figure 6-3:
Emission-time profile for 1 ng (1 yL of 1 yg/mL) copper
at 324.7 nm.
A - Analyte emission
B - Background emission
Re Iat ive Intens ity
S>
T ime
Csec.
l\>
081
181
RESULTS AND DISCUSSION
Desolvation Technique
A factor that limited reproducibility and signal stability in
initial microarc-MINDAP trials was uncertainty in the duration of the
desolvation process.
If desolvation were incomplete, initiating the
arc resulted in intense broadband emission from the MINDAP, caused
presumably by tungsten oxide liberated from the sample electrode (18).
On the other hand, waiting a sufficient period of time to ensure
complete desolvation decreased the throughput of sample analysis.
It
was decided that the best way to overcome this difficulty would be to
monitor the desolvation process.
The voltage-derivative method developed by Layman and Hieftje (19)
records the desolvation process electronically by monitoring changes in
the microarc filament resistance as the solvent evaporates.
Unfortu­
nately this technique was not successful when incorporated into the
microarc-MINDAP system.
Erratic derivative signals were produced
during the sample desolvation, caused probably by the high flow rate of
gas and by the consequent instability in the solvent evaporation rate.
Instead, it was found that desolvation could be monitored
independently of gas flow and solvent evaporation rates by observing
the emission of OH at 306.4 nm.
During desolvation of an aqueous
sample, water vapor is continuously swept into the MINDAP to yield
strong emission at 306.4 nm; a pronounced reduction in this signal
indicates the end of desolvation.
This spectroscopic monitor was found
to be as sensitive and accurate as the voltage-derivative technique for
182
determining the end of solvent evaporation but is more compatible with
the flowing-nitrogen system.
Compared to waiting a fixed length of
time for desolvation, the spectroscopic technique increased sample
throughput by as much as 50%.
Analytical Figures-of-Merit
Detection Limits.
Limits of detection for the microarc-MINDAP system
were determined at the 95% confidence level for eight elements of
varying excitation potential.
The method used is similar to that
previously described (3) and is based on a noise value equal to the
standard deviation obtained from thirty background traces (N=30,
a=*0.05, t=2.045) (20).
The values reported in Table 6-1 are the
averages of at least five separate determinations of the detection
limit.
For comparison, Table 6-1 includes detection limits obtained
with the MINDAP coupled with a nebulizer sample-introduction system.
The values in Table 6-1 are consistent with the general trend that
elements with higher excitation potential have higher detection limits
than those of lower potential.
The results indicate also an
improvement in detection limits when the analyte is introduced in a
preatomized form by the microarc.
Similar improvement has been
reported (1,3,4,20) when the microarc was used for sample introduction
into other plasma systems.
A comparison of detection limits offered
Table 6-1:
Detection limits (ng/mL) for the MINDAP obtained
with microarc and nebulization/desolvation
sample-introduction systems.
Element
Ca(I)
Q
Wavelength(nm)
422.7
Excitation
Energy (eV)
2.92
Detection Limits
Nebulizer1'
Microarc"
1 . 0
1 . 2
0.13
4.4
0.4
5.4
0 . 2
0 . 2 2
Cu(I)
324.7
3.80
K (I)
766.5
• 1.61
Li(I)
670.8
1.84
Mg(l)
285.2
4.33
0.5
Na(I)
589.0
2.09
0.072 (c)
Pb(I)
405.8
3.04
56
Zn(l)
213.8
5.77
166
•
•
•
all volumes were 1 pL, unless otherwise indicated.
^Chapter 2
(c) 0.5 pL sample volume.
1 2
0.29
82
1 2 0
184
by several discrete-sample-introduction / multielement-analysis systems
is offered in Table 6-2.
The microarc-MINDAP combination compares
favorably with the far more elaborate and expensive systems.
The increased sensitivity offered by the microarc over the
nebulizer system is not surprising.
The microarc concentrates the
sample introduced into the plasma as a "plug" of analyte vapor much as
a carbon furnace does in AAS.
plasma is nearly
1 0 0
In addition, sample delivery to the
% efficient with the microarc whereas common
nebulizers are at best 5% efficient.
One of the major advantages of
using the microarc-MINDAP is that it is mass-sensitive rather than
concentration-sensitive. Therefore, preconcentration of sample
solutions onto the microarc filament should improve detection limits
even further.
Finally, the separation of sample desolvation and atomization by
the microarc promotes MINDAP stability and analyte excitation.
When
water vapor is introduced into the MINDAP, the plasma background and
noise level increase, as shown in Chapter 5.
This shift in background
level was noticed as early as 1917 by Strutt, who reported an increase
in the NO band systems in the nitrogen afterglow upon the addition of
water vapor (22).
It has recently been suggested that the NH bands in
the background spectrum are derived from the reaction of nitrogen with
the dissociation products of water (23,24).
Introduction of water vapor into the MINDAP might have the added
consequence of quenching the production of excited nitrogen species
necessary to sustain the nitrogen afterglow and which might be related
185
Table 6-2:
Comparison of detection limits (pg) offered
by several discrete-sample-introduction /
multielement-analysis systems.
Element
Ca (I)
microarc
MINDAP
0.05
microarc
Ar-MIPa
1.0
Ca(Il)
ETA
ICPC
-
AAS
1.0
0.06
Cu (I)
0.13
K
(I)
0.4
Li (I)
0.2
-
Mg (I)
0.5
0.045
0.16
3.9
-
-
Mg(II)
Na (I)
microarc
Conv-ICPb
-
0.87
0.036
0.01
Pb (I)
56
0.38
Zn (I)
166
0.92
-
0.4
-
1.0
-
-
0.4
-
3.6
280
0.12
0.5
100
20
0.02
0.4
0.4
0.7
1000
^icroarc coupled to an argon-supported Microwave-Induced Plasma (1).
bMicroarc coupled to an Inductively Coupled Plasma (ICP) (3).
g
Electrothermal atomization into an ICP (7).
^Carbon-furnace Atomic-Absorption Spectroscopy (21).
186
to analyte excitation (25,26,27).
Of the elements listed in Table 6-1,
lithium and sodium should not have been affected significantly by the
influence of water vapor on background emission, since both lie in a
low-background spectral region.
Yet, the presence of water vapor
hardly affected the detection limit for lithium, but reduced that for
sodium by almost an order of magnitude.
This change might be due to a
transfer of energy to sodium from nitrogen in the first positive system
of the Lewis-Rayleigh afterglow (28).
Starr has demonstrated that
vibrationally excited nitrogen molecules can transfer energy to the
electronic levels of sodium by a collisional process (29).
Working Curves.
Figure 6-4 shows calibration curves for zinc
(213.8 nm), copper (324.7 nm), lead (405.8 nm), and lithium (670.8 nm)
with lines drawn to an order of magnitude above their respective
detection limits.
These curves have slopes of approximately unity over
a concentration range of three-to-five orders of magnitude which are
typical for most elements studied.
The measurements at 3000 Wg/ml were
performed using the preconcentration technique (4); three 1 ML aliquots
of 1000 Mg/rnL solution were dispensed onto the cathode before the arc
was struck.
Precision.
The precision of the microarc-MINDAP combination was
ascertained from five successive determinations of
each of five different days.
1 0
ng of copper on
The relative standard deviation on each
day was between 3 and 7% whereas the reproducibility among days varied
between 1 and 4%.
At least three factors appeared to affect the
precision of the measurement:
the electrode spacing and relative
187
Figure 6-4
Calibration curves for the microarc-MINDAP system.
Curves for copper and lithium are superimposed.
A - Zn(I) 213.8nm
o - Cu(I) 324.7nm
* - Pb(l) 405.8nm
* - Li(I) 670.8nm
10
Re Iat ive
Intens
ity
10
10
-2
1 0 -3'-------- L—
1 .0E-3 0.01
10
100
1000
10000
188
Analyte
Concentration
CMg/mL)
189
geometry, the position of the sample droplet on the cathode and the
surface characteristics of the sample electrode.
These factors have
been excellently characterized by Bystroff, et al., who operated the
microarc under different conditions than presented here (18).
As a
result, the qualitative discussion below will dwell on the effects the
flowing nitrogen atmosphere has on these parameters.
The electrode spacing affected the microarc characteristics
-greatly.
If the electrodes were too close ( <0.5 mm), anode
contamination was common, and the breakdown of nitrogen and the
decomposition of the sample affected.
When the electrodes were too far
apart ( > 1 mm), the breakdown was not reproducible.
The relative
geometry of the electrodes influenced arc stability by controlling the
position where the arc contacted the cathode.
Greatest precision was
found empirically when the electrodes were aligned so the arc was
directed at the tip of the hairpin-shaped cathode (cf. Figure 6-1).
The position of the sample droplet on the cathode determined both
precision and sample vaporization efficiency.
The more centralized the
droplet was at the tip, the more complete became the vaporization
step.
When a determination was attempted before the solvent was
completely evaporated, two effects occurred:
broadband emission from
the electrode material and difficulty of the sample aliquot adhering to
the cathode.
The droplet would bead-up on the wire and be blown off by
the flowing nitrogen gas.
It took between five and ten one-second
strikes of the arc before the sample would adhere reproducibly to the
cathode.
An investigation of the electrode surface might be able to
explain these effects.
190
Interferences
Two classical interferences on calcium emission were examined:
the ionization effect caused by the addition of sodium (Fig. 6-5) and
the vaporization effect from the addition of phosphate (Fig.
6
-6 ).
The
absence of interferences is evident from the constant calcium atom
emission intensity as the interferent concentration is increased.
The
ionization interference is essentially absent even at a concomitant
solution concentration of 0.1%.
Similarly, vaporization effects are
not present even at a phosphate/calcium molar ratio of 60.
In
comparison to the nebulizer system, which was severely influenced by
the solution matrix (Chap. 3), the microarc exhibited no interference
even in the absence of releasing agents.
These results are consistent
with previous work using the microarc (1 ) and are attributable to its
ability to separate and efficiently perform the processes involved in
decomposing the sample into its atomic constituents.
CONCLUSION
The microarc-MINDAP system is an economical and sensitive
spectroscopic tool for microvolume sample analysis.
between 0.1-10 pL can be easily analyzed.
Sample volumes
Importantly, only the volume
dispensed onto the electrode is analyzed, unlike the situation with
nebulizer systems which are seldom greater than 5% efficient.
Because
the microarc- MINDAP combination responds to sample mass and exhibits
minimal matrix interferences, calibration curves can be prepared using
only a single solution standard.
A broad linear dynamic range of
191
Figure 6-5:
Sodium interference on 10 ng (1 yL of 10 yg/mL) Calcium
at 422.7 nm.
300
1
...
1
!
-
-
S igna
250
200
}-
CD
Ca
------ g -----
o
150
-
1 0 0
-
.
1
1
10
100
Sodium
...
Concentration
_,l
...
1000
Cjug/mL)
10000
193
Figure
6
-6 :
Phosphate interference on 10 ng (1 yL of 10 yg/mL)
Calcium at 422.7 nm.
300
200
Ca
CD
S i gna
250
150
-
1 0 0
0.01
1 0 0
P04 /
Ca M o l a r
Ratio
195
three-to-five orders of magnitude is typical for these curves.
Unlike
the argon (1) and helium (5) MIPs, the microarc-MINDAP system can
handle sample masses up to at least 3 yg.
This discrete
sample-introduction system exhibits detection limits which are
comparable to those produced by other microwave-induced plasmas (1,5),
inductively coupled plasmas (3-4), and flameless atomic absorption (21)
techniques.
t
The system is virtually free from matrix (phosphate and
sodium) interferences, unlike the ICP-microarc combination and
nebulizer-MINDAP systems.
The overall precision is good and ranges
between 3 and 7%.
The operational features of this new combination make it
convenient to use.
The sample introduction and emission systems are
inexpensive to construct and operate because of their low power
requirements and the use of nitrogen for the plasma and sample-delivery
gas.
Both the microarc and detection electronics can be easily
automated (1,3) for accurate and precise measurements.
analysis time is short - approximately
1
Moreover,
minute from dispensing the
sample until the computer finishes calculating and writing the results
on a storage disk.
Although inherent sensitivity and economy make this new
combination attractive, it is by nature a discrete-sampling technique.
Continuous sample introduction is therefore impossible and signal
averaging is time-consuming.
Fortunately, the MINDAP is dominated by
low-frequency noise and, as indicated in Chapter 5, time constants
longer than approximately two seconds do not improve precision.
Because the MINDAP is a used in an emission mode, simultaneous
multielement detection is possible even for discrete sample analysis.
196
Other minor drawbacks could be minimized.
The precision and
sample throughput rate could be improved further by measuring
automatically the mass of the analyte dispensed onto the cathode using
the described OH-band monitoring technique.
The microarc-MINDAP
combination could be improved also by optimizing the applied power and
nitrogen flow rate for greatest signal-to-background ratio.
197
References
1.
L. Layman and G. M. Hieftje, Anal. Chem., 4 7 , 194(1975).
2.
H. Falk, E. Hoffmann, 1. Jaekel and Ch. Ludke, Spectrochim. Acta,
34B, 333 (1979).
3.
J. P. Keilsohn, R. D. Deutsch and G. M. Hieftje, Appl.Spectrosc.,
37, 101 (1983).
4.
R. D. Deutsch and G. M. Hieftje, "Microarc Microsampling for a
Mini ICP", paper #179, IX Annual Meeting of the Federation of
Analytical Chemistry and Spectroscopy Societies, Philadelphia, PA.
1982.
5. A. T. Zander and G. M. Hieftje, Anal. Chem., 50, 1257 (1978).
6
. D. E. Nixon, V. A. Fassel, R. N. Knisely, Anal. Chem., 46, 210
(1974).
7. A. M. Gunn, D. L. Millard, and G. F. Kirkbright, Analyst, 103,
1066 (1978).
8
. R. L. Dahlquist, J. W. Knoll, and R. E. Hoyt, Paper #341, 26th
Pittsburgh Conference, 1975.
9.
A. Aziz, J. A. C. Broekaert, and F. Leis, Spectrochim. Acta, 36B,
251 (1981).
10. C. I. M. Beenakker, Spectrochim. Acta, 32B, 173 (1978).
11. B. D. Quimby, P. C. Uden, and R. M. Barnes, Anal. Chem., 5£, 2112
(1978).
12. F. L. Fricke, 0. Rose, and J. A. Caruso, Talanta, 23, 317 (1975).
13. J. H. Runnels, and J. H. Gibson, Anal. Chem., 39, 1399 (1967).
14.
W. B. Robbins, J. A. Caruso, and F. L. Fricke, Analyst, 104, 35
(1979).
15.
P. Barett and T. R. Copeland, "Applications of Plasma Emission
Spectroscopy", 139, R. M. Barnes ed., Heyden Publishers, 1979.
16.
T. Ishizuka and Y. Uwamino, Anal. Chem., 52, 125 (1980).
17.
F. L. Lichte and R. K. Skogerboe, Anal. Chem., 45, 399 (1973).
18.
R. I. Bystroff, L. R. Layman, and G. M. Hieftje, Appl. Spectrosc.,
33, 230 (1979).
19.
L. Layman and G. M. Hieftje, Anal. Chem., 46, 332 (1974).
198
20. J. D. Winefordner and T. J. Vickers, Anal. Chem.,
36, 1939 (1964).
21. Instrumentation Laboratories, Publication AID 91,
Mass.
Wilmington,
22.
R. J. Strutt, Proc. Roy. Soc., A93, 254 (1917).
23.
P. Goudmand, G. Pannetier, 0. Dessaux, and L. Marsigny, Compt.
Rend., 25£, 422 (1963).
24.
G. Pannetier, P. Goudmand, 0. Dessaux, and N. Tavernier, J. Chim.
Phys., 61, 395 (1964).
25.
E. P. Lewis, Phys. Rev., 18, 125 (1904).
26.
R. J. Strutt, Proc. Roy. Soc., A85, 219
(1911).
27.
R. J. Strutt, Proc. Roy. Soc., A88, 539
(1913).
28.
A. N. Wright and C. A. Winkler, "Active Nitrogen", Academic Press,
N. Y., 1968.
29.
W. L. Starr, J. Chem. Phys., 43, 73 (1965).
199
CHAPTER 7
Conclusion and Future Work
As has hopefully been demonstrated in this thesis, the MINDAP has
excellent potential as an inexpensive, compact, and sensitive elementselective detector for routine analytical measurements.
Other compa­
rable methods are either very expensive, require special procedures for
introducing samples, do not have high excitation temperatures or
complicate the analysis with intense background spectral features.
The MINDAP system possesses many of the qualities of an ideal
emission source for routine trace elemental determinations:
it is
sustained through a low flow ( < 2 L/min) of replenishable and
economical molecular nitrogen gas.
The principal component of the
power supply used to generate the plasma (a magnetron tube) is
inexpensive and commercially available, being used in both medical
diathermy units and microwave ovens.
instrumentation and operation are low.
Consequently, costs of both
The plasma has both high
thermal and excitation temperatures, suggesting its ability to
decompose and excite the analyte.
As a result, it provides low
detection limits (ppb) and linear working curves for analyte introduced
in the form of either an aerosol or atomic vapor.
Available precision
( < 2%) is comparable to that of other more sophisticated and expensive
techniques.
Other qualities of the new plasma system include its
ability to analyze both macro and micro aliquots of analyte and its
relative freedom from interfering matrix effects.
Although these
characteristics are somewhat dependent on the operating conditions, the
200
MINDAP exhibits also dominant noise at low frequencies.
For analytical
measurements this feature means that longer integration times need not
be used to obtain useful precision and sensitivity.
In the development and characterization of a new technique, many
new and interesting questions arise and a host of further research
opportunities appear.
In the following section, a few of these
questions and ideas are listed, along with a brief description of their
importance for the development and understanding of the nitrogen
discharge in the presence of a microwave field.
FUTURE WORK
The plasma should be sustained and characterized using a smaller
diameter torch (4 mm o.d.) which has already been constructed.
A
decrease in the size of the torch should increase its interaction with
the high electric field centered within the microwave cavity.
Such a
smaller torch would be expected to produce an increased energy density
in the plasma at any desired applied power or, alternatively, an
analytically useful plasma at lower applied powers and nitrogen flow
rates.
A thorough investigation of the smaller plasma's analytical and
physical figures-of-merit might be a significant scientific
contribution.
The MINDAP should be coupled to a flow-independent nebulizer
(i.e., ultrasonic).
This combination would enable the optimum
operating conditions of the MINDAP to be determined because the
introduction of analyte would then be independent of gas flow rates.
201
Such a gas-flow race independent nebulizer would also enable Che MINDAP
Co be sustained as an annular plasma and Che performance of Che
resulting system compared to work presented in this thesis.
Another aid to understanding the plasma would be to isolate the
energetic species in the plasma and determine the extent to which each
contributes to the excitation of analyte.
In afterglow studies two
species have been found to be principally responsible for .excitation:
vibrationally excited nitrogen molecules (N£ ) and ground-state
nitrogen atoms (N).
Chemical separation of either of these species in
the discharge is possible through the addition of N^O (to remove
) or NO (to remove N) gas.
Results from these experiments
should help to identify and explain a mechanism by which analyte is
excited in the plasma.
An interesting study would be to look at the spatial-timedependent emission signal from the microarc atomizer in the MINDAP
tail-flame with a gated vidicon detector.
This investigation would
provide spatial and temporal information about the distribution of
atoms in the tail flame.
possibly
Results from this type of an experiment could
suggest better methods for introducing and concentrating
sample into the plasma.
Identifying and determining the magnitude and effect of other
noise sources will be essential for optimal operation of the MINDAP.
Cross-correlation studies involving signal noise, from either
background or analyte emission, with fluctuations from such operational
202
parameters as gas pressure, flow rate, and solution nebulization are
necessary for understanding the origins of multiplicative noise
observed in the MINDAP signals.
A practical application of the MINDAP would be to couple a GC,
capillary GC (through the central tube), or even an LC or HPLC system
to it.
The ease with which the two could be interfaced make this an
attractive, inexpensive, yet sensitive detector for chromatographic
separations.
It would be extremely interesting and helpful to elucidate the
characteristics of the plasma within the microwave cavity itself.
Such
measurements could be performed by viewing through one of the cooling
holes drilled in the side of the cavity.
Temperatures and analytical
figures-of-merit could be obtained using a "remote sensing" fiber optic
bundle to transmit the radiation from inside the cavity.
This plasma system probably will not get the Hieftje "Seal of
Approval" if a droplet generator system is not some how incorporated
into it.
Droplets, separated in time, could either be introduced into
one of the gas flows entering the plasma torch and experience the
breakdown region or be shot into the MINDAP tail flame and encounter
the energy transferred from the main discharge region.
Understanding
atomization and vaporization processes in the MINDAP would be aided
with this approach.
203
VITA
Robert David Deutsch
Born:
August 16, 1956, Far Rockaway, New York.
Undergraduate Education:
University of Rhode Island
Kingston, Rhode Island (1974-1978)
Professional Societies:
Society of Applied Spectroscopy
Optical Society of America
Positions:
Research Associate, Indiana Univ. (1980-1983)
Associate Instructor, Indiana Univ. (1978-1980)
Undergraduate Research Associate, URI (1976-1978)
Undergraduate Teaching Assistant, URI (1977-1978)
PRESENTATIONS
"Instrumental and Operational Characteristics of an AtraosphericPressure Microwave Induced Nitrogen Plasma," 8th annual Federation of
Analytical Chemistry and Spectroscopy Societies meeting (Philadelphia,
PA, September 1981)
R. D. Deutsch and G. M. Hieftje.
"Analytical Characteristics of an Atmospheric-Pressure Microwave
Induced Nitrogen Plasma," 8th annual Federation of Analytical
Chemistry and Spectroscopy Societies meeting (Philadelphia, PA,
September 1981)
R. D. Deutsch, J. P. Keilsohn and G. M. Hieftje.
Near Infrared Emission from Nonmetal Atoms in a Helium Microwave
Induced Plasma," 8th annual Federation of Analytical Chemistry and
Spectroscopy Societies meeting (Philadelphia, PA, September 1981)
J. E. Freeman, R. D. Deutsch and G. M. Hieftje.
"Time Resolved Atomic Fluorescence in a Microwave Induced Plasma,"
Winter Conference on Plasma Spectrochemistry (Orlando, FL, January
1982)
G. M. Hieftje, J. P. Keilsohn and R. D. Deutsch.
"The Use of a Microarc Atomizer for Sample Introduction into the
Inductively Coupled Plasma," Pittsburgh Conference (Atlantic City, NJ,
March 1982
J. P. Keilsohn, R. D. Deutsch and G. M. Hieftje.
"Interelement Interferences and Temperature Measurements in a
Microwave-Induced Nitrogen Discharge at Atmospheric Pressure
(MINDAP)," 24th Rocky Mountain Conference (Denver, CO, August 1982)
R. D. Deutsch and G. M. Hieftje.
204
"Microarc Microsampling for a Mini ICP," 9th annual Federation of
Analytical Chemistry and Spectroscopy Societies meeting (Philadelphia,
PA, September 1982)
R. D. Deutsch and G. M. Hieftje.
"Microarc Sample Introduction for the Microwave Induced Nitrogen
Discharge at Atmospheric Pressure (MINDAP)," 9th annual Federation of
Analytical Chemistry and Spectroscopy Societies meeting (Philadelphia,
PA, September 1982)
R. D. Deutsch and G. M. Hieftje.
"New Techniques and Instrumentation for Time-Resolved Fluorimetry,"
Pittsburgh Conference (Atlantic City, NJ, March 1982)
G. M. Hieftje, G. R. Haugen, E. E. Vogelstein, R. E. Russo, R. D.
Deutsch and J. P. Keilsohn.
"Analysis of Noise in the MINDAP (Microwave Induced Nitrogen Discharge
at Atmospheric Pressure)," 10th annual Federation of Analytical
Chemistry and Spectroscopy Societies meeting (Philadelphia, PA,
September 1983)
R. D. Deutsch and G. M. Hieftje.
"Vidicon Based Detection Systems for Spatially Resolved Studies of
Atomic Sources," 10th annual Federation of Analytical Chemistry and
Spectroscopy Societies meeting (Philadelphia, PA, September 1983)
J. W. Olesik, R. D. Deutsch, G. M. Hieftje and J. P. Walters.
"Analysis of Noise in the MINDAP (Microwave Induced Nitrogen Discharge
at Atmospheric Pressure)," (Vancouver, Canada, October 1983)
R. D. Deutsch and G. M. Hieftje.
PUBLICATIONS
J. P. Keilsohn, R. D. Deutsch and G. M. Hieftje, "The Use of a
Microarc Atomizer for Sample Introduction into the Inductively Coupled
Plasma," Appl. Spectrosc., 37_, 101 (1983).
R. D. Deutsch and G. M. Hieftje, "Development and Characterization of
a Microwave Induced Nitrogen Discharge at Atmospheric Pressure
(MINDAP)," submitted to Applied Spectroscopy.
R. D. Deutsch, J. P. Keilsohn, and G. M. Hieftje, "Analytical
Characterization of a Microwave Induced Nitrogen Discharge at
Atmospheric Pressure (MINDAP)," submitted to Applied Spectroscopy.
R. D. Deutsch and G. M. Hieftje, "Temperature Measurements in the
MINDAP System," submitted to Applied Spectroscopy.
R. D. Deutsch and G. M. Hieftje, "Microarc Microsampling for a Mini
ICP," in preparation.
R. D. Deutsch and G. M. Hieftje, "Microarc Sample Introduction for the
Microwave Induced Nitrogen Discharge at Atmospheric Pressure
(MINDAP)," submitted to Applied Spectroscopy.
205
R. D« Deutsch and G. M. Hieftje, "Analysis of the Noise Components in
the MINDAP System," submitted to Applied Spectroscopy.
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