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Laser-excitation atomic fluorescence spectroscopy in a helium microwave -induced plasma

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ABSTRACT
Name: Timothy S. Schroeder
Department: Chemistry and Biochemistry
Title: Laser Excitation Atomic Fluorescence Spectroscopy in a Helium Microwave
Induced Plasma
Major: Analytical Chemistry
Degree: Doctor o f Philosophy
Approved by:
Date:
f y /ir /V
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dissertation Director
NORTHERN ILLINOIS UNIVERSITY
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ABSTRACT
The focus of this dissertation is to report the first documented coupling o f helium
microwave induced plasmas (MIPs) to laser excitation atomic fluorescence spectroscopy.
The ability to effectively produce intense atomic emission from both metal and nonmetal
analytes gives helium microwave induced plasmas a greater flexibility than the more
commonly utilized argon inductively coupled plasma (ICP). Originally designed as an
element selective detector for non-aqueous chromatography applications at low applied
powers (< 100W), the helium microwave plasma has been applied to aqueous sample
determinations at higher applied powers (> 500 W). The helium MIP has been shown to
be a very powerful analytical atomic spectroscopy tool.
The development of the pulsed dye laser offered an improved method of
excitation in the field o f atomic fluorescence. The use o f laser excitation for atomic
fluorescence was a logical successor to the conventional excitation methods involving
hollow cathode lamps and continuum sources. The highly intense, directional, and
monochromatic nature o f laser radiation results in an increased population of atomic
species in excited electronic states where atomic fluorescence can occur.
The application of laser excitation atomic fluorescence to the analysis of metals in
a helium microwave induced plasma with ultrasonic sample nebulization was the initial
focus of this work. Experimental conditions and results are included for the aqueous
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characterization o f manganese, lead, thallium, and iron in the helium MIP-LEAFS
system. These results are compared to previous laser excitation atomic fluorescence
experimentation. The effect o f matrix interferences on the analytical fluorescence signal
was also investigated for each element.
The advantage o f helium MIPs over argon ICPs in the determination of nonmetals
in solution indicates that the helium MIP is an excellent candidate for laser excitation
atomic fluorescence experiments involving nonmetals such as chlorine, bromine, iodine,
and sulfur. Preliminary investigations into this area are reported, including
documentation o f all excitation and fluorescence lines investigated for chlorine and
iodine in the helium MIP.
Also discussed is the modification of the microwave resonator cavity used in these
experiments in an effort to achieve atomic fluorescence signal from nonmetals. Holes
were drilled in the sides o f the resonator cavity to align with holes placed in the sides of
the plasma torch to allow the laser beam to interact with the plasma while inside the
microwave cavity.
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NORTHERN ILLINOIS UNIVERSITY
LASER EXCITATION ATOMIC FLUORESCENCE SPECTROSCOPY
IN A HELIUM MICROWAVE INDUCED PLASMA
A DISSERTATION SUBMITTED TO THE GRADUATE SCHOOL
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE
DOCTOR OF PHILOSOPHY
DEPARTMENT OF CHEMISTRY AND BIOCHEMISTRY
BY
TIMOTHY S. SCHROEDER
© 1999 Timothy S. Schroeder
DEKALB, ILLINOIS
AUGUST 1999
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UMI Number 9948285
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ACKNOWLEDGEMENTS
I would like to thank my research advisor, Dr. Jon W. Carnahan, for his time,
patience and assistance in the completion o f this work. I also wish to thank the members
o f my committee: Dr. David Ballanitine, Dr. Elizabeth Gaillard, Dr. Gaiy Baker, and Dr.
Susan Mini.
I would like to thank Dr. David L. McCurdy. This work could not have been
completed without his past assistance, guidance, and most of all friendship.
I would like to thank the Northern Illinois University Graduate School and the
Northern Illinois University Department of Chemistry and Biochemistry for their
financial support of this project.
Lastly, I wish to thank my family. Shannin and Bailey, you are the real reasons
that this work was done.
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DEDICATION
To Shannin and Bailey, with much love and gratitude
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TABLE OF CONTENTS
Page
LIST OF T A B L E S.......................................................................................................
viii
LIST OF FIGURES .....................................................................................................
ix
Chapter
1.
2.
SURVEY OF PLASMA ATOMIC SPEC TRO SCO PY .............................
1
1.1 Introduction ......................................................................................
1
1.2 Development of Plasmas as Analytical Tools ...............................
2
1.3 Microwave Induced Plasma Atomic Emission Spectrometry . . . .
3
1.4 Aqueous Sample Introduction into Analytical Plasmas
...............
5
1.5 Kilowatt-Plus Microwave Induced Plasma ....................................
7
1.6 Theory o f Laser Excitation Atomic Fluorescence Spectroscopy . .
8
1.7 Historical Background of Atomic Fluorescence Spectroscopy . . .
10
1.8 Direction o f Research ......................................................................
18
EXPERIMENTAL INSTRUMENTATION AND ARRANGEMENTS . .
26
2.1 Molecular Fluorescence Using a Commercial
Spectrofluorimeter......................................................................
26
2.2 Laser Excitation Atomic and Molecular
Fluorescence Arrangement..........................................................
27
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vi
Chapter
3.
Page
2.2.1 Laser Instrumentation...............................................................
30
2.2.2 Laser Fluorescence C e ll s .......................................................
32
2.2.3 Double Monochromator and Detection S y ste m ...................
35
2.2.4 Data C ollectio n ......................................................................
36
2.3 Laser Pathways for Laser Excitation Atomic Fluorescence
in the Helium Microwave Induced Plasma ................................
38
LASER EXCITATION ATOMIC FLUORESCENCE OF MANGANESE,
LEAD, THALLIUM, AND IRON IN THE HELIUM MEP .......................
48
3.1 Introduction.....................................................................................
48
3.2 Experimental ...................................................................................
48
3.3 Data Collection and M anipulation.................................................
49
3.4 Evaluation of Plasma Parameters and Analytical Figures
o f M e rit...........................................................................................
54
3.4.1 Limit o f Detection Calculation...............................................
54
3.4.2 Optimization o f Helium Flow Rates ....................................
55
3.4.3 Linearity o f Analytical Fluorescence S ignals.......................
56
3.5 Matrix Interferences on the Analytical Fluorescence Signal . . . .
57
3.6 Laser Excitation Atomic Fluorescence of M anganese.................
58
3.7. Laser Excitation Atomic Fluorescence of Lead .........................
69
3.8. Laser Excitation Atomic Fluorescence o f T h a lliu m ...................
84
3.9 Laser Excitation Atomic Fluorescence of Iron ..........................
97
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vii
Chapter
Page
3.10 Comparison of Mn, Pb, Tl, and Fe LEAFS Limits of
Detection in the Helium MIP with Other Atomic
Fluorescence M ethods..................................................................
101
APPLICATION OF LEAFS TO THE DETERMINATION OF
NONMETALS IN THE HELIUM MIP ........................................................
Ill
4.1 In tro d u ctio n .....................................................................................
Ill
4.2 The Population of High Energy Nonmetal Excited States in a
Helium Microwave Induced Plasma .............................................
Ill
4.3 Fluorescence o f Naphthalene in Methanol Solution for
Instrumentation Optimization ........................................................
118
4.4 Laser Excitation Atomic Fluorescence Investigations
o f Chlorine in the Helium MIP ......................................................
119
4.4.1 Instrum ental............................................................................
129
4.4.2 Chlorine LEAFS; Excitation at 283 nm and Fluorescence
at 479.454 n m ..........................................................................
129
4.4.3 Chlorine LEAFS; Excitation at 267 nm and Fluorescence
at 521 nm ..............................................................................
131
4.5 Laser Excitation Atomic Fluorescence Investigations of Iodine
in the Helium M IP ..........................................................................
135
4.6 C o n clu sio n s....................................................................................
141
FUTURE CONSIDERATIONS ..............................................................
142
REFERENCES ..............................................................................................................
144
4.
5.
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LIST OF TABLES
Table
Page
1. LEAFS with Graphite Furnace A tom ization..................................................
14
2. Atomic Fluorescence Using Flame Atomization...........................................
16
3. Atomic Fluorescence in Plasma A tom izers....................................................
19
4. Optimized Instrumental Parameters for Mn L EA FS.......................................
66
5. Optimized Instrumental Parameters for Pb LEAFS .......................................
79
6. Optimized Instrumental Parameters for T1 L E A FS.........................................
92
7. Optimized Instrumental Parameters for Fe LEAFS......................................
102
8. Comparison o f Limits of Detection from This Work to Limits o f
Detection o f Other Atomic Fluorescence M ethods.......................................
107
9. Operational Parameters for Laser Excitation Molecular Fluorescence of
Naphthalene in Methanol Solution ................................................................
120
10. Tabulated Excitation Energy Levels for the 283/479 nm Chlorine
Excitation/Fluorescence Schem e....................................................................
126
11. Fixed Parameters for the Investigation o f Chlorine LEAFS
in the Helium M I P ...........................................................................................
130
12. Tabulated Energy Levels for the 267/521 nm Chlorine
Excitation/Fluorescence Schem e....................................................................
134
13. Tabulated Energy Levels for the Iodine Laser Excitation Atomic
Fluorescence Scheme .....................................................................................
139
14. Fixed Parameters for the Investigation of Iodine LEAFS in
the Helium M I P ...............................................................................................
140
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LIST OF FIGURES
Figure
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Page
Schematic of Laser Excitation Atomic Fluorescence in a Graphite
Furnace..............................................................................................................
12
Block Diagram of the Helium Microwave Induced Plasma Laser
Excitation Atomic Fluorescence Instrumental Arrangem ent.......................
28
Diagram of the Laser Beam-Microwave Resonator Cavity Interface
Used for Laser Excitation Atomic Fluorescence o f M e tals...................
40
Diagrams of the Laser Beam-Microwave Resonator Cavity Interface
Used to Examine Laser Excitation Atomic Fluorescence o f Nonmetals . . .
42
Diagram of the Holed Torch Design Used for Nonmetal Laser Excitation
Atomic Fluorescence Experiments ................................................................
45
Comparison of Collected Atomic Fluorescence Spectra o f 1.0 ppm Mn
and Distilled Water B la n k ........................................................................
52
Partial Energy Level Diagram for Manganese Showing the Electronic
Transitions Used for Manganese LEAFS in the Helium M IP ...............
59
Helium Plasma Gas Flow Rate Study for Manganese Laser Excitation
Atomic Fluorescence in the Helium MIP .....................................................
62
Ultrasonic Nebulizer Helium Carrier Gas Flow Rate Study for
Manganese Laser Excitation Atomic Fluorescence in the Helium MIP
64
...
Linearity of Manganese Laser Excitation Atomic Fluorescence in the
Helium MDP...............................................................................................
67
Interference of the Matrix Components Ca, Na, S 0 42\ and P 0 43' on the
Helium Microwave Induced Plasma Laser Excitation Atomic
Fluorescence of M anganese......................................................................
70
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X
Figure
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
Page
Partial Energy Level Diagram for Lead Showing the Electronic
Transitions Used for Lead LEAFS in the Helium M I P .................................
72
Helium Plasma Gas Flow Rate Study for Lead Laser Excitation Atomic
Fluorescence in the Helium Microwave Induced P la sm a ............................
75
Ultrasonic Nebulizer Helium Carrier Gas Flow Rate Study for Lead Laser
Excitation Atomic Fluorescence in the Helium M IP .....................................
77
Linearity o f Lead Laser Excitation Atomic Fluorescence in the
Helium M I P ......................................................................................................
80
Interference o f the Matrix Components Ca, Na, S 042', and P 0 43' on the
Helium Microwave Induced Plasma Laser Excitation Atomic
Fluorescence of L ea d ........................................................................................
82
Partial Energy Level Diagram for Thallium Showing the Electronic
Transitions Used for Thallium LEAFS in the Helium MIP ........................
85
Helium Plasma Gas Flow Rate Study for Thallium Laser Excitation
Atomic Fluorescence in the Helium MIP ......................................................
88
Ultrasonic Nebulizer Helium Carrier Gas Flow Rate Study for Thallium
Laser Excitation Atomic Fluorescence in the Helium M IP ..........................
90
Linearity of Thallium Laser Excitation Atomic Fluorescence in the
Helium M I P ......................................................................................................
93
Interference o f the Matrix Components Ca, Na, S 0 42\ and P 0 43‘ on the
Helium Microwave Induced Plasma Laser Excitation Atomic
Fluorescence of Thallium ...............................................................................
95
Partial Energy Level Diagram for Iron Showing the Electronic Transitions
Used for Iron LEAFS in the Helium M I P ......................................................
99
Linearity o f Iron Laser Excitation Atomic Fluorescence in the
Helium M I P .........................................................................................................
103
Interference o f the Matrix Components Ca, Na, and P 0 43' on the
Helium Microwave Induced Plasma Laser Excitation Atomic
Fluorescence o f Iron ..........................................................................................
105
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xi
Figure
25.
26.
27.
28.
29.
30.
Page
Partial Energy Level Diagram for Chlorine Illustrating the Electronic
States Used in Determining Thermodynamic Overpopulations .................
114
Calibration Curve for the Laser Excited Molecular Fluorescence of
Naphthalene in Methanol Solution ................................................................
121
Partial Energy Level Diagram of Chlorine Illustrating a Sample o f the
Electronic Transitions Located Within Seven Electron Volts above the
Charge Transfer Energy L evel........................................................................
124
Partial Energy Level Diagram of Chlorine Illustrating the Electronic
Transitions Associated with the 283/479 nm Excitation/Fluorescence
Scheme ............................................................................................................
127
Partial Energy Level Diagram of Chlorine Illustrating the Electronic
Transitions Associated with the 267/521 nm Excitation/Fluorescence
Scheme ............................................................................................................
132
Partial Energy Level Diagram of Iodine Illustrating the Energy Levels
Used in Laser Excitation Atomic Fluorescence Experim ents.....................
137
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CHAPTER 1
SURVEY OF PLASMA ATOMIC SPECTROSCOPY
1.1 Introduction
In this dissertation, the development and application of helium microwave
induced plasmas for atomic fluorescence spectroscopy is discussed. Technique
background, applications with atomic fluorescence of metals and nonmetals, and future
directions are discussed. The first chapter discusses the development of analytical
plasmas and aqueous sample introduction techniques and reviews atomic fluorescence
works appearing in the literature. Chapter 2 details the instrumentation used to perform
the atomic fluorescence experiments reported in this dissertation. Chapter 3 details
atomic fluorescence experiments performed on manganese, lead, thallium, and iron.
Chapter 4 discusses system characterization and development with solution phase
molecular fluorescence and the potential o f nonmetal atomic fluorescence with the
helium microwave induced plasma. The last chapter discusses future directions and
considerations for the helium microwave induced plasma - laser excitation atomic
fluorescence instrument.
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2
1.2 Development of Plasmas as Analytical Tools
The late 1960's and early 1970's was an extraordinary period in the area of
elemental analysis by optical emission spectroscopy techniques. During this time, the
development of plasma sources and their applications to analytical chemistry
revolutionized the ways in which routine elemental analyses were performed. Before this
time, the predominant methods of elemental analysis using optical emission techniques
were flame emission and atomic absorption spectroscopy. These early methods of
analysis had a number o f drawbacks. They were very prone to matrix interferences,
making suitable calibration difficult. They often exhibited complex spectral
backgrounds, which are a detriment to detection limits and applications to real samples.
In many cases, they provided insufficient energy to successfully atomize and excite all
elements, thus limiting their effectiveness as broad range analytical tools. Plasmas have
shown themselves to be free o f most of the problems associated with flame emission and
atomic absorption spectroscopies. In addition, linear dynamic ranges are significantly
improved using analytical plasma techniques. The plasma techniques developed in the
1960's and 1970's are continuously being refined and improved as new applications are
constantly being developed.
There were three main analytical plasmas developed during this period: the
inductively coupled plasma (ICP), the direct current plasma (DCP), and the microwave
induced plasma (MIP). Each type o f plasma has advantages not possessed by the others.
The ICP, simultaneously introduced in 1965 by Fassel and Kniseley [1, 2], and
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3
Greenfield, McGreachin, and Smith [3] is currently the most widely used analytical
plasma. The ICP provides excellent linearity, long linear dynamic range, and a simple
spectral background. The DCP was introduced simultaneously by Korelov and
Vainshtein [4] and Margoshes and Scribner [5] in 1959. The DCP is a very rugged
plasma and is mainly used to analyze geological samples, as it is robust and able to
tolerate complex sample matrices while remaining stable. Both the ICP and DCP are
operated using argon as the plasma support gas and have been characterized as being
sensitive techniques for the determination o f most relevant metals. However, neither
system has been effectively applied to the determination of nonmetallic species.
Microwave induced plasmas were developed as an element specific emission detector for
gas chromatography, and were introduced by McCormack, Tong, and Cooke [6] and
Bache and Lisk [7] in 1965. Microwave induced plasmas have performed well when
applied to the analysis o f both metallic and nonmetallic samples, but are still most
commonly used as a detector for chromatography [8-22].
1.3 Microwave Induced Plasma Atomic Emission Spectrometry
The ability to effectively excite nonmetals, metalloids, and metals gives the
helium microwave induced plasma a greater flexibility than argon inductively coupled
plasmas. Helium MIPs produce intense nonmetal atomic and ionic emission in the
vacuum ultraviolet (VUV), ultraviolet (UV), visible (Vis), and near infrared (NIR)
regions o f the spectrum. Helium discharges can also be operated at relatively low gas
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4
flows (<0.2 L/min) [23] and applied powers (as low as 25 Watts) [10,23, 24]. The
capability to perform sensitive nonmetal analyses at low applied powers and helium flows
has resulted in the helium discharges being used as powerful, element selective detectors
for gas chromatography [8-15,22], supercritical fluid chromatography [16-20], and liquid
chromatography [10, 15,21].
The use o f helium microwave induced plasmas for atomic emission spectroscopy
with liquid samples requires some changes in the operational parameters used for
chromatography detection. The most significant o f these changes are the increased
helium flow rates and microwave powers used. In general, helium flow rates must be
increased as applied microwave powers and plasma torch diameters increase. For the
kilowatt plus microwave induced plasma used in these studies, optimum nonmetal
emission has been observed at helium flow rates o f 16-24 L/min [25,26].
The capabilities of these helium discharges to perform well with a variety of
elements has been well detailed. Alvarado and Carnahan [27] investigated the VUV
detection of bromine, chlorine, iodine, sulfur, antimony, arsenic, selenium, and lead in a
high power MIP. In all cases o f that study, detection limits were at the part per million
level or better. A number o f authors have discussed the UV-Vis detection of a variety of
elements (metals and nonmetals) using a helium microwave induced plasma [28-36].
Tanabe, Haraguchi, and Fuwa have published a comprehensive tabulation of UV-vis
emission lines observed from most relevant nonmetals in a helium microwave induced
plasma [37]. Freeman and Hieftje[38-40] and Pivonka, Schleisman, Fateley, and Fry [41]
have investigated the capabilities of helium discharges to perform atomic emission
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5
experiments in the red to NIR region.
1.4 Aqueous Sample Introduction into Analytical Plasmas
Historically, the most common method of aqueous sample introduction into
analytical plasmas involves aqueous sample nebulization. In this process, analytes and
samples are dissolved in aqueous solution and introduced into the plasma via a nebulizer
and spray chamber assembly interface. The nebulizer creates a fine mist (aerosol) of the
solution. A gas flow is used to carry this mist into the plasma region. The fate o f these
fine aerosol droplets varies from nebulizer to nebulizer, but not all o f the droplets are
introduced into the plasma. In general, one or more techniques are used to select a
specific droplet size distribution which is ultimately swept into the plasma.
A variety o f spray chamber configurations has been successfully applied to
regulate droplet size distribution. Analyte droplets can be targeted at an impactor bead to
break larger droplets into smaller ones as well as give large droplets a surface to adhere
for removal from the aerosol. The droplets can be forced to traverse a tortuous pathway,
resulting in a narrow droplet size distribution. In some cases, gravity is employed to
restrict the droplet size, as the aerosol is forced to travel upwards to survive the spray
chamber. Aerosol droplets which do not exit the spray chamber typically condense on the
inside surfaces and are removed.
Droplet size can be crucial in atomic emission and fluorescence spectroscopies.
Droplets which are too small can result in an insufficient amount of analyte entering the
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6
plasma. Large droplets may not spend sufficient time in the plasma to effectively
evaporate the droplet and atomize the analyte. Both these situations result in a decrease
in analytical signal. The latter situation is additionally detrimental because it results in
the introduction o f large amounts o f solvent into the plasma. An excess o f solvent causes
“cooling” of the plasma which may make the plasma unstable and increase plasma flicker
noise.
Conventional pneumatic nebulization techniques use only the methods described
above to restrict droplet size and control the amount o f solvent entering the plasma.
Pneumatic nebulizers are considered the simplest aqueous sample introduction technique.
They use a rapid flow o f gas through a small orifice to convert a stream o f aqueous
solution into an aerosol. The methods used to convert the bulk solution to aerosol vary
greatly from nebulizer to nebulizer. The result, however, is a fine mist of aerosol
introduced into the spray chamber. Pneumatic nebulization techniques, though often
adequate, often result in an excess amount of solvent entering into and cooling the
plasma.
Modem ultrasonic nebulization (USN), developed by Fassel and Bear [42], with
associated desolvation techniques are able to remove the majority of the solvent and keep
it from entering the plasma. An ultrasonic nebulizer uses a vibrating crystal to create an
aerosol mist. The aerosol is more uniform in droplet size than the mist created with
conventional pneumatic nebulizers. This aerosol mist is passed through a heated tube and
the solvent droplets are evaporated. As the aerosol exits the heated tube, the sample mist
has been converted to dry analyte particles and solvent vapor. The analyte
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7
particle/solvent vapor mixture is then passed through a cooled condenser. The solvent
molecules are condensed back to liquid form and are routed to waste. The analyte
particles pass through the condenser to enter the plasma. While this nebulization method
does not remove all o f the solvent, it does eliminate the majority. Additionally,
nebulization efficiencies are roughly ten times greater with ultrasonic nebulizers than
with pneumatic nebulization. As a result, detection limits are generally improved when
using ultrasonic nebulizers as compared to conventional nebulization techniques.
1.5 Kilowatt-Plus Microwave Induced Plasma
In addition to the elimination of solvent entering the plasma, another option for
improving stability o f the plasma is to increase the applied power o f the plasma source.
Microwave discharges operated at low applied powers (< 100 W) and gas flow rates (< 3
L/min) are very sensitive to the presence o f water in the plasma. They are easily
extinguished if too much water is introduced. These plasmas are often used as detectors
for a variety o f chromatography techniques [8-22, 43,44], The presence o f water is
generally not a concern with gas chromatography.
To use microwave induced discharges for analytical atomic emission purposes
with aqueous sample introduction, higher power plasmas offer distinct advantages. These
more robust plasmas are capable of tolerating greater amounts o f solvent while remaining
stable. They are larger in diameter and the plasma volume is significantly larger. This
characteristic allows more time for analyte-containing droplets to be vaporized. This
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8
results in an increased number o f atomized analyte species in the viewing zone of the
plasma and an increased analytical signal collected at the detector. Previously, a very high
power microwave plasma system was developed in our laboratory [24, 25]. It is capable
of operating at applied powers o f up to 3000 W. The microwave plasma source used in
conjunction with sample introduction via ultrasonic nebulization provides an excellent
means to perform sensitive atomic spectroscopic analyses [33].
1.6 Theory o f Laser Excitation Atomic Fluorescence Spectroscopy
Atomic fluorescence spectroscopy involves the absorption of radiation, generally
as photon(s) o f light, by an atom. Subsequent re-emission of all or part of that energy as
light can be monitored. A key component in the usefulness o f atomic fluorescence is that
the wavelength of light absorbed is generally different than the wavelength o f light
monitored during the fluorescence process. This fundamental property generally leads to
signal modulation which may be used to eliminate effects of background radiation and
noise.
The use o f a tunable pulsed dye laser as an excitation source for atomic
fluorescence spectroscopy offers some attractive advantages when compared to other
conventional methods o f excitation such as hollow cathode lamps. Laser emission is
directional, coherent, and extremely intense. These factors combine to yield an excitation
source which can be capable o f saturating an atomic excited state. Saturation may occur
when laser power is great enough to produce a ratio of excited to ground state atoms
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9
(N2/N,) equal to the ratio of the statistical g values of the Boltzmann equation for the
excited and ground state (g2/gt). When N2/N, is equal to g2/g„ the rate o f absorbance of
incident laser radiation is equal to the rate o f emission of radiation as fluorescence. In
this situation, the atom population experiences the maximum theoretical population of the
excited state and intense stimulated fluorescence is produced.
An important consideration when performing atomic fluorescence spectroscopy is
the need for the excitation source to be capable o f producing highly populated excited
states. Highly populated excited states are necessary because often the atom is capable of
fluorescing via several distinctly different pathways from a single excited state. Since
fluorescence intensity is directly proportional to the excited state population, each
different pathway that can be followed results in a loss of collected fluorescence signal
and a decrease in quantum efficiency for the monitored electronic transition. Before the
development o f tunable dye laser excitation sources, it was difficult to excite a sufficient
population o f analyte atoms to achieve sensitive analytical results.
Atomic fluorescence excitation is generally achieved by exciting ground state
atoms to an excited state (resonance transition) as opposed to pumping atoms from one
excited state to another (nonresonance transition). According to Boltzmann theory,
higher energy excited states have a much lower naturally occurring population compared
to the ground state energy level. Using a laser to pump atoms from the ground state to an
excited state means that the majority o f analyte atoms are capable o f absorbing the
incident laser radiation. The larger the number of atoms excited, the larger and more
sensitive the analytical fluorescence signal. Conversely, in some o f the hotter atomizers
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10
such as inductively coupled plasmas, researchers have been able to successfully pump
atoms which are in low-lying excited states rather than the ground state. This is due, in
part, to the high temperatures in the inductively coupled plasma extensively populating
these lower energy excited states.
1.7 Historical Background of Atomic Fluorescence Spectroscopy
A great deal o f work has been performed in the general area of atomic
fluorescence. A variety o f different types of atomizers have been investigated. A review
article by Sjostrom [45] discusses a variety of laser excitation atomic fluorescence topics
including theory, processes, detection schemes, and background correction. This paper is
targeted towards laser excitation atomic fluorescence systems using graphite furnace
atomizers. Another paper by Smith, Glick, Spears, and Winefordner [46] offers a
comprehensive list o f elements which have been determined by atomic fluorescence
spectroscopy along with their respective limits of detection. Many of the elements listed
have been evaluated by several different atomic fluorescence instrumental arrangements.
In these cases, multiple results for each element are included in the table with specifics o f
each different type o f experiment (atomizer, excitation source, and excitation/emission
wavelengths). This list is organized by element and is not biased toward any one
particular type o f atomizer or excitation source.
There have been multiple instrumental arrangements utilized for laser excitation
atomic fluorescence using pre-existing graphite furnaces as atomizers. These works
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11
involved passing a laser beam through a heated graphite tube [47-54], over a heated
graphite rod or cup [55-60], or through a flame [61-67]. Other thermal (non-flame)
atomization techniques have also been investigated [68, 69].
The majority of laser excited atomic fluorescence work has been performed using
graphite furnace atomizers employing heated graphite tubes or cups. Figure 1 illustrates
the graphite tube-laser beam interface. The graphite tube is clamped between two
electrodes. A current is passed from one electrode to the other through the graphite tube.
This current causes the graphite tube to be resistively heated. First, the solvent is
removed at a relatively low temperature (100 - 200 °C). Residual organics are then
removed by combustion at about 400 - 700 °C. After the organics are removed, the
analyte is generally present in the graphite tube as a metal salt. Finally, the graphite tube
is heated to between 1500-2500 °C and the analyte molecules are vaporized and
atomized. These free gaseous atoms absorb incident excitation laser radiation, and
fluoresce. Fluorescence radiation is monitored off-axis from the incident laser beam to
reduce the amount of scattered light entering the monochromator. Fluorescence can be
monitored at either end of the tube or through a specially designed fluorescence port
machined into the tube.
Table 1 contains many o f the elements which have been characterized for laser
excitation with graphite furnace atomization. Listed in the table are references and
specifics about the experiments: excitation wavelength (A.^, fluorescence wavelength
(A.nuor)5atomizer type, and limit o f detection (LOD). All elements listed in Table 1 are
metals. Very little experimental work has been noted in which laser excitation atomic
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Figure 1.
Schematic of Laser Excitation Atomic Fluorescence in a Graphite Furnace
Typical graphite tube furnace is shown. Solid line indicates the laser beam
pathlength. Dotted lines indicate possible pathways for observation of
atomic fluorescence.
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13
Laser Beam Out
Laser Beam In
Fluorescence Out
\
Fluorescence Out
Fluorescence Out
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14
Table 1
LEAFS with Graphite Furnace Atomization
Element
Ag
Al
Bi
Cd
Co
Cu
Eu
Fe
Ir
Mn
Na
Pb
Pt
Sb
Te
TI
nm
328.1
257.510
223.1
228.8
217.460
304.400
304.4
304.400
324.7
287.9
296.69
296.7
295.1
279.5
589.6
283.3
283.31
283.3
283
283.306
214.281
293.0
212.7
214.3
213.618
276.79
^■fiuon nm
Atomizer type
LOD, ppb Reference
338.3
309
299.3
228.8
235.0
340.5
340.5
340.5
510.5
536.1
373.49
373.5
322.1
279.5
589.6
405.7
405.78
405.7
406
405.783
241.0
299.7
259.8
238.4
238.4
351.92
GC
GT
GC
GC
PCGT
PCGT
GC
GT
GC
GC
GT
GC
GC
GC
GC
GC
GT
GC
GT
GT
PCGT
GC
GT
GT
PCGT
GT
0.003*
0.10
0.00005
0.00007
0.15
0.001
0.002*
0.08
0.002*
10.0*
0.025*
0.001*
0.20*
0.006*
0.02*
0.018
0.0025*
0.000025*
0.001**
0.002
4.0*
0.50
1.0
0.00001
56
53
58-60
57, 58
54
54
56
52
56
56
47
56
56
56
56
58
47
56
51
53
53
56
50
50
54
49
* = LOD determined by extrapolation
** = LOD based on 2 a
= no LOD listed in publication
Atomizer types: GC = graphite cup, GT = graphite tube, PCGT = pressure controlled
graphite tube
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15
fluorescence spectroscopy has been applied to the analysis o f nonmetals. Suitable laser
sources are not readily available to produce the desired resonance excitation in the deep
ultraviolet and vacuum ultraviolet regions.
Other than graphite furnace atomization, the other major methods of analyte
atomization involve flames or plasmas. Laser excitation atomic fluorescence studies
using these atom reservoirs have been previously reported. Flame atomizers employing a
variety of mixed gases have been extensively investigated [59-67]. In addition, laser
excitation atomic fluorescence experiments involving inductively coupled plasmas [66,
67, 70-76], direct current plasmas [77], argon microwave induced plasmas [78], and other
methods [79, 80] have been reported. The ability of the plasmas to more thoroughly
atomize analyte-containing samples in many cases makes it a superior choice over flame
atomization techniques.
Atomic fluorescence has been observed in a variety o f different flames using a
variety of different excitation sources. Flames comprised o f air-H2, air-acetylene, and
N,0-acetylene have been used in atomic fluorescence experimentation. Excitation
sources for flame atomic fluorescence consist of mainly pulsed dye lasers (PDL) [62-65,
67], though other excitation methods have been utilized as well [66]. Table 2 lists a
survey of the elements which have been characterized and experimental conditions
reported for atomic fluorescence in a flame atomizer. For cases in which a hollow
cathode lamp was used as an excitation source, no data is given for excitation
wavelength. In these instances it is possible that more than one wavelength o f light
emitted from the HCL contributes to analyte excitation.
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2
Atomic Fluorescence Using Flame Atomization
Element
Excitation
X, nm
Fluorescence
X, nm
Excitation
Source
Atomizer
Type
Detection
Limit, ppb
As
Ce
Co
Cu
Dy
Er
Eu
Gd
Ho
Lu
Mn
Nd
293.7
371.64
304.400
324.754
364.54
400.80
459.40
376.84
405.39
465.80
279.482
463.42
245.65
394.21
340.5
510.5
353.6
400.80
462.72
336.22
410.38
513.51
403.3
489.69
ArF
PDL
OPO
OPO
PDL
PDL
PDL
PDL
PDL
PDL
OPO
PDL
Air/H
Nitrous/Acet
Air/Acet
Air/Acet
Nitrous/Acet
Nitrous/Acet
Nitrous/Acet
Nitrous/Acet
Nitrous/Acet
Nitrous/Acet
Air/acet
Nitrous/Acet
20 ppb
500*
2.0
2.0
300*
500*
20*
800*
150*
3000*
0.2
2000*
(continued on following page)
Reference
66
62
64
64
62
62
62
62
62
62
64
62
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Table 2 (continued)
Element
Excitation
X, nm
Fluorescence
X, nm
Excitation
Source
Atomizer
Type
Detection
Limit, ppb
Pb
283.306
283.306
427.23
366.14
370.28
276.787
276.787
371.79
398.80
405.8
405.783
430.58
375.64
350.92
352.9
352.943
409.42
346.44
OPO
PDL
PDL
PDL
PDL
OPO
PDL
PDL
PDL
Air/Acet
Air/Acet
Nitrous/Acet
Nitrous/Acet
Nitrous/Acet
Air/Acet
Air/Acet
Nitrous/Acet
Nitrous/Acet
0.40
0.02
1000*
150*
500*
0.90
0.80
100*
10*
Pr
Sm
Tb
T1
Tm
Yb
Excitation sources:
Atomizer types:
ArF = Argon fluoride eximer laser
PDL = Pulsed dye laser
OPO = Optical parametric oscillator laser
Air/H = Air - hydrogfn flame
Air/Acet = Air - acetylene flame
Nitrous/Acet = Nitrous oxide - acetylene flame
* Signifies detection limit determined using 2o
Reference
64
67
62
62
62
64
67
62
62
18
By far the most commonly cited plasma atomizer for laser excitation atomic
fluorescence is the inductively coupled plasma. In this instance the inductively coupled
plasma is used solely as the atom reservoir for atomic fluorescence. Analyte excitation is
accomplished in a number of different ways in the references listed above. Pulsed dye
lasers, continuous wave dye lasers, hollow cathode lamps, xenon-arc lamps, and even line
emission from a second inductively coupled plasma have been used to provide incident
excitation radiation. Table 3 lists many o f the elements which have been characterized by
atomic fluorescence in a plasma atomizer. As in Table 2, determinations involving
hollow cathode lamps do not list an excitation wavelength since it is likely that there is
more than one excitation wavelength. Table 3 does not include results from literature
sources which were not accompanied by all experimental conditions.
1.8 Direction o f Research
Past work involving helium microwave induced plasmas have shown that they are
sensitive analytical tools. A number o f advantages are associated with helium microwave
discharges as compared to other plasma techniques. Their ability to effectively excite
nonmetal analyte species has been characterized [22-24, 27, 37-41]. They can be
operated at much lower powers (as low as 25 W) and gas flows (as low as 1 L/min) than
inductively coupled plasmas and direct current plasmas.
The use of laser excitation in atomic fluorescence spectroscopy has been well
characterized in the literature [61-67, 70-74, 77, 79-83]. Laser excitation has several
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Table 3
Atomic Fluorescence in Plasma Atomizers
Element
Excitation
X, nm
Ag
A1
394.403
309.278
As
193.7
Au
B
Ba
249.678
455.403
585.4
Fluorescence Excitation
X, nm
Source
Atomizer
Type
Detection
Limit, ppb
328.0
328.1
328.1
396.153
309.278
396.2
396.2
309.2
197.21
190
267.6
249.773
614.172
455.4
455.4
553.5
553.5
ICP
Ar-MIP
Ar-MIP
ICP
ICP
Ar-MIP
Ar-MIP
ICP
ICP
ICP
ICP
ICP
ICP
ICP
ICP
Ar-MIP
Ar-MIP
2.0*
70*
40*
0.40
8000
1000*
700*
20*
25
200*
10*
4.0
0.7
6.0
50*
40*
20*
HCL
Xe-arc
HCL
PDL
ICP
Xe-arc
HCL
HCL
ArF
HCL
HCL
PDL
PDL
CWDL
HCL
Xe-arc
HCL
Reference
81
78
78
67
75
78
78
81
66
81
81
67
67
73
81
78
78
(continued on following page)
VO
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Table 3 (continued)
Element
Be
Bi
Ca
Ca
Excitation
X, nm
393.4
393.366
Cd
Co
240.725
Cr
357.869
(continued on following page)
Fluorescence Excitation
X, nm
Source
234.7
306.8
393.4
393.366
422.7
422.7
422.7
228.8
240.7
240.725
240.7
240.7
357.9
357.869
357.9
357.9
HCL
HCL
PDL
ICP
HCL
HCL
Xe-arc
HCL
HCL
ICP
HCL
Xe-arc
HCL
ICP
HCL
Xe-arc
Atomizer
Type
Detection
Limit, ppb
ICP
ICP
ICP
ICP
ICP
Ar-MIP
Ar-MIP
ICP
ICP
ICP
Ar-MIP
Ar-MIP
ICP
ICP
Ar-MIP
Ar-MIP
0.80*
50*
1.0
2.0
0.08*
20*
20*
0.80*
5.0*
40
1000*
1000*
10*
900
2000*
2000*
Reference
81
81
71
75
81
78
78
81
81
75
78
78
81
75
78
78
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3 (continued)
Element
Excitation
X, nm
Fluorescence Excitation
X, nm
Source
Atomizer
Type
Detection
Limit, ppb
Cu
324.754
Fe
296.7
342.754
324.8
373.5
248.3
259.940
248.3
248.3
417.206
294.4
253.6
451.1
410.2
766.5
766.5
670.8
670.8
ICP
ICP
ICP
ICP
ICP
Ar-MIP
Ar-MIP
ICP
ICP
ICP
ICP
ICP
Ar-MIP
Ar-MIP
Ar-MIP
Ar-MIP
30
1.0*
50
10*
100
600*
1000*
5.0
5.0
25*
10*
300
50*
20*
20*
50*
259.940
Ga
403.298
287.424
Hg
In
410.2
K
Li
ICP
HCL
PDL
HCL
ICP
HCL
Xe-arc
PDL
PDL
HCL
HCL
PDL
Xe-arc
HCL
HCL
Xe-arc
Reference
75
81
71
81
75
78
78
67
67
81
81
71
78
78
78
78
(continued on following page)
to
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Table 3 (continued)
Element
Excitation
A, nm
Mg
Mn
257.610
Mo
287.151
386.411
Na
588.995
Ni
231.604
Fluorescence Excitation
A, nm
Source
Atomizer
Type
Detection
Limit, ppb
285.2
285.2
285.2
279.5
279.5
257.610
279.5
291.192
386.411
313.3
588.995
589.0
589.0
589.0
231.604
232.0
Ar-MIP
Ar-MIP
ICP
Ar-MIP
ICP
ICP
Ar-MIP
ICP
ICP
ICP
ICP
ICP
Ar-MIP
Ar-MIP
ICP
ICP
20*
70*
0.20*
500*
0.3*
9.0
400*
40
1500
30*
100
0.5*
10*
50*
100
10*
HCL
Xe-arc
HCL
HCL
HCL
ICP
Xe-arc
PDL
ICP
HCL
ICP
HCL
HCL
Xe-arc
ICP
HCL
Reference
78
78
81
78
81
75
78
67
75
81
75
81
78
78
75
81
(continued on following page)
to
to
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Table 3 (continued)
Element
Excitation
A, nm
Pb
283.306
Sb
Sc
Se
Si
Sm
Sn
364.3
286.160
363.429
300.914
Sr
Ti
T1
407.8
307.864
276.787
352.943
377.6
(continued on following page)
Fluorescence Excitation
A, nm
Source
Atomizer
Type
Detection
Limit, ppb
283.3
405.783
231.1
364.3
200
251.5
363.429
303.4
317.505
460.7
460.7
460.7
407.8
316.7
377.572
535.046
377.6
535.0
ICP
ICP
ICP
ICP
ICP
ICP
ICP
ICP
ICP
ICP
Ar-MIP
Ar-MIP
ICP
ICP
ICP
ICP
ICP
ICP
25*
1.0
40*
30
150*
1.0
30,000
60*
3.0
0.7*
20*
80*
0.50
1.0
7.0
4.0
7.0*
10
HCL
PDL
HCL
PDL
HCL
PDL
ICP
HCL
PDL
HCL
HCL
Xe-arc
PDL
PDL
PDL
PDL
HCL
PDL
Reference
81
67
81
71
81
67
75
81
67
81
78
78
71
67
67
67
81
71
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Table 3 (continued)
Element
Excitation
X, nm
Fluorescence Excitation
Source
X, nm
Atomizer
Type
Detection
Limit, ppb
U
V
Yb
Zn
409.014
268.796
318.398
309.311
508.742
508.7
369.419
213.856
Zr
310.658
385.958
290.882
318.398
309.311
371.030
371.0
369.419
213.856
213.8
213.9
213.9
257.
ICP
ICP
ICP
ICP
ICP
ICP
ICP
ICP
ICP
Ar-MIP
Ar-MIP
ICP
20
3.0
5.0
1000
0.60
70
30
6.0
0.50*
40*
40*
3.0
Excitation sources:
HCL = Hollow cathode lamp
Xe-arc = Xenon arc lamp
Y
PDL
PDL
PDL
ICP
PDL
PDL
ICP
ICP
HCL
HCL
Xe-arc
PDL
PDL = Pulsed dye laser
ICP = Inductively coupled plasma
ArF = Argon fluoride eximer laser
CWDL = Continuous wave dye laser
Atomizer types:
Reference
67
67
67
75
67
71
75
75
81
78
78
67
ICP = Inductively coupled plasma
Ar-MIP = Argon microwave plasma
* Signifies detection limit determined using 2o
to
25
advantages as compared to other excitation techniques such as hollow cathode lamps.
Laser emission is extremely intense and in many cases is capable of saturating an atomic
excited state. Laser emission is directional and coherent, meaning that there is little or
no need to focus the beam on the plasma. Laser emission is also monochromatic;
therefore it is unnecessary to filter unwanted nonmonochromatic radiation coming from
the excitation source.
The work described in this dissertation focuses on the coupling o f a helium
microwave induced plasma with laser excitation atomic fluorescence spectroscopy for the
determination o f metals and, ultimately, nonmetals. The helium MIP is used as an atom
reservoir instead o f an excitation source. The use o f intense laser radiation to induce
atomic fluorescence in the helium has the potential to serve as a sensitive optical method
for the determination o f metals and nonmetals in aqueous solution. The helium
microwave induced plasma laser excitation atomic fluorescence (MIP-LEAF)
spectrometer is not likely to replace the use o f graphite furnaces as metal analyte
atomizers, since graphite furnace systems have low noise and small sample volumes. It
can, however, offer an effective alternative option for metal and nonmetal analysis. Of
course, the potential o f the helium MIP for nonmetal determinations is unmatched by any
atomic spectrometric technique. Success in this area would produce a significant advance
in atomic spectrometry.
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CHAPTER 2
EXPERIMENTAL INSTRUMENTATION AND ARRANGEMENTS
This chapter discusses the general instrumentation configurations and equipment
used for data collection during all phases of this project. The specific settings for each
component vary from experiment to experiment. These settings are noted as appropriate
in later chapters.
2.1 Molecular Fluorescence Using a Commercial Spectrofluorimeter
For experiments involving solution-phase molecular fluorescence of organic
compounds, proper excitation and fluorescence wavelengths were determined. To obtain
these initial excitation and fluorescence spectra, a commercial “continuum” lamp based
spectrofluorimeter (American Instrument Company, Silver Spring, MD) was used. This
system used a xenon-arc lamp and a scanning monochromator to produce the excitation
wavelength and a scanning monochromator with a PMT to collect fluorescence signals.
Output signals from the fluorimeter were recorded using a model SE 790 X-Y plotter
(Asea Brown and Boveri, Austria). Plotter movement in the X direction was
synchronized with the wavelength o f the fluorimeter monochromators. Movement in the
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27
Y direction was dictated by the PMT signal.
The fluorimeter cells were manufactured by Stama Cells, Inc. (Atascadero, CA).
These cells were standard 1 cm pathlength cuvettes with four polished and transparent
sides. According to the manufacturer, all cuvette cells were produced identically.
Several cuvette cells were used without the need to compensate for differences from one
cell to another.
2.2 Laser Excitation Atomic and Molecular Fluorescence Arrangement
Figure 2 is a block diagram of the kilowatt-plus helium microwave induced
plasma laser excitation atomic fluorescence arrangement. The Nd:YAG power supply
operates the Nd: YAG pump laser. It also triggers the data collection components via a
synchronous pulse generated each time the laser pulse is initiated. Output from the
Nd:YAG laser enters the dye laser and frequency doubler labeled UVT-2. Here NdrYAG
laser harmonics are converted to dye laser fundamental and harmonic outputs. Output
from the dye laser and UVT-2 is directed to the plasma cavity. Fluorescence signal is
focused onto the entrance slit of a double monochromator. The PMT signal is directed to
one of a combination of data collection and manipulation devices: oscilloscope, boxcar
averager, or chart recorder.
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Figure 2.
Block Diagram o f the Helium Microwave Induced Plasma Laser
Excitation Atomic Fluorescence Instrumental Arrangement
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Trigger Pulse Synchronized with Nd:YA G Output
Oscilloscope
M icrowave Generator
Boxcar
A verager
W aveguide
Stripchart
Recorder
Com puter
and Cavity
Dye Laser
Control
^Beam-dump
Nd:YAG
UVT-2
Double ^ —
Monochromator
Ultrasonic
Nebulizer
Dye
Nd:YAG
Laser
Laser
Power
Supply
30
2.2.1 Laser Instrumentation
A Continuum (Santa Clara, CA) Model 8010 Nd:YAG laser is used as the pump
laser. It is a flashlamp pumped laser with fundamental output at 1064 nm. The pulse
width is 6 - 8 nanoseconds (ns). It is equipped with two NdrYAG rods: one in the
oscillator and one in the amplifier. It utilizes three flashlamps: one in the oscillator and
two in the amplifier. Manufacturer specifications list an average output power of 14.5
watts at the 1064 nm fundamental wavelength. At 10 Hz, this results in .1450 millijoules
(mJ) of energy per pulse. Harmonics are generated by passing the NdrYAG output
through one or more frequency doubling crystals. One crystal is used to generate output
at 532 nm and a second crystal is used to generate output at 355 nm. Manufacturer rated
energy outputs at 532 and 355 nm are 720 and 410 mJ, respectively.
The NdrYAG harmonics output is used to pump a Continuum model ND6000 dye
laser. For most of the work discussed, the 532 nm NdrYAG output was used. Dye laser
power varies according to the dye used. Pumping at 532 nm, the ND6000 gives a
maximum power o f 200 mJ at 560 nm with rhodamine 590 (also known as rhodamine
6G) laser dye. Each laser dye has an associated maximum energy output wavelength
known as the center wavelength. The center wavelength is different for all laser dyes.
Tunability varies from dye to dye, but it is normally possible to tune over a range o f
approximately +/-15 nm from the center wavelength.
In general, the dye laser output is in the visible region o f the electromagnetic
spectrum when using common rhodamine dyes. In order to gain access to the ultraviolet
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31
region, where many o f the spectroscopically useful atomic transitions occur, harmonics of
the dye laser fundamental must be generated. Harmonic generation is accomplished in a
fashion similar to that o f the NdrYAG pump laser. A Continuum model UVT-2 harmonic
generator is used to convert the dye laser fundamental output to shorter wavelengths. To
double the frequency (halve the wavelength) the dye laser fundamental is sent through a
KDP crystal. This process is inefficient, resulting in the generation o f heat within the
crystal. As the crystal temperature rises, the frequency doubling characteristics change.
This variation must be compensated by rotating the crystal to change the entrance angle
of the laser beam slightly. To provide a monitoring/feedback system to regulate the laser
power (a function o f output angle), a small portion of the resulting frequency doubled dye
laser beam is focused onto light-sensitive diodes. As the energy o f the frequency doubled
output drops, the diodes send a signal to a microprocessor which adjusts the orientation of
the crystal to maintain maximum output signal. The ND 6000 dye laser output maximum
(200 mJ at 560 nm) when doubled results in a maximum rated energy o f 50 mJ at 280
nm.
All laboratory laser power energy values are determined using a volume-absorbing
laser power probe. The probe is a Molectron (Molectron Detector, Inc., Portland, OR)
model PM30V1 and is connected to a Molectron model 500A analog power meter. This
probe is used to measure the energy o f all laser outputs except the NdrYAG fundamental.
The high energy 1064 nm harmonic wavelength is damaging to the active material
of the laser power probe. The NdrYAG energy is determined by measuring the energy of
the second or third harmonic at 532 nm or 355 nm, respectively. In addition, these
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32
wavelengths are the most relevant and useful when pumping a dye laser.
2.2.2 Laser Fluorescence Cells
For these experiments, two distinctly different fluorescence cells were used.
Solution-phase laser excitation molecular fluorescence o f organic compounds was
performed using a conventional quartz cuvette fluorimeter cell. Laser excitation atomic
fluorescence was performed using a helium microwave induced plasma as the atom
reservoir.
2.2.2.1 Laser Excited Molecular Fluorescence
A simple fluorimeter sample housing o f approximately 12 by 18 inches was
assembled for molecular laser excited fluorescence experiments. This assembly was
located in place o f the helium MIP system in Figure 2. Thin-gauge sheet metal was used
to create the front and back of the cavity. The sheet metal was painted black to reduce
scattered light. One side and the top of the cavity were covered with a double layer of
heavy black felt material purchased locally. This material allowed easy access to the
cavity while effectively blocking room lighting. The other side of the box was attached to
the Kratos double monochromator.
A fused silica lens (51 mm focal length) was used to expand the laser beam
diameter. The lens was placed in the laser beam path and canted at approximately a 20°
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33
angle. This angle was necessary to avoid the focused laser beam creating laser induced
breakdown plasma sparks in the air at the focal point. At distances greater than the focal
length o f the lens, the beam was larger in diameter and expanded continuously. Without
beam expansion, as the laser passed through the cuvette, a solid deposit would quickly
form on the inside cuvette surface. This process occurred only when sample-containing
solutions were in the cuvette. Blank cuvettes (solvent only) did not spot. It is thought
that the full-power laser was causing significant heating of the solution at the cuvette
interface. This heating was causing a small amount of solvent to evaporate, leaving the
solute behind. Alternately, it is possible that the laser energy induced decomposition of
the solute molecules into insoluble precipitates.
The expanding beam was directed into the fluorimeter cavity through a 0.75 inch
hole drilled through one of the pieces of sheet metal. The liquid sample was held in a
fluorimeter cuvette in line with the laser beam. Fluorescence was monitored at a 90°
angle to the laser beam path and focused on the entrance slit of the double
monochromator. The fluorescence signal was recorded using a Soltec (San Fernando,
CA) model 1241 stripchart recorder.
2.2.2.2 Helium Microwave Induced Plasma Assembly
The helium microwave induced plasma power transfer system is similar to that
described previously by Alvarado and Carnahan [27]. The microwave generator operates
at a frequency of 2.45 GHz and at powers up to 3.0 kW (Cober Electronics, Stamford,
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34
CT). The generator was manufactured as a pulsed output instrument and modified for
continuous output [84]. An in-line slotted waveguide with worm-driven baffles (Cober
Electronics, Stamford, CT) was used for forward and reflected power tuning o f the
plasma system. Daily tuning adjustments were not necessary with this plasma system.
Rectangular waveguides were used to transfer microwave power from the generator and
slotted tuner to the microwave cavity (Gerling Laboratory, Modesto, CA). Waveguide to
1.625 inch coaxial coupler and 1.625 to 0.875 inch coaxial reducers (Andrews
Corporation, Orland Park, EL) are used to connect the microwave cavity to the
waveguides. External microwave power meters (Gerling Laboratory, Modesto, CA) were
used in conjunction with power meters on the microwave generator to measure forward
and reflected power. All helium supplies were purchased from BOC Gases (Carol
Stream, EL).
The helium plasma is formed in a TMq10 resonator cavity o f a modified Beenakker
design [22,25-26, 85-91]. The resonator cavity has an internal diameter of 88.8 mm and
a depth of 2 cm. A 13 mm hole was drilled axially through the center o f the cavity to
insert a demountable fused silica torch. The torch consists of a 10 mm o.d., 8 mm i.d.
quartz tube with a four threads per inch, doubly threaded teflon insert approximately 30
mm in length and 8 mm o.d.. Sample mist is introduced through a 3 mm o.d., 2 mm i.d.
ceramic tube inserted through the center of the threaded insert. The helium plasma is
routinely operated at 2 kW applied power with helium plasma gas flow rates between 12
and 30 L/min.
Sample introduction into the helium microwave induced plasma was
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35
accomplished using a Cetac Technologies, Inc. (Omaha, NE) model U5000AT ultrasonic
nebulizer. Adjustable settings for the heated U-tube and condenser coolant temperatures
were kept constant at the manufacturer’s suggested settings o f 145 and 4°C, respectively.
The peristaltic pump-driven (Rainin, Woburn, MA) sample solution uptake rate was 2.1
mL/min for all experiments. The sample transport helium flow rate was optimized
experimentally for different elements and will be discussed later.
2.2.3 Double Monochromator and Detection System
The wavelength selection device is a double monochromator assembly. Two
model GM-252 0.25 meter monochromators (Kratos Analytical, Chestnut Ridge, NY)
were positioned and secured on a 10 mm thick aluminum platform. The platform rests on
three one-inch diameter legs, each adjustable between four and eight inches. With this
assembly, the monochromator can be easily raised and lowered to the plasma cavity level.
In this double monochromator system, the output from the first monochromator is fed
directly into the entrance slit o f the second monochromator. This monochromator system
has three slits: entrance, connecting, and exit. All three of the slit settings are width and
height adjustable.
The detector used in all experiments was a photomultiplier tube (PMT). The
photomultiplier tubes were a Hamamatsu (Bridgewater, NJ) model R212 UH for
wavelengths below 600 nm, and a model R955 for wavelengths above 600 nm. The
photomultiplier tube housing attached to the exit slit o f the double monochromator was
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36
machined at Northern Illinois University to fit the monochromator. The input PMT
voltage is provided with an adjustable high voltage photomultiplier tube power supply
assembled at Northern Illinois University.
2.2.4 Data Collection
With these instrumental arrangements, several different devices were used to
collect data. All atomic emission data was collected using a current-to-voltage converter
and stripchart recorder. For molecular fluorescence and nonmetal atomic fluorescence
experiments, data was collected using a 175 MHZ model 9400A LeCroy (Chestnut Ridge,
NY) digital oscilloscope, a Stanford Research Systems (Sunnyvale, CA) model SR 250
boxcar averager, and the Soltec stripchart recorder. For the transition metal atomic
fluorescence experiments, a 500 MHZ model TDS 620B oscilloscope (Tektronix,
Wilsonville, OR) was used to collect and store digital data without the use o f a boxcar
averager or stripchart recorder.
2.2.4.1 Collection of Atomic Emission Data
To investigate atomic emission spectra, a current-to-voltage converter was used to
send the PMT signal to a stripchart recorder. The current-to-voltage converter was
assembled at Northern Illinois University. It has selectable settings for gain (0.1 -100),
d.c. offset, and noise filtration (0.01 - 1 second time constant). The Soltec stripchart
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37
recorder was used for data collection.
2.2.4.2 Collection o f Solution-Phase Laser Molecular Fluorescence Data
Initial system characterization experiments utilized the laser excited molecular
fluorescence cell described earlier. Data was collected with the use o f an oscilloscope,
boxcar averager and stripchart recorder. In this arrangement, the PMT signal was split
and directed to both the boxcar averager and oscilloscope. The boxcar averager was used
to collect integrated data and send it to the stripchart recorder. The oscilloscope was used
to monitor instantaneous fluorescence signals. The oscilloscope was triggered by the
same synchronized laser pulse which triggered the boxcar averager.
The boxcar averager has adjustability for triggering, delay, width, d.c. offset,
scale, and number averaging. Triggering allows the boxcar averager to start collecting
data. Triggering is accomplished using the laser power supply. The delay is the amount
of time after the trigger pulse that the boxcar averager “waits” before collecting data. The
delay time is critical because it allows time for the laser pulse to travel to its destination
and for the electronics to process the signal. These times are small (on the order o f 80
ns), but not negligible. Width settings fix the amount o f time that the boxcar averager
records data during each triggered cycle. For the 6 - 8 ns laser pulses, width values were
generally set near 6 ns. DC offset and scale adjustments allow the averaged signal output
to be monitored. Settings varied somewhat from day to day and experiment to
experiment. The number averaged setting fixes the number of consecutive signals which
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38
are averaged and sent out through the averaged signal port to the stripchart recorder.
2.2.4.3 Collection o f Laser Excitation Atomic Fluorescence Data
The use o f the Tektronix oscilloscope, obtained in the course o f this work,
simplified data collection for laser excitation atomic fluorescence experiments. The PMT
signal was sent directly to the oscilloscope for all signal averaging and data collection. A
3.5 inch floppy disk drive built in to the oscilloscope allowed collected fluorescence
spectra to be stored as digital files. These files were then uploaded to a spreadsheet
program for data analysis and manipulation.
Adjustable settings for d.c. offset and scale were trivial and not recorded. Settings
for number o f consecutive fluorescence signals averaged were optimized and noted where
appropriate.
2.3 Laser Pathways for Laser Excitation Atomic Fluorescence
in the Helium Microwave Induced Plasma
During the laser excitation atomic fluorescence experiments, there were three
different optical orientations used to interact the laser beam with the microwave induced
plasma. These three orientations were necessary to observe laser excitation atomic
fluorescence in different regions o f the helium plasma. Because population distributions
o f atomic species vary greatly according to plasma location, it was necessary to direct the
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39
laser through the plasma plume across the outside edge o f the cavity for some
experiments. For other elements, it was necessary for the laser beam to be passed into the
plasma within the cavity.
The simplest arrangement for laser excitation atomic fluorescence is shown in
Figure 3. The microwave resonator cavity is oriented “vertically” (the torch is positioned
vertically). The laser beam is steered across the surface o f the cavity using turning
prisms. The laser beam travels through the protruding plume of the microwave plasma
approximately 2 mm above the surface o f the microwave cavity and plasma torch. In this
region of the plasma, many transition metals are sufficiently atomized so that laser
excitation atomic fluorescence can occur. Fluorescence is monitored at an angle o f
approximately 30° relative to the laser beam path. Once the laser beam has traversed the
cavity face, it is absorbed by a beam-dump. This orientation was used for all transition
metal laser excitation atomic fluorescence experiments.
To investigate laser excitation atomic fluorescence o f nonmetallic elements, a
second approach to the laser beam-plasma interface was investigated. Because o f the
high excitation energies required for the nonmetallic atomic and ionic species
investigated, it was necessary to pass the laser beam into the plasma within the
microwave cavity. Prior experiments indicate that the ionic nonmetal species to be
monitored do not exist in sufficient populations outside the microwave cavity. Simply
passing the laser through the plume of the plasma is not acceptable.
Two methods o f examining fluorescence within the cavity are illustrated in Figure
4. The first method was to direct the laser slightly off-axis through the plasma torch
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Figure 3.
Diagram of the Laser Beam-Microwave Resonator Cavity Interface Used
for Laser Excitation Atomic Fluorescence o f Metals
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41
Fluorescence to IVfonochromator
Cavity
Plasma Torch and
Threaded Insert
Laser Beam to Cavity
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Figure 4.
Diagrams o f the Laser Beam-Microwave Resonator Cavity Interface Used
to Examine Laser Excitation Atomic Fluorescence of Nonmetals
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43
C a v ity
F lu o r e s c e n c e to M o n o c h r o m a to r
P la s m a T o rc h and
T h re a d e d In se rt
L aser B eam
to C a v ity
1
I
C a vity
F l u o r e s c e n c e to M o n o c h r o m a t o r
P la s m a T o rc h a n d
T h re a d e d In se rt
L aser B eam
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T o C a v ity
44
toward the threaded teflon insert. This method resulted in the laser interacting with the
largest possible plasma volume. Atomic fluorescence was monitored at approximately
10° relative to the laser beam path. This method resulted in a significant amount of
scattered laser radiation. In addition, the large amounts (-40 mJ, 6 ns pulse) o f ultraviolet
energy produced by the laser caused the quartz to produce intense broad-band
fluorescence in the 340 - 450 nm region. The high backgrounds produced by these two
problems make sensitive analysis difficult with this configuration.
The second method to pass the beam into the resonator cavity was to direct the
laser beam through the cavity perpendicular to the plasma orientation. This arrangement
is illustrated in the lower portion of Figure 4. This trans-cavity laser path was made
possible by drilling two holes in the sides of the resonator cavity. The holes are 5.0 mm
in diameter and oriented 180° apart at the midpoints o f the sides of the cavity. This
orientation allows the laser beam to interact with the plasma inside the cavity.
To bypass the problem of interfering fluorescence from the quartz torch, specially
designed plasma torches were developed. Figure 5 illustrates the design of these torches.
Standard torches were modified to allow the laser to pass through without contacting
quartz by creating two 5.0 mm holes 180° apart approximately 1.5 cm from the end o f the
torch. These holes in the torch were aligned with the laser ports in the cavity walls. The
resulting system allows the laser beam to pass through the cavity with a minimum of
scattered radiation and quartz fluorescence.
However, the holed torches allowed the resonator cavity to fill with helium, which
caused microwave arcing within the cavity. To alleviate this problem, a second set o f 5.0
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Figure 5.
Diagram o f the Holed Torch Design Used for Nonmetal Laser Excitation
Atomic Fluorescence Experiments
Torch is shown from side and top view.
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46
Side View
i
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mm holes were drilled into the sides of the microwave cavity. Compressed air was
introduced through the second set o f holes to flush the helium out o f the cavity. An
flow of 0.5 TVmin allowed the microwave induced plasma to operate normally. No
additional tuning was required to operate the plasma system in this manner.
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CHAPTER 3
LASER EXCITATION ATOMIC FLUORESCENCE OF MANGANESE,
LEAD, THALLIUM, AND IRON IN THE HELIUM MIP
3.1 Introduction
This chapter discusses the determination of aqueous manganese, lead, thallium,
and iron by laser excitation atomic fluorescence spectroscopy. The method o f data
collection and manipulation will be discussed. Specific details and results o f laser
excitation atomic fluorescence experiments on Mn, Pb, Tl, and Fe are detailed separately.
Optimization o f plasma parameters, the linearity o f the fluorescence signals, detection
limits, and matrix effects are outlined for each element.
3.2 Experimental
The instrumental arrangement described in Chapter 2 (see Figure 2) for laser
excitation atomic fluorescence of metals in the helium MIP was used for all four metals.
For these experiments the plasma cavity was oriented vertically (see Figure 3). All
aqueous metal solutions were prepared by dilution from a weight/volume stock solution
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prepared from high purity (>99.9%) metal salts: M nS04, Pb(N 03)2, and T12C 0 3 dissolved
in 1% (v/v) nitric acid. Iron solutions were made by dissolving Fe(NH4)2(S 04)2 in 0.1%
(w/v) hydroxylamine hydrochloride to prohibit the Fe2+ from oxidizing to the less soluble
Fe3+. Fluorescence signal collected by the PMT was directed to the Tektronix
oscilloscope for averaging and storage.
3.3 Data Collection and Manipulation
The Tektronix oscilloscope used for data collection was a significant
improvement compared to the boxcar averager-stripchart recorder combination. Its
integrated 3.5 inch diskette drive provided a convenient method o f digital data storage.
This approach removed the human error in reading peak heights from stripchart paper.
The oscilloscope was also preprogramed to perform boxcar signal averaging functions.
The result of using the Tektronix oscilloscope for data collection was an increase in the
speed of data manipulation and the accuracy in the calculation o f the analytical signal.
The oscilloscope was used to save averaged data as spreadsheet files. These
spreadsheet files were uploaded into a spreadsheet program (QuattroPro) on a personal
computer. Each collected PMT signal versus time data collection was saved as a separate
data file. Data files consisted o f 500 data points, each with a time and a PMT signal
value. Once the data files were uploaded to the spreadsheet program, unneeded data
points were discarded to save diskette space. The only points retained for further data
analysis were the points which determined the baseline o f the fluorescence signal and the
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50
points which determined the height of the fluorescence peak. No permanent changes
were made to the original data files, which were stored for future reference.
One important consideration in the analysis o f the analytical fluorescence data is
that the fluorescence signal is collected in the presence o f the normal atomic emission
signal; atomic emission was occurring in the helium MIP at a steady rate and the laser
modulated atomic fluorescence signal was observed as a temporal luminescence increase.
As the analyte concentration increased, the emission signal increased and shifted the
baseline o f the fluorescence signal. This problem needed to be taken into account when
calculating the intensity o f the analytical fluorescence signal.
A companion blank data file was collected immediately before each sample data
file. The blank solution was prepared from distilled deionized water with no analyte
present. The analyte and blank oscilloscope data were manipulated simultaneously when
uploaded to the spreadsheet program. Ten data points preceding the fluorescence signal
(points 240-249 o f all data files, corresponding to the time period o f 42 to 6 ps before the
laser pulse) were averaged and used to establish the baseline for both data files. The
difference between the baselines o f the blank and analyte PMT signal versus time plots
was due to the atomic emission signal from that concentration o f analyte. These
differences were accounted for when the peak height for the fluorescence signal was
determined. Ten data points across the peak of the fluorescence signal (points 253-262 of
all data files, corresponding to 10 to 46 ps after the laser pulse) were averaged to obtain
luminescence information during the laser pulse for both the analyte and the blank. The
following equation was used to determine the analytical fluorescence signal for a set of
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51
companion analyte and blank data files:
O
fluorescence
_ /C
' peak
_ O
\ _ /D
baseline'
' peak
_ f>
baseline'
where Snuorescence is the analytical atomic fluorescence signal,
\
13-11
the peak height o f the
analyte fluorescence peak, Sbaseline is the signal height o f the blank fluorescence peak,
is the baseline level of the analyte spectra, and
is the baseline level o f the blank
spectra.
Figure 6 is a set o f 100 data point files (10 ps/point) including 403.3 nm PMT
signals collected during two acquisition cycles and a background corrected file. This
example illustrates the need to perform the correction for the atomic emission signal. The
middle trace is the detector signal during the laser firing with distilled water (blank)
nebulized into the plasma. The lower trace is the signal obtained with 1.0 ppm Mn.
Clearly, there is a significant shift in the overall baseline of the 1.0 ppm manganese trace
compared to the distilled water trace. The additional baseline shift which appears during
the laser pulse is due to the data collection electronics and scattered laser radiation
reaching the detector. The upper trace (dark line) in Figure 6 is the signal due to
manganese fluorescence with the blank spectra subtracted from it. If not accounted for,
the baseline shift due to the emission signal results in a systematic error which increases
in magnitude with analyte concentration.
For subsequent experiments, the same method of data manipulation was used.
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Figure 6.
Comparison o f Collected Atomic Fluorescence Spectra of 1.0 ppm Mn and
Distilled Water Blank
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53
PMT Signal, Volts
Difference
-
0.04
-
0.08
-
-
Distilled Water
1.0 ppm Mn
0.12
0.16
- 0 . 2 -I------
0.0002
0
0.0002
0.0004
0.0006
0.0008
Tim e, sec
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0.001
54
Triplicate analyte atomic fluorescence data files were collected along with the associated
blank files for each concentration o f analyte. The analytical fluorescence signal was
determined for each sample. The triplicate values were averaged and plotted as single
calibration points.
3.4 Evaluation of Plasma Parameters and Analytical Figures of Merit
For three o f the four elements characterized and discussed in this chapter, flow
rates for helium plasma gas and ultrasonic nebulizer carrier gas were optimized
experimentally. After similar results were observed for the first three elements, it was
assumed that the optimum flow rates would be similar for subsequent elemental
determinations. After plasma parameters were evaluated, experiments were performed to
characterize the analytical figures of merit, including limits of detection and linearities o f
fluorescence signals. The limits o f detection will be discussed first because this
parameter was optimized to evaluate the proper plasma and ultrasonic nebulizer gas flow
rates.
3.4.1 Limit of Detection Calculation
For the evaluation of limits of detection, a similar method of data collection as
described previously was used, but with a somewhat different manipulation. A minimum
o f six sample data files were collected, alternating blank and analyte. The analyte
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55
concentrations ( C ) were chosen to be less than 50 times the expected detection limit to
insure that the analytical fluorescence signal observed was in the linear portion o f the
calibration plot. The experimental noise was determined by calculating the standard
deviation o f the ten points used to establish the baseline. The standard deviation of
baseline points was determined to n-1 degrees o f freedom. The value o f the standard
deviation is considered to be the noise (N) o f the system. The analytical fluorescence
signal was determined using the method described in equation 3-1. The limit of detection
(LOD) was determined to be the analyte concentration which produces analytical signal
equal to a signal to noise ratio of three by the following equation.
LU D ~
_
LOD
3 NC
(3 2 )
fluorescence
3.4.2 Optimization o f Helium Flow Rates
The flow rates used for helium microwave induced plasmas can have significant
effects on the magnitudes of the analytical signals and the baseline noise. The plasma gas
flow rates were the first plasma parameter to be optimized, followed by ultrasonic
nebulizer helium flow rates. Flow rate optimization was accomplished by varying the
respective flow rate and monitoring the analyte LOD.
For these studies, analyte concentrations were chosen which gave substantial
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56
analytical signal, not necessarily concentrations within 50 times the expected detection
limit as discussed in section 3.3. This was done so that reliable detection limit
calculations could be performed even if a significant decrease in analytical signal was
observed by changing the plasma conditions.
Plasma gas flow rates were varied between 15 and 18.6 L/min. In separate
experiments, ultrasonic nebulizer carrier gas flow rates were varied between 0.9 and 2.1
L/min. After each change in helium gas flow, no data were collected for a period o f five
minutes to allow the plasma to re-equilibrate at the new flow rate. Data were plotted as
the analyte LOD versus gas flow rate. Optimal conditions resulted in the minimum limit
o f detection.
3.4.3 Linearity of Analytical Fluorescence Signals
Evaluation o f the linearity of the analytical fluorescence signal for each element
was determined after the limit of detection was established. A number of solutions
ranging in concentration from just above the optimum limit o f detection to approximately
10,000 times the limit o f detection limit were used. Data were collected and manipulated
as described in section 3.3 and plotted as analytical fluorescence signal versus analyte
concentration.
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57
3.5 Matrix Interferences on the Analytical Fluorescence Signal
The final experiments performed on each element were to determine the effects of
matrix components on the intensities o f the atomic fluorescence signals. The application
of this project to real-world samples would undoubtedly result in samples which are not
simple aqueous solutions. The effects o f four different, commonly-occurring matrix
components were analyzed individually: calcium, sodium, sulfate, and phosphate. These
matrix components are likely concomitants o f real-world samples. Prior knowledge o f
possible interferences is an advantage when analyzing actual field samples.
For each matrix component o f interest, ten solution-containing flasks were
prepared. In five o f these flasks, a known and constant amount of the analyte metal was
pipetted. The other five flasks were prepared without analyte metal. One analytecontaining flask at the proper matrix interferant concentration, and one analyte blank
flask at the same interferant concentration were used for each concentration of matrix
interferant investigated. The concentrations o f the matrix component were 0, 1, 10,100,
and 1000 ppm. Calcium and sodium stock solutions were prepared from nitrate salts.
Sulfate and phosphate stock solutions were prepared from sulfuric and phosphoric acids,
respectively.
Data were collected and manipulated as described in section 3.3. The solutions
which contained interferant were assigned a analytical fluorescence signal of 1.0. All the
subsequent data for each experiment were normalized to the value o f the fluorescence
signal with no interferant. In this way, a direct comparison o f the magnitude o f signal
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58
change caused by the matrix component could be easily observed. Data were plotted as
relative fluorescence signal versus matrix interferant concentration.
3.6 Laser Excitation Atomic Fluorescence o f Manganese
Laser excitation atomic fluorescence o f manganese has been reported previously
using a graphite cup atomizer [56] and in an air/acetylene flame [64], With the graphite
cup atomizer, the laser excitation wavelength was 279.482 nm and the atomic
fluorescence wavelength was 279.482. For manganese LEAFS in the air/acetylene flame,
the laser excitation wavelength was 279.482 nm with atomic fluorescence at 403.307 nm.
In addition to these LEAFS determinations o f manganese, atomic fluorescence of
manganese has also been reported using hollow cathode lamp [78, 81], xenon arc lamp
[78], and tandem ICP emission excitation [75].
Atomic fluorescence measurements involving excitation and fluorescence
wavelengths which were identical were not an option for the helium microwave induced
plasma system. For the instrumental arrangement described in Figure 2, an excitation
wavelength identical to the monochromator wavelength resulted in a large amount o f
scattered laser excitation radiation observed at the PMT. This resulted in a very high
background signal with a great deal of associated noise.
For the manganese experiments discussed here, the wavelength of laser excitation
was set at 279.482 nm and the wavelength o f monitored fluorescence was set at 403.307
nm. Figure 7 is a partial energy diagram of manganese illustrating the excitation and
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Figure 7.
Partial Energy Level Diagram for Manganese Showing the Electronic
Transitions Used for Manganese LEAFS in the Helium MIP
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60
Energy (eV) Above the Ground State Atom
3 d 54 s 4 p , 6P °7/,
2 7 9 .4 8 2 / n m
fig
1---------------------------
6p0
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61
fluorescence transitions. Transition probabilities for the excitation and fluorescence
electronic transitions have been tabulated previously [92]. The transition probability for
the excitation wavelength at 279.482 nm (0-35570 cm'1) is 8.3 X 10® /second. The
fluorescence transition at 403.307 nm (24788-0 cm'1) has a transition probability o f 0.95
X 10® /second.
The optimizations o f helium flow rates are shown in Figures 8 and 9. A
manganese concentration of 0.5 ppm was used for this study. Figure 8 is a plot o f helium
plasma gas flow rate versus the manganese limit of detection. As the figure indicates, the
lowest detection limit was observed with a plasma gas flow rate o f 17.5 L/min. The
optimization o f the ultrasonic nebulizer carrier gas flow rate with a plasma gas flow rate
o f 17.5 L/min is illustrated in Figure 9. This figure indicates that the optimum ultrasonic
nebulizer carrier gas flow rate is approximately 1.5 L/min, but little change occurs in the
range of 1.5 to 2.4 L/min.
Table 4 lists the optimized parameters for the manganese experiments. Other
settings not tabulated such as oscilloscope scaling were trivial and not recorded since they
changed from solution to solution during each experiment. The laser energy is listed as
an energy range because the day-to-day output energy o f the frequency doubled dye laser
fluctuated. However, when the laser had fired a sufficient number of times to warm up
all the dye laser frequency doubling optics, the laser power output was constant
throughout any given day.
The LOD and fluorescence signal linearity determinations were performed after
optimization o f plasma parameters. The limit of detection for Mn was 6 ppb. Figure 10
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 8.
Helium Plasma Gas Flow Rate Study for Manganese Laser Excitation
Atomic Fluorescence in the Helium MIP
The manganese limit o f detection is plotted versus the helium flow rate.
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63
Manganese Limit of Detection, ppb
14
13
12
11
10
9
15
17
18
16
Helium Plasma Gas Flow Rate, L/min
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
19
Figure 9.
Ultrasonic Nebulizer Helium Carrier Gas Flow Rate Study for Manganese
Laser Excitation Atomic Fluorescence in the Helium MIP
The manganese limit of detection is plotted versus the helium flow rate.
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65
Manganese Limit of Detection, ppb
50
40
30
20
10
0
0.8
1
1.2
1.4
1.6
1.8
2
Ultrasonic Nebulizer Helium Flow rate, L/min
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.2
2.4
66
Table 4
Optimized Instrumental Parameters for Mn LEAFS
Instrumental Component
Fixed Setting
Microwave Induced Plasma
Forward Power:
Helium Plasma Gas Flow rate:
2 Kilowatts
17.5 L/min
Ultrasonic Nebulizer
Helium Carrier Gas Flow rate:
U-Tube Temperature:
Liquid Coolant Temperature:
Sample Uptake Rate:
1.53 L/min
145 °C
4 °C
2.1 mL/min
Dve Laser
Laser Dye:
Nd:YAG Pump Wavelength:
Final Output Beam Wavelength:
Output Beam Energy:
Rhodamine 590
532 nm
279.482 nm
24 - 33 mJ/pulse
Monochromator
Wavelength:
Slit Dimensions
Entrance:
Connecting:
Exit:
PMT Voltage:
Oscilloscope
Number o f Samples Averaged:
403.3 nm
20 pm
50 pm
15 pm
-1000 Volts
50
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 10.
Linearity o f Manganese Laser Excitation Atomic Fluorescence in the
Helium MIP
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Manganese Atomic Fluorescence Signal, Volts
68
2 -
1r
,A '
kr'
q f t ___________ I_____________ I_____________ I_____________ !_____________ I_____________ i
0
5
10
15
20
Manganese Concentration, ppm
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
25
30
69
is a calibration plot for manganese laser excitation atomic fluorescence. It is observed to
be linear from just above the limit o f detection to 20 ppm Mn. The correlation coefficient
(r2) for this plot is 0.9997 with a slope o f 0.118 volts/ppm and a y-intercept o f 0.0085
volts.
Interference effects o f matrix components are shown in Figure 11. The presence
o f sodium and sulfate generally resulted in an increase in fluorescence signal. Caicium
and phosphate caused the fluorescence signal to decline. For these and subsequent matrix
interference studies, the mechanisms involved with the interferences were not
investigated. These studies were performed only to obtain an empirical awareness of the
interferences. The variability and severity o f these interferences indicate that the method
of standard additions would be useful when performing real-world analyses.
3.7. Laser Excitation Atomic Fluorescence o f Lead
Laser excitation atomic fluorescence of lead has been extensively investigated in
graphite cup atomizers [56, 58], graphite tube atomizers [51, 53, 58], air/acetylene flames
[64,67], and inductively coupled plasmas [67]. Atomic fluorescence in the ICP using
hollow cathode excitation has also been reported [81]. In all of the laser excitation
atomic fluorescence experiments, the same laser excitation and fluorescence lines were
used. Lead atoms were excited at the 283.305 nm resonance line and atomic fluorescence
was observed at 405.8 nm.
For laser excitation atomic fluorescence in the helium MIP, the same lines used in
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 11.
Interference of the Matrix Components Ca, Na, S 0 42\ and P 0 43' on the
Helium Microwave Induced Plasma Laser Excitation Atomic
Fluorescence of Manganese
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71
* — Calcium
~ *— Sodium
— Sulfate
+
Phosphate
Relative Manganese Fluorescence Signal
3
2
1
0
100
Interferant Concentration, ppm
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1000
Figure 12.
Partial Energy Level Diagram for Lead Showing the Electronic Transitions
Used for Lead LEAFS in the Helium MIP
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73
Energy (eV) Above the Ground State Atom
6 s ’6 p 7 s , ’P°,
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74
the previous studies were used. Figure 12 is a partial energy diagram o f lead which
shows the transitions for excitation and fluorescence. Transition probabilities for these
transitions have been previously tabulated [92]. The excitation transition at 283.305 nm
(0-35287 cm'1) has a transition probability o f 1.8 X 108 /second. The fluorescence
transition at 405.8 nm (35287-10650 cm'1) has a transition probability of 9.2 X 108
/second.
Optimization and characterization o f laser excitation atomic fluorescence
experiments of lead were performed in the same manner and sequence as the manganese
experiments. Initial characterization of helium flows was followed by the limit of
detection determination, linearity studies, and finally an investigation of the matrix effects
on the Pb LEAF signal.
Figures 13 and 14 show the optimizations o f helium flows for lead. Pb solutions
at a concentration o f 0.1 ppm were used to optimize the flow rates in these studies.
Figure 13 indicates that the optimum plasma gas flow rate is 18.6 L/min. Figure 14
indicates that the best lead signal is collected with the ultrasonic nebulizer helium flow at
1.75 L/min. These settings were used throughout the remainder o f the lead laser
excitation atomic fluorescence experiments. Table 5 lists the fixed instrumental settings
used for lead laser excitation atomic fluorescence in the helium MIP.
The limit o f detection for lead LEAFS in the helium MIP was 0.7 ppb. Excellent
linearity was observed for lead atomic fluorescence from just above the detection limit
through 2 ppm Pb. Figure 15 is a calibration plot for Pb LEAFS. The correlation
coefficient (r2) for this plot is 0.998 with a slope o f 2.18 volts/ppm and a y-intercept of
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 13.
Helium Plasma Gas Flow Rate Study for Lead Laser Excitation Atomic
Fluorescence in the Helium Microwave Induced Plasma
The lead limit o f detection is plotted versus the plasma gas flow rate.
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76
Lead Limit of Detection, ppb
6
5
4
3
15
16
17
18
Helium Plasma Gas Flow Rate, L/min
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19
Figure 14.
Ultrasonic Nebulizer Helium Carrier Gas Flow Rate Study for Lead Laser
Excitation Atomic Fluorescence in the Helium MIP
The lead limit o f detection is plotted versus the USN gas flow rate.
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78
Lead Limit of Detection, ppb
40
30
20
10
0
0.8
1
1.2
1.4
1.6
1.8
Ultrasonic Nebulizer Helium Flow Rate, L/min
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2
2.2
79
Table 5
Optimized Instrumental Parameters for Pb LEAFS
Instrumental Component
Fixed Setting
Microwave Induced Plasma
Forward Power:
Helium Plasma Gas Flow rate:
2 Kilowatts
18.6 L/min
Ultrasonic Nebulizer
Helium Carrier Gas Flow rate:
U-Tube Temperature:
Liquid Coolant Temperature:
Sample Uptake Rate:
1.75 L/min
145 °C
4 °C
2.1 mL/min
Dve Laser
Laser Dye:
Nd:YAG Pump Wavelength:
Final Output Beam Wavelength:
Output Beam Energy:
Rhodamine 590
532 nm
283.305 nm
2 1 - 2 6 mJ/pulse
Monochromator
Slit Dimensions
Entrance:
Connecting:
Exit:
PMT Voltage:
405.8 nm
20 pm
50 pm
15 pm
-1000 Volts
Oscilloscope
Number o f Samples Averaged:
50
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Figure 15.
Linearity o f Lead Laser Excitation Atomic Fluorescence in the
Helium MIP
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81
Lead Atomic Fluorescence Signal, Volts
5
2-
Q
kA
b ,------------------------------------------------------------------------------ I-------------------------------------------------------------------------- L _
0
I
2
Lead Concentration, ppm
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 16.
Interference o f the Matrix Components Ca, Na, S 0 42', and P 0 43' on the
Helium Microwave Induced Plasma Laser Excitation Atomic
Fluorescence o f Lead
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83
■—-
C a lc iu m
— *—
S o d iu m
— • —
S u lf a te
— +—
P h o s p h a te
Relative Lead Fluorescence Signal
3
2
----- A
l
o
10
100
Interferant Concentration, ppm
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1000
84
0.076 volts.
Matrix effects for lead are illustrated in Figure 16. The presence o f sodium had
very little effect on the atomic fluorescence signal. Phosphate and sulfate were observed
to increase Pb fluorescence signals. Calcium suppressed Pb atomic fluorescence.
3.8. Laser Excitation Atomic Fluorescence o f Thallium
Thallium laser excitation atomic fluorescence has been previously observed in a
graphite tube atomizer [49], air/acetylene flames [64, 67], and the ICP [67, 71]. Thallium
atomic fluorescence has also been reported in the ICP using hollow cathode lamp
excitation [81]. For the graphite tube, air/acetylene flames, and one instance [67] in the
ICP, thallium atoms were excited at the 276.787 nm resonance line. Fluorescence
observation wavelengths differed in these experiments. Previously, atomic fluorescence
was observed at 351.92 nm in the graphite tube, 352.943 nm in the air/acetylene flame,
and 377.573 nm in the ICP. Nonresonance laser excitation has also been reported in the
ICP [67,71]. Laser excitation was achieved at 352.943 and 377.573 nm with atomic
fluorescence observed at 535.046 nm in both instances.
For these experiments, the resonance excitation at 276.787 nm was used and
atomic fluorescence was monitored at 351.924 nm. Initial characterization o f thallium
LEAFS in the helium MIP using resonance line excitation would allow for a more direct
comparison of limits o f detection, both for other elements in the helium MIP-LEAFS
system and for thallium LEAFS using other atomization sources. In addition, laser
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Figure 17.
Partial Energy Level Diagram for Thallium Showing the Electronic
Transitions Used for Thallium LEAFS in the Helium MIP
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Energy (eV) Above the Ground State Atom
86
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
87
excitation using a resonance line results in a greater population o f thallium atoms which
are able to absorb incident radiation. Figure 17 is a partial thallium energy level diagram
which illustrates the transitions used for thallium LEAFS experiments in the helium MIP.
The transition probabilities for these transitions have been previously tabulated [92]. The
excitation transition probability at 276.787 nm (0-36118 cm'1) is listed as 4.1 X 108
/second. The transition probability for the fluorescence transition at 352.943 nm (361187793 cm '1) is 5.9 X 108/second.
Characterization o f the helium flow rate parameters for thallium LEAFS in the
helium plasma are shown in Figures 18 and 19. The thallium concentration used in these
studies is 0.1 ppm. Figure 18 is a plot from which the optimum helium plasma gas flow
rate was determined to be 18.6 L/min. In Figure 19, the ultrasonic nebulizer helium
carrier gas flow rate was determined to be 1.75 L/min. These settings were used for all
further thallium data collection purposes. Table 6 lists the important fixed parameters
used in the thallium laser excitation atomic fluorescence experiments in the helium MIP.
The limit of detection for thallium LEAFS in the helium microwave induced
plasma was 1 ppb. The linearity o f the thallium LEAFS signal is shown in the calibration
plot in Figure 20. The plot is linear with a correlation coefficient (r2) of 0.995 from 10
ppb through 10 ppm. The slope o f the calibration plot is 0.816 volts/ppm and the yintercept is 0.078 volts.
Figure 21 is the result o f investigations of matrix interferant effects on the
thallium LEAFS signal in the helium MIP. The presence of calcium showed little effect
on the fluorescence signal. The effect of sodium on the atomic fluorescence signal was
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Figure 18.
Helium Plasma Gas Flow Rate Study for Thallium Laser Excitation
Atomic Fluorescence in the Helium MLP
The thallium limit of detection is plotted versus the plasma gas flow rate.
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89
Thallium Limit of Detection, ppb
7
6
5
4
3
2
1
0
15
16
17
18
Helium Plasma Gas Flow Rate, L/min
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19
Figure 19.
Ultrasonic Nebulizer Helium Carrier Gas Flow Rate Study for Thallium
Laser Excitation Atomic Fluorescence in the Helium MIP
The thallium limit of detection is plotted versus the USN gas flow rate.
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91
Thallium Limit of Detection, ppb
25
20
15
10
5
0
0.8
1
1.2
1.4
1.6
1.8
Ultrasonic Nebulizer Helium Flow Rate, L/min
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2
2.2
92
Table 6
Optimized Instrumental Parameters for T1 LEAFS
Instrumental Component
Fixed Setting
Microwave Induced Plasma
Forward Power:
Helium Plasma Gas Flow rate:
2 Kilowatts
18.6 L/min
Ultrasonic Nebulizer
Helium Carrier Gas Flow rate:
U-Tube Temperature:
Liquid Coolant Temperature:
Sample Uptake Rate:
1.75 L/min
145 °C
4 °C
2.1 mL/min
Dve Laser
Laser Dye:
Nd:YAG Pump Wavelength:
Final Output Beam Wavelength:
Output Beam Energy:
Rhodamine 590
532 nm
276.787 nm
13-15 mJ/pulse
Monochromator
Wavelength:
Slit Dimensions
Entrance:
Connecting:
Exit:
PMT Voltage:
20 pm
50 pm
15 pm
-1000 Volts
Oscilloscope
Number o f Samples Averaged:
50
351.92 nm
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Figure 20.
Linearity o f Thallium Laser Excitation Atomic Fluorescence in the
Helium MIP
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Thallium Atomic Fluorescence Signal, Volts
94
5 r-
4 r
2 h
^
1
^ 4 __________________ |_____________________ j____________________ _>_____________________ t_____________________ |_____________________ i
00
2
4
6
8
Thallium Concentration, ppm
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10
12
Figure 21.
Interference of the Matrix Components Ca, Na, S 0 42', and P 0 43' on the
Helium Microwave Induced Plasma Laser Excitation Atomic
Fluorescence of Thallium
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96
Ca/cium
Sodium
1
10
Sulfate
Phosphate
Relative Thallium Fluorescence Signal
3
2
0
100
Interferant Concentration, ppm
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1000
97
inconsistent as the concentration of sodium was increased. Sulfate and phosphate served
to cause a general increase in thallium atomic fluorescence signal.
3.9 Laser Excitation Atomic Fluorescence o f Iron
Laser excitation atomic fluorescence experiments done previously on iron
included the use o f graphite tube atomizers [47], graphite cup atomizers [56], and ICPs
[71].
In both the graphite furnace and the ICP LEAFS papers mentioned above, laser
excitation was performed at 296.69 nm and iron atomic fluorescence observed at 373.49
nm. Additional iron atomic fluorescence experiments reported in plasmas involved the
use o f hollow cathode lamps [78, 81], xenon-arc lamps [78], and ICP emission [75] as
excitation sources.
Initial iron LEAFS experiments in the helium MIP were performed using the same
296.69 nm laser excitation, 373.49 nm fluorescence wavelengths. Transition
probabilities for these electronic transitions have been tabulated [92]. The excitation at
296.69 nm (0 - 33695 cm'1) transition probability is 3.9 X 108 /second. The value for the
fluorescence line at 373.49 nm (33695-6928 cm*1) is 20 X 108 /second.
During the initial investigations, low fluorescence signal and a very high
background resulted in detection limits near 100 ppb. This value is much higher than the
single-digit parts-per-billion ranges observed for Mn, Pb, and Tl. The low signal and
high background may be due, in part, to nitrogen absorption bands located in the 365 375 nm range. Atmospheric nitrogen, which can enter the plasma plume located outside
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98
the plasma cavity and torch, could absorb a great deal o f the laser radiation. This would
result in a significant decrease in the number o f laser photons which reach iron atoms for
excitation. An increase in background noise was also observed and attributed to the
interfering nitrogen molecular background. Any significant decrease in the number of
iron atoms in the excited state results in an observable decrease in the analytical
fluorescence signal.
It was decided that perhaps the manifold of iron emission lines observed in the
helium MIP could produce a completely different set o f wavelengths which could be
more effectively applied to iron LEAFS.
By comparing iron emission spectra with tabulated energy levels, a second set of
excitation and emission wavelengths was chosen. With laser excitation at 302.064 nm (033096 cm'1), iron fluoresces at 382.043 nm (33096-6928 cm'1). Figure 22 is a partial
energy level diagram of iron which shows these excitation and fluorescence transitions.
The tabulated transition probabilities for these transitions are 5.5 X 108/second, and 12
X 108/second, respectively. Investigations into the usefulness of these lines showed
some improvement compared to the 296.69/373.49 nm lines. The fluorescence signal
was somewhat more sensitive and the background was reduced marginally. This set of
lines was used throughout the rest of the iron atomic fluorescence experiments.
As discussed earlier, only limit of detection, linearity, and matrix interference
effects were determined for iron LEAFS in the helium MIP. The results o f plasma gas
flow rate and ultrasonic nebulizer helium flow rates for Mn, Pb, and T1 gave very similar
results. For iron, the fluorescence signal was observed on the oscilloscope at plasma gas
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Figure 22.
Partial Energy Level Diagram for Iron Showing the Electronic Transitions
Used for Iron LEAFS in the Helium MIP
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100
Energy (eV) Above the Ground State Atom
3 d 74 p , 5D °
3 0 2 .0 6 4 / n m
SD
3 8 2 .0 4 3 \ n m
5D°
SF
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101
flowrates near the optimum values for Mn, Pb, and Tl. The flow rate which gave the
largest observable signal on the oscilloscope was used. By inspection, the plasma gas
flow rate was set at 18.6 L/min. In a similar fashion, ultrasonic nebulizer helium flow
rate was set at 1.75 L/min. Table 7 shows the instrumental parameters used for the
collection of iron atomic fluorescence data.
The limit o f detection for iron laser excitation atomic fluorescence in the helium
plasma was 40 ppb. The linearity of the iron LEAFS signal is illustrated in Figure 23. It
is linear from 50 ppb through 2 ppm. The correlation coefficient (r2) for this plot is
0.994. The slope is 0.434 volts/ppm and the y-intercept is 0.0078 volts.
Figure 24 shows the results of matrix interference effects studies for iron. Note
that in this plot only three interferants were investigated: calcium, sodium, and phosphate.
Sulfate was not investigated since the stock iron solutions were prepared from ferrous
ammonium sulfate. In general, all three interferants resulted in a decrease in analytical
signal at all interferant concentrations.
3.10 Comparison of Mn, Pb, Tl, and Fe LEAFS Limits o f Detection in
the Helium MIP with Other Atomic Fluorescence Methods
Table 8 is a comparison o f detection limits for manganese, lead, thallium, and iron
by atomic fluorescence methods. Compared in Table 8 are the limits of detection
determined in this work with limits of detection found using other similar methods
including LEAFS in flames and plasmas, LEAFS in graphite furnaces, the use o f hollow
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102
Table 7
Optimized Instrumental Parameters for Fe LEAFS
Instrumental Component
Fixed Setting
Microwave Induced Plasma
Forward Power:
Helium Plasma Gas Flow rate:
2 Kilowatts
18.6 L/min
Ultrasonic Nebulizer
Helium Carrier Gas Flow rate:
U-Tube Temperature:
Liquid Coolant Temperature:
Sample Uptake Rate:
1.75 L/min
145 °C
4 °C
2.1 mL/min
Dve Laser
Laser Dye:
Nd:YAG Pump Wavelength:
Final Output Beam Wavelength:
Output Beam Energy:
Rhodamine 640
532 nm
302.064 nm
21- 2 4 mJ/pulse
Monochromator
Wavelength:
Slit Dimensions
Entrance:
Connecting:
Exit:
PMT Voltage:
60 pm
100 pm
50 pm
-1000 Volts
Oscilloscope
Number o f Samples Averaged:
100
382.043 nm
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Figure 23.
Linearity of Iron Laser Excitation Atomic Fluorescence in the Helium MIP
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104
Iron Atomic Fluorescence Signal, Volts
1.00
0.90
i
0.80 -
!
0.70
0.60
0.50
0.40
0.30 h
0.20 r
I
0.10 j -
I
0.00
0
1
2
Iron Concentration, ppm
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 24.
Interference o f the Matrix Components Ca, Na, and P 0 43' on the
Helium Microwave Induced Plasma Laser Excitation Atomic
Fluorescence o f Iron
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106
Calcium
Sodium
Phosphate
Relative Iron Fluorescence Signal
2
1
0
100
Interferant Concentration, ppm
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1000
Table 8
Comparison of Limits of Detection from This Work to Limits of Detection of Other Atomic Fluorescence Methods
Fluorescence
Type
Wavelength Scheme
Excitation/Fluorescence, nm
Manganese
MIP-LEAFS
Air/Acet-LEAFS
ICP-ICP
HCL-ICP
HCL-ArMIP
XeArc-ArMIP
GC-LEAFS
279.482/403.307
279.482/403.307
257.610/257.610
--------- /279.482
-------- -/279.482
--------- /279.482
279.482/279.482
6
0.2
9.0
0.3*
500*
400*
0.006*
This Work
64
75
81
78
78
56
Thallium
MIP-LEAFS
Air/Acet-LEAFS
Air/Acet-LEAFS
ICP-LEAFS
ICP-LEAFS
ICP-LEAFS
HCL-ICP
GT-LEAFS
276.787/351.92
276.787/352.943
276.787/352.943
276.787/377.572
352.943/535.046
377.573/535.046
--------- /377.573
276.787/351.92
1
0.9
0.8
7.
4.
10
7.*
0.00001
This Work
64
67
67
67
71
81
49
Element
(Continued on next page)
Limit of Detection
ppb
Reference
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Table 8 (continued)
Element
Lead
Fluorescence
Type
Wavelength Scheme
Excitation/Fluorescence, nm
MIP-LEAFS
Air/Acet-LEAFS
Air/Acet-LEAFS
ICP-LEAFS
HCL-ICP
GC-LEAFS
GC-LEAFS
GT-LEAFS
GT-LEAFS
GT-LEAFS
283.305/405.8
283.305/405.8
283.305/405.8
283.305/405.8
--------- /405.8
283.305/405.8
283.305/405.8
283.305/405.8
283.305/405.8
283.305/405.8
(Continued on next page)
Limit of Detection
PPb
0.7
0.4
0.02
1
20*
0.02
0.00002*
0.002*
0.001*
0.002
Reference
This Work
64
67
67
81
58
56
47
51
53
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 8 (continued)
Element
Iron
Fluorescence
Type
Wavelength Scheme
Excitation/Fluorescence, nm
MIP-LEAFS
ICP-LEAFS
ICP-ICP
HCL-ICP
HCL-ArMIP
XeArc-ArMIP
302.064/382.043
296.69/373.49
259.940/259.940
--------- /248.3
--------- /248.3
--------- Z248.3
Limit of Detection
PPb
40
50
100
10
600*
1000*
Reference
This Work
71
75
81
78
78
Fluorescence Types: MIP-LEAFS = Helium Microwave Induce Plasma Laser Excitation Atomic Fluorescence Spectroscopy
Air/Acet-LEAFS= Air/acetylene Flame Laser Excitation Atomic Fluorescence Spectroscopy
ICP-ICP = Tandem Inductively Coupled Plasma Atomic Fluorescence
HCL-ICP = Hollow Cathode Lamp Excitation Inductively Coupled Plasma Atomic Fluorescence
HCL-ArMIP = Hollow Cathode Lamp Excitation Argon Microwave Induced Plasma Atomic
Fluorescence
XeArc-ArMIP = Xenon Arc Lamp Excitation Argon Microwave Induced Plasma Atomic Fluorescence
GC-LEAFS = Graphite Cup Laser Excitation Atomic Fluorescence Spectroscopy
ICP-LEAFS = Inductively Coupled Plasma Laser Excitation Atomic Fluorescence Spectroscopy
GT-LEAFS = Graphite Tube Laser Excitation Atomic Fluorescence Spectroscopy
* Signifies that the limit of detection was determined using 2 a .
110
cathode lamps, xenon-arc lamps, and ICP emission as excitation sources for atomic
fluorescence in flames and plasmas.
The limits of detection for Mn, Pb, Tl, and Fe in the helium microwave induced
plasma laser excitation atomic fluorescence instrument are, in general, comparable or
slightly better than LEAFS in flames or plasmas reported elsewhere. In the cases o f
xenon-arc lamp and hollow cathode lamp excitation in an argon microwave induced
plasma, there was significant advantage to the MIP-LEAFS system discussed here.
Limits of detection for these metals in the helium MIP-LEAF instrument do not approach
the limits of detection reported for LEAFS using graphite furnaces. This is likely due to
the extremely low background noise o f graphite furnace systems compared to plasma
systems and the ability to preconcentrate the analyte in graphite furnace atom reservoirs.
The net result is that these initial experiments indicate that helium MIP LEAFS is
a viable and competitive technique for the analysis of aqueous Mn, Pb, Tl, and Fe. The
further characterization of the system for other elements is expected to yield similar
successful conclusions.
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CHAPTER 4
APPLICATION OF LEAFS TO THE DETERMINATION OF
NONMETALS IN THE HELIUM MIP
4.1 Introduction
This chapter discusses the potential and application o f laser excitation atomic
fluorescence for nonmetal determinations in the helium microwave induced plasma. A
brief overview o f the processes involved in populating high energy nonmetal atom and
ion excited states is presented. A discussion o f the experiments performed in an attempt
to observe nonmetal laser excitation atomic fluorescence follows. While these
experiments were not successful, documentation may provide hints and a guide for future
research in this area.
4.2 The Population of High Energy Nonmetal Excited States in a
Helium Microwave Induced Plasma
Nonmetal atoms require large amounts o f energy to populate even the lowestenergy excited states. Resonance lines for nonmetal atoms are generally in the vacuum
ultraviolet region o f the spectrum [27]. These emission lines are difficult to observe
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112
because the spectrometer must have special non-absorbing optics in the far ultraviolet
region and must be purged o f atmospheric oxygen and nitrogen to avoid broadband
absorption of the nonmetal emission.
Microwave induced plasmas generally incorporate the use o f helium as the
support gas. There is a very large difference in the ionization potentials between argon
(15.76 eV) and helium (24.59 eV) [84]. This large difference facilitates the production o f
higher energy plasma species in a helium microwave induced plasma as compared to an
argon inductively coupled plasma. These higher energy species result in better excitation
o f nonmetallic species in the helium microwave plasma.
Nonmetal emission in the helium microwave discharge has been studied
previously [27-41]. For many nonmetals in a helium microwave induced plasma, the
most intense and accessible emission lines arise from positively ionized species. The
fundamental processes involved in producing these intense ion emission lines have been
investigated [84, 93-96]. The most plausible explanation for the intense emission
resulting from electronic transitions between high energy states is that charge transfer
(CT) within a helium microwave induced plasma results in electronically excited
nonmetal ions. The general charge transfer reaction can be written:
X° +
-► X * ' + G ° + A£
C4 -1)
where X° is the ground state analyte species which undergoes charge transfer, G+ is a
plasma support gas ion, X "* is the electronically excited analyte ion following charge
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113
transfer, G° is the resulting ground state plasma gas atom, and AE is the energy defect o f
the charge transfer process. The energy defect is the deviation from resonance: the
difference in energy between the energy level populated in the species experiencing
charge transfer and the plasma support gas ion. In an ideal charge transfer reaction, the
energy o f the plasma gas ion exactly matches the energy of the excited analyte ion
(resonance) and the energy defect is zero. Various authors have discussed the maximum
energy defect values which will give rise to a significant likelihood o f charge transfer [29,
84, 93, 95, 96]. Whatever the criteria, as the energy defect decreases, the likelihood o f
charge transfer increases.
Charge transfer theory can be used to explain certain instances in which intense
emission is observed from an ion electronic state which is not likely to achieve significant
thermal population in a plasma. One such example is the well-characterized chlorine
emission in a helium microwave discharge. The most intense chlorine emission line is at
479.454 nm and is due to a chlorine ion (Cl II) electronic transition from 29.0 eV to 26.41
eV above ground state chlorine atom. The chlorine charge transfer reaction may be
written:
C /°(0 eV) + tfe *(24.58 eV) -*■ C/**(24.57 eV) + H e ° ( 0 eV) + 0.01 e V
Note that the energy defect is only 0.01 eV.
Figure 25 is a partial energy level diagram of chlorine adapted from reference
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(4.2)
Figure 25.
Partial Energy Level Diagram for Chlorine Illustrating the Electronic
States Used in Determining Thermodynamic Overpopulations
Included are the ionization potentials for helium and chlorine, along
with thermally accessible electronic states for chlorine atoms and ions.
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115
29.00 eV.
3s2 3p3 4p 5P
2&41 eV .
479.454 rm
.3s2 3p3 4s 5S°
_3s_3jj 3 3P ° __________
24.56 eV .
3s j p S 3P
Ground State Ffe (II) Ion
107.105 nm
Q xxnd State Q (D) Ion
13.01 e V ___________________ __3s23 p _ 3P .
ll.96eV.
J s 2 3p3 5p 2P°
452.621 nm
3s2 3p4 4s 2P
92 2 eV.
922 eVJ
.3s2 3p4 4s 2P
134.724nm
OeV.
3s2 3g4_2P°
Ground State Q (I) Atom
Gound State
(T) Atom
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116
[84]. The figure illustrates energies of the ground state chlorine atom (Cl I), the ground
state chlorine ion (Cl II), and the ground state helium ion (He II). Also included in Figure
25 are selected energy levels for chlorine atoms and ions which could be thermally
populated from the ground state atom, ground state ion, or proposed charge transfer
energy level.
If charge transfer processes are not considered, straightforward thermodynamic
calculations may be used to approximate nonmetal electronic excited atom and excited
ion state population ratios. The theoretical population of chlorine ions present in the
plasma can be calculated using the Saha equation. The Saha equation defines the extent
o f ionization of a species in the plasma as:
(
log
Nx .
v " ,°
\
(
-5 040 E.
-------------22 + 2.5 log T - 6.49 + log
Saha
\
SX* SeSx°
- log P .
(4.3)
t
where the N terms represent number densities, Elon is the ionization potential in electron
volts, T is the Kelvin temperature, g terms are statistical factors, and P is density
(pressure) in atmospheres. The subscripts X+, X°, and e" represent ionic and atomic
species and electrons.
Boltzmann theory can be used to predict the ratio of populations o f atoms or ions
in various electronic states. The equation is written:
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where N2 is the atom or ion number density in the upper electronic state, N, is the number
density in the lower electronic state,
is the energy difference between the upper and
lower electronic states, k is the Boltzmann constant (1.38 X 10'23 J/K), T is temperature in
Kelvins, and g2 and g, are statistical factors relating to the number o f degenerate states of
the upper and lower electronic states respectively. Key terms o f importance in this
equation are E2, and T. The ratio N /N , exhibits exponential relationships with E21 and T.
Thus the higher Ei, is at a given temperature, the smaller the population o f the upper
excited electronic state. For ionic species in the plasma, the Saha equation can be used to
approximate the fraction of ionic species. Then, Boltzmann calculations can be used to
approximate the excited atom and ion populations. Knowledge o f transition probabilities
can be used to relate these expected ratios to line emission intensities.
Experimental and theoretical Saha and Boltzmann calculations indicate that there
is a tremendous overpopulation o f the energy levels near the chlorine charge transfer
energy level [84]. Calculations were performed that compared the expected population
ratios o f the excited states of chlorine ions (3s23p34p, 29.0 eV) and atoms (3s23p35p,
11.96 eV). The ratio of the populations o f these states is denoted [ C r s|E/Cl,,']theory. This
calculated ratio is about 1.2 X 10'12, meaning that the population o f Cl* should be 8.3 X
10" times larger than the CT* population. Calculations performed using experimental
ratios of emission line intensities and transition probability information indicate that the
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118
experimentally observed ratio [C rV C l*],^ is approximately 1.2. The energy matches of
the charge transfer reactions indicate that the overpopulations are caused by charge
transfer reactions. Similar overpopulation phenomena are known to occur in helium
microwave induced discharges for other nonmetals such as sulfur, phosphorus, iodine,
and bromine [94, 96].
4.3 Fluorescence of Naphthalene in Methanol Solution for
Instrumentation Optimization
These experiments were performed before the Tektronix oscilloscope was
acquired. As such, the boxcar averager was relied upon for data collection purposes.
These experiments on naphthalene in methanol solution were done to experimentally
optimize the boxcar averager settings o f delay, width, and number averaged. Laser
excited fluorescence of naphthalene was used to optimize these settings to minimize the
inherent noise that the plasma system contributes to the PMT signal. Naphthalene was
utilized because naphthalene absorbs radiation near 280 nm and fluoresces near 330 nm
[97]. The commercial spectrofluorimeter described in section 2.1 was used to
characterize molecular fluorescence of naphthalene in methanol solution. This
instrument was used to confirm the proper excitation and fluorescence wavelength
maxima. The 280 nm excitation worked very well with the laser output maximum energy
at 280 nm with frequency doubled rhodamine 590 laser dye. The boxcar settings were
optimized by monitoring fluorescence signal at each setting. The excitation wavelength
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119
which produced the greatest fluorescence signal was 280 nm. The fluorescence emission
wavelength maximum was 350 nm.
Once the proper wavelengths were determined, they were used to observe laser
excitation atomic fluorescence and optimize instrumental settings. The general
instrumental arrangement used in these studies is illustrated in Figure 2. The fluorimeter
sample housing prepared for these studies is described in section 2.2.2.1 and was put in
place o f the microwave plasma cavity o f Figure 2. The settings used for these molecular
fluorescence studies are shown in Table 9.
These optimized experimental settings for the boxcar averager were confirmed by
obtaining the calibration curve shown in Figure 26. It contains only three points because
the focus of these experiments was only to optimize settings for future work with the
boxcar averager. The correlation coefficient (r2) for this plot is 0.995 with a slope o f 7.10
volts/10'7 M, and a y-intercept o f 10.67 volts.
4.4 Laser Excitation Atomic Fluorescence Investigations
o f Chlorine in the Helium MIP
The charge transfer process which occurs between chlorine atoms and helium ions
in the helium microwave induced plasma has been characterized and described previously
[84, 93 - 96], The large overpopulation of the charge transfer state (3s3p5, 3P°) makes
chlorine a good candidate for laser excitation atomic fluorescence. If the chlorine atoms
which have undergone charge transfer can be excited by laser radiation, there are a
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120
Table 9
Operational Parameters for Laser Excitation Molecular Fluorescence o f Naphthalene in
Methanol Solution
Instrumental Component
Fixed Setting
Dve Laser
Laser Dye:
NdrYAG Pump Wavelength:
Final Output Beam Wavelength:
Output Beam Energy:
Rhodamine 590
532 nm
278 nm
15-18 mJ/pulse
Boxcar Averager
Triggering Frequency:
Delay:
Width:
Samples Averaged:
10 Hz (laser controlled)
82 ns
1 ns
30
Monochromator
Wavelength:
Slit Dimensions
Entrance:
Connecting:
Exit:
PMT Voltage:
350 nm
75 pm
100 pm
50 pm
-1000 Volts
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Figure 26.
Calibration Curve for the Laser Excited Molecular Fluorescence of
Naphthalene in Methanol Solution
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Analytical Fluorescence Signal
122
10
1
2
3
Naphthalene Concentration, X 10'7 Molar
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4
123
number o f possible transitions available for UV-Vis atomic fluorescence. Figure 27 is a
partial energy level diagram for chlorine adapted from Carnahan and Hieftje [93]. It
illustrates several o f the energy levels within seven electron volts above the charge
transfer level (3s3p5, 3P°).
The first fluorescence experiments were designed to excite chlorine ions using the
283 nm intercombination multiplet transitions, followed by fluorescence at the 479 nm
triplet. Though the intercombination transition is thermodynamically forbidden, it is
thought to occur because the emission lines at 479 nm are the brightest visible emission
lines for chlorine in the helium MIP. These forbidden intercombination lines are noted in
energy level diagrams, but not well characterized in the helium MIP.
Table 10 shows the energy levels associated with the possible transitions
associated with these states, including the J values which accompany them. The J value
and wavenumber information was taken from tables published by Moore [98]. Both the
upper and lower state o f the intercombination transitions have three J values resulting in
nine possible emission lines. The upper nine rows represent the lines considered as
possibilities for excitation transitions. The three lower rows represent possible
fluorescence lines. These three transitions have been well characterized as atomic
emission lines in the helium MIP and are the only emission lines normally observed in the
helium MIP for the transition from the 4p, 5P to 4s, 5S° electronic states.
The energy differences for all transitions were calculated in wavenumbers and
converted to wavelength while compensating for the tabulated data being collected in
vacuum. These transitions are shown in Figure 28 as an energy level diagram. The
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Figure 27.
Partial Energy Level Diagram o f Chlorine Illustrating a Sample of the
Electronic Transitions Located Within Seven Electron Volts above the
Charge Transfer Energy Level
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125
Energy (eV) Above Ground State Atom
34-
Quintet
Triplet
32-
4p'
4p'
( 2 ,3 ,4 )
nm
4 3 4 \n m
30180
nm
(1.2,0'
( 1 ,2 ,3 )
28-
2 6 7 Inm,
475/ nm
188,
26-
5 4 5 \n m
—
283 nm
In te rc o m b in a tio n
24-
22
-
0
,0
o
fl
Term Symbol
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fi
126
Table 10
Tabulated Excitation Energy Levels for the 283/479 nm Chlorine
Excitation/Fluorescence Scheme
Lower State
Upper State
Energy Difference, (cm'1)
3s3p5, 3P° o
3s3p5, 3P° o
3s3p5, 3P°0
4p, 5P,
4p, 5P2
4p, SP3
34289.1
34329.7
34396.2
291.553
291.208
290.645
3s3p5, 3P°,
3s3p5, 3P°,
3s3p5, 3P°,
4p, 5P,
4p, 5P2
4p, SP3
34623.2
34663.8
34731.1
288.740
288.401
287.842
3s3p5, 3P°,
3s3p5, 3P°,
3s3p5, 3P°2
4p, 5P,
4p, 5P2
4p, SP3
35255.3
35295.9
35363.2
283.562
283.107
282.697
4s, 5S°,
4s, 5S°,
4s, 5S°‘
4p, SP3
4p, 5P2
4p, 5P,
20851.3
20784.0
20743.4
479.454
481.006
481.946
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Wavelength, nm
Figure 28.
Partial Energy Level Diagram of Chlorine Illustrating the Electronic
Transitions Associated with the 283/479 nm Excitation/Fluorescence
Scheme
Please note the scale change.
Wavelengths are in nanometers.
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128
29.00 -
4 7 9 .4 5 4
28.99 -
28.98 ,481.00i
291.20!
28.97 -
2 9 0 .6 4 5
2 9 1 .5 5 3
Energy (eV) Above the Ground State Atom
2 8 7 .8 4 2 ,
2 8 8 .7 4 0
4 8 1 .9 4 6
26.50 2 8 8 .4 0 1
26.00 -
25.50 -
25.00 -
24.50 -
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129
transition probabilities for the excitation transitions are currently unknown.
4.4.1 Instrumental
To investigate the possibilities of laser excitation atomic fluorescence of chlorine
in the helium MIP, the instrumental arrangement described in Figure 2 was used. The
plasma cavity was positioned in the axial orientation (plasma torch was horizontal) and
the laser was directed axially down the torch. Aqueous chloride solutions were prepared
by dissolving high purity magnesium chloride hexahydrate in water. Signal from the
PMT was directed to both the boxcar averager and the LeCroy oscilloscope. Averaged
signal was sent from the boxcar averager to the Soltec stripchart recorder.
4.4.2 Chlorine LEAFS: Excitation at 283 nm and Fluorescence at 479.454 nm
Table 11 lists the fixed settings for the chlorine LEAFS experiments. The plasma
and monochromator were optimized for chlorine atomic emission at 479.454 nm. Plasma
and ultrasonic nebulizer gas optimum flows were determined experimentally for chlorine
emission, and maintained for the chlorine LEAFS investigations.
Attempts to observe atomic fluorescence at 479.454 nm while exciting at
wavelengths between 282 and 293 nm were unsuccessful. It was observed that in the
absence of the plasma, there was an intense blue light flashing from the plasma torch.
The intense UV laser radiation was causing the quartz plasma torch to fluoresce in the
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130
Table 11
Fixed Parameters for the Investigation o f Chlorine LEAFS in the Helium MIP
Instrumental Component
Fixed Setting
Microwave Induced Plasma
Forward Power:
Plasma Gas Flow Rate:
2 kW
28 L/min
Ultrasonic Nebulizer
Helium Carrier Gas Flow rate:
U-Tube Temperature:
Liquid Coolant Temperature:
Sample Uptake Rate:
1.75 L/min
145 °C
4 °C
2.1 mL/min
Dve Laser
Laser Dye:
Nd:YAG Pump Wavelength:
Final Output Beam Wavelength:
Output Beam Energy:
Rhodamine 590
532 nm
282 - 293 nm
10-35 mJ/pulse
Boxcar Averager
Triggering Frequency:
Delay:
Width:
Samples Averaged:
10 Hz (laser controlled)
82 ns
6 ns
30
Monochromator
Wavelength:
Slit Dimensions
Entrance:
Connecting:
Exit:
PMT Voltage:
479.454 nm
10 pm
30 pm
10 pm
-1000 Volts
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131
360 - 490 nm range. To combat the quartz fluorescence, the modified plasma cavity and
plasma torches described in section 2.3 were used. The helium MIP was operated
normally with the modified torches and holes which were drilled in the sides o f the
cavity. This approach greatly reduced the background level and noise of the PMT signal.
Unfortunately, these modifications did not result in observable atomic fluorescence.
The inability to observe chlorine laser excitation atomic fluorescence at 479.454
nm with excitation using the lines o f the intercombination multiplet could be the result of
the intercombination being a forbidden transition. It is reasonable to consider that the
forbidden transition could not be driven by laser radiation.
4.4.3 Chlorine LEAFS: Excitation at 267 nm and Fluorescence at 521 nm
An attempt was made to observe chlorine LEAFS signal using a wavelength
scheme which did not involve the forbidden change in multiplicity. By examining the
chlorine energy level diagrams o f Carnahan and Hieftje [93] shown in Figure 27, it was
thought that the next best option for observing chlorine LEAFS signal would be to use
transitions within the triplet region o f the energy level diagram.
A second possible scheme involves laser excitation at 267 nm and fluorescence at
521 nm. This scheme is illustrated as an partial energy level diagram of chlorine in
Figure 29. The energy levels involved in the excitation process are the overpopulated
charge transfer energy level 3s3p5, 3P° and the 4p, 3P. Fluorescence at 521 nm involves a
transition from the 4p, 3P state to the 4s, 3S° state. Table 12 lists all o f the possible
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Figure 29.
Partial Energy Level Diagram of Chlorine Illustrating the Electronic
Transitions Associated with the 267/521 nm Excitation/Fluorescence
Scheme
Please note the scale changes.
Wavelengths are in nanometers.
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133
30.0
4 p 3P
Energy (eV) Above the Ground State Atom
(0, 1,2)
29.0
2 6 7 .0 5 0
2 6 7 .1 4 4
2 6 7 .0 5 4
28.0
5 2 1 .7 6 8 N .
5 2 2 .1 2 8 \
5 2 1 .7 8 4
2 6 4 .6 8 8
2 6 4 .7 8 0
2 6 4 .6 9 2
27.0
4 s , 3S ‘
26.5
2 6 0 .3 3 1
2 6 0 .4 2 1
2 6 0 .3 5 5
26.0
4
Z
25.0
24.5
(0, 1,2)
3p0
3p
3g0
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134
Table 12
Tabulated Energy Levels for the 267/521 run Chlorine Excitation/Fluorescence Scheme
Lower State
Upper State
3s3p5, 3P°0
3s3p5 3P°0
3s3p5, 3P°0
4 p 3P0
4p 3P i
4 p 3P2
37435.2
37442.0
37434.6
267.050
267.144
267.054
3s3p5, 3P°,
3s3p5, 3P °,
3s3p5, 3P °,
4 p 3P0
4 p 3P,
4 p 3P2
37769.3
37756.1
37768.7
264.688
264.780
264.692
3s3ps, 3P°2
3s3p5, 3P°2
3s3p5, 3P°2
4 p 3P0
4 p 3P,
4p 3P2
38401.4
38388.2
38400.8
260.331
260.421
260.335
4s, 3S°,
4s, 3S°,
4s, 3S°1
4 p 3P0
4 p 3P,
4 p 3P2
19160.0
19146.8
19159.4
521.768
522.128
521.784
Energy Difference, (cm'1)
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Wavelength, nm
135
transitions which correspond to the excitation and fluorescence processes. Data for the
energy levels and wavenumber values are from atomic energy level tables published by
Moore [98]. The wavenumber values were converted to wavelength and corrected for
differences between vacuum and air.
Experiments designed to observe chlorine LEAFS signal were nearly identical to
the experiments described in section 4.4.1 and Table 11. All flows helium flow settings
were identical to the previous chlorine LEAFS study. The laser dye was changed from
rhodamine 590 to coumarin 153 with pumping at 355 nm from the Nd:YAG laser. This
arrangement yielded approximately 5 mJ/pulse in the 260 - 270 nm range. Scans of
excitation wavelengths from 260 - 268 nm while monitoring at the appropriate
wavelengths o f the 521 nm triplet gave no atomic fluorescence signal.
4.5 Laser Excitation Atomic Fluorescence Investigations
o f Iodine in the Helium MIP
Iodine charge transfer in the helium microwave induced plasma has been
characterized by Jones and Carnahan [94]. As with chlorine, several high energy excited
states are populated which are not thermodynamically favorable according to Boltzmann
and Saha calculations. In a publication by Tanabe et al. [37], the UV-Vis emission lines
for a variety o f nonmetals in the helium plasma were determined and tabulated. Also
included in the tables are energy levels, and wavenumber values for the transitions.
These studies indicated that in the helium MIP, the second most intense emission line in
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136
the iodine UV-Vis spectrum at 516.119 nm is due to singly ionized iodine. It was thought
that the lower state o f this transition would be particularly overpopulated and a likely
candidate for atomic fluorescence excitation.
The wavelength scheme used for investigations of iodine atomic fluorescence is
shown in Figure 30. The energy levels, energy differences in wavenumbers, and
wavelengths are shown in Table 13. Excitation could occur at 516.119 nm from the 6s,
5S level to the 6p, 5P3 level followed by fluorescence from the 6p, 5P3 level to the 5d,
° 2
5D energy level at 695.878 nm. Fluorescence could also occur at 681.256 nm from the
° 4
same excited state to the 5d, 5D°3 energy level. The energy levels of these transitions
including J values were matched to the energy level tables published by Moore [98]. This
data was used to calculate the energy differences and wavelengths. Only the transitions
which were observed by Tanabe et. al. are included in Figure 30 since they are likely the
most meaningful transitions.
Table 14 lists the fixed parameters for instrumental components for the iodine
LEAFS experiments in the helium MIP. They are very similar to the settings used for
chlorine experiments. The PMT signal was directed to both the boxcar
averager/stripchart recorder and the LeCroy oscilloscope. The dye laser output was used
directly with no frequency doubling in order to excite iodine ions at 516.119 nm. The
plasma cavity was oriented in the axial position (horizontal torch placement). The laser
beam was directed down the torch axially for some experiments and directed through the
laser ports in the cavity and modified torches for other experiments.
A great deal o f structured background was observed while collecting iodine
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Figure 30.
Partial Energy Level Diagram of Iodine Illustrating the Energy Levels
Used in Laser Excitation Atomic Fluorescence Experiments
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138
23 —
Energy (eV) Above Ground State Atom
5 4 6 .4 6 1 nm .
5 4 9 .6 9 2 nm .
22 —
'5 1 6 .1 1 9 n m
6 8 1 .2 5 6 n m
6 9 5 .8 7 8 n m
21
20 —
5o0
5P
Term Symbols
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SD°
139
Table 13
Tabulated Energy Levels for the Iodine Laser Excitation Atomic Fluorescence Scheme
Lower State
Upper State
Energy Difference, (cm'1)
6s, 5S°,
6s, sS°2
6s, sS°2
6p, 5P,
6p, SP2
6p, SP3
18186.0
18294.4
19369.9
549.686
546.455
516.114
5 d ,sD°4
5d, 5D°3
6p, SP3
6p, SP3
14366.3
14674.6
695.867
681.249
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Wavelength, nm
140
Table 14
Fixed Parameters for the Investigation o f Iodine LEAFS in the Helium MIP
Instrumental Component
Fixed Setting
Microwave Induced Plasma
Forward Power:
Plasma Gas Flow Rate:
2 kW
28 L/min
Ultrasonic Nebulizer
Helium Carrier Gas Flow rate:
U-Tube Temperature:
Liquid Coolant Temperature:
Sample Uptake Rate:
2.1 mL/min
Dve Laser
Laser Dye:
Nd:YAG Pump Wavelength:
Final Output Beam Wavelength:
Output Beam Energy:
Coumarin 153
355 nm
516.114 nm
3 0 - 4 5 mJ/pulse
Boxcar Averager
Triggering Frequency:
Delay:
Width:
Samples Averaged:
10 Hz (laser controlled)
82 ns
6 ns
30
oU
Monochromator
Wavelength:
Slit Dimensions
Entrance:
Connecting:
Exit:
PMT Voltage:
1.75 L/min
145 °C
681.249, 695.867 nm
10 pm
30 pm
10 pm
-1000 Volts
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141
emission spectra between 680 - 696 nm. A narrow bandpass interference filter (Melles
Griot, Irvine, CA) was placed in front of the monochromator to alleviate some of this
background. Weak emission signal was recorded for the 695.878 and 681.256 nm iodine
lines.
The iodine LEAFS experiments were unsuccessful in obtaining iodine atomic
fluorescence. In numerous scans of laser output beam wavelength at fixed
monochromator wavelength and scans o f monochromator wavelength at fixed laser
output beam wavelength, no laser excitation atomic fluorescence was observed.
4.6 Conclusions
The inability to observe chlorine atomic fluorescence at 479 nm, though
disappointing, was not a failure. The design and implementation of the modified plasma
torches and microwave resonator cavity in these studies will be useful for further
investigation. The laser excitation atomic fluorescence scheme at 283/479 nm
excitation/fluorescence is still the most promising nonmetal atomic fluorescence
possibility. The use o f the newer and faster Tektronix oscilloscope along with further
plasma modification and characterization should yield successful atomic fluorescence
signal. Once chlorine atomic fluorescence signal is observed and characterized, the
atomic fluorescence determination of other nonmetals including iodine should be much
simpler.
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CHAPTER 5
FUTURE CONSIDERATIONS
In this work, the first documented application o f laser excitation atomic
fluorescence spectroscopy to metallic species in a helium microwave induced plasma was
successfully accomplished. The determinations of aqueous Mn, Pb, Tl, and Fe were
performed with the plasma cavity in the vertical position (torch inserted vertically through
the resonator cavity). Future work in this area includes the determination o f other metals
by LEAFS in the helium MIP. Additionally, the metal determination experiments could
be reproduced with different plasma orientations. The use o f an axial-view cavity (torch
inserted horizontally) may offer a greater plasma excitation volume for laser interaction,
resulting in greater analytical sensitivity and better detection limits.
The experimental determination o f nonmetallic elements in the helium MIP by
laser excitation atomic fluorescence has not yet been successfully accomplished. It is
likely that these elements can be determined by LEAF spectroscopy, but by using
optimum experimental conditions different than those used for atomic emission, as was
done in this study. For chlorine, in particular, using a laser to probe the overpopulation
calculated at the charge transfer energy level should result in atomic fluorescence signal.
The experimental conditions need to be further investigated. Once chlorine LEAFS
experiments are successfully completed, the experience gained will be an invaluable asset
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143
for the determinations o f other charge transfer species such as iodine, sulfur, phosphorus,
and bromine in the helium microwave induced plasma.
On a related topic, the use o f a modified torch used in conjunction with the
nonmetallic laser excitation atomic fluorescence experiments has been described. This
idea appears to be very promising for the determination o f nonmetals by LEAFS. The
holes in these torches were aligned with holes in the cavity to allow the laser to pass
through the cavity without interacting with the quartz. The position o f these holes in
relation to the plasma location within the cavity may be critical. At the present time,
however, only one position is available for this unobstructed pass-through and it is
somewhat to the rear o f the plasma. If slotted ports were machined into the plasma cavity
without altering the resonator characteristics, then the holes in the torch could be placed
in a variety of positions inside the cavity. This would allow different volumes o f the
plasma to be probed by the laser in an effort to observe atomic fluorescence from energy
levels associated with nonmetal charge transfer in the helium MIP.
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