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ec, w, 1946.
H. w. wAsHBURN
2,412,236
MASS SPECTROMETHY
Filed Deo. 9, v1943»
' 2 Sheets-Sheet l
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A TTOÍFNEYS
Deco i0, 1946.
H, W, WASHBURN
‘ 2,412,236 f
MASS SPECTROMETRY
INVENTOR.
H/wœ n /4./ M45/@amv
Patented Dec. l0, 1946
- 2,412,236
UNITED STATES PATENT OFFICE
2,412,236
\
Mass sPEcTnoMETRY
Harold W. Washburn, Pasadena, Calii'., assignor
to Consolidated Engineering Corporation,
Pasadena, Calif., a corporation of California.
Application December 9, 1943, Serial No. 513,526
20 Claims.
This invention relates to gas analysis and par
ticularly to quantitative analysis of gaseous mix
tures by mass spectrometry.
This application is a continuation in part of
my co-pending application Serial No. 320,802,
(Cl. 73-18)
2
simple quantitative relationship between the
composition of the mixture to be analyzed and
the various ions formed therefrom in the mass
spectrometer. Thus I have found that, if the
pressure in the sample chamber from which the
filed February 26, 1940.
gas mixture is admitted into the ionization cham
A mass spectrometer is an apparatus employed
ber is suiliciently low the various components of
for producing and sorting ions. One known
a gaseous mixture to be analyzed will flow into
form of mass spectrometer comprises a sample
the ionization chamber at mutually independent
chamber. an ionization chamber, an analyzer, 10 rates, i. e. the rate of ñow of each component
and a collector. A 'gas mixture to be analyzed
will be in accordance with the partial pressure
is introduced from the sample chamber through
of the component in the mixture and independ
an orifice into the ionization chamber and is
ent of the partial pressures of the other compo
there bombarded by electrons emitted by a fila
nents. Hence the portion of the mixture which
ment, so that molecules in the mixture become
enters the ionization chamber will be quantita
positive ions. As a result of their charge, the
tively as well as qualitatively representative of
positive ions are accelerated toward an exit slit
the mixture.
in the ionization chamber. After passing through
The pressure in the sample chamber should
this slit the ions are accelerated further toward
be reduced so far that the mean free path of
a second slit, which is kept at a large negative 20 the molecules in the region of the conduit>
potential with respect to the iirst slit. Hence the
through which the mixture is admitted into the
positive ions pass through the second slit at high
ionization chamber is large compared to the least
velocity and enter the analyzer, where they arel
cross sectional dimension of this'conduit. It ap
subjected to the action of a magnetic ñeld that
pears that optimum results are obtained if the
causes them to pursue a curved path. The ra 25 pressure in the sample chamber is so low that
dius of curvature of this path for a -given ac
the mean free path of each type of molecule
celerating voltage depends upon the ratio of the
present is at least twice the least cross section
charge on the ion to its mass or atomic weight,
dimension of the conduit. However, irrespective
which ratio is hereinafter sometimes referred to
of the particular ratio between the mean free
as “speciiic mass.” In consequence, the ions of 30 path of the molecules and the cross section of
low mass follow a path of short radius in the
the conduit through which they enter the ioniza
analyzer, while those of larger mass follow a
tion chamber, the fact remains that in- any in
path of greater radius.
stance the pressure in the sample chamber can
At the exit end of the analyzer there is an
be reduced to a point below which the molecules
exit slit, and by proper adjustment of the mag
will ñow from sample chamber to ionization
netic ñeld or the accelerating voltage or both,
chamber at mutually independent rates, and
the radius of path for ions of a given mass can
that when this condition prevails it becomes
be adjusted so that these ions are directed at
much simpler to determine the quantitative com
the split, pass through it and strike a collector,
position of the gas mixture from its spectrum.
where their quantity is measured, for example 40 In short. conditions of “molecular ñow” into the
by a galvanometer connected to the collecter
ionization chamber can be obtained by reducing
through a suitable vacuum tube amplifier.
the pressure in the sample chamber and when
By varying the magnetic field or the acceler
molecular ilow is established quantitative an
ating voltage, the diverging ion beams of dif
alysis with a mass spectrometer becomes rela
ferent speciñc mass formed from the gas mole 45 tively simple. In-order to obtain a uniform
cules in the mixture can be brought successively
mixture within the sample chamber, the diifu~
through the exit slit and discharged to „produce
sion within the latter must be rapid, which is
a series of ion currents which represent the mass
the case when the pressure is low and the shape
spectrum. If a quantitative relationship can be
of the chamber is appropriate, i. e., relatively
found between the molecular components of the- 50 wide in proportion to its length.
mixture and the ions formed therefrom, the mass
In summary, my invention contemplates :dow
ing the mixture (preferably by pressure) from
analysis of the mixture.
the sample chamber into the ionization chamber.
As the result of my investigation, I have found
while maintaining the pressures in both cham-.
that it is possible to establish and maintain a 55
bers. Si? 112W thateach component flows into the
spectrum becomes a means for the quantitative
2,412,236
3
quantitatively and qualitatively the same as that
in any other portion of the chamber. How
ever, the rates of diiïusion of the components of
gas sample at those temperatures and pressures
` ionization chamber at a rate dependent on the
partial pressure of that component in the mix
` ture and independent of the partial pressure of
any othîí component. Moreover, the net with
drawal of each component from the ionization
' which I prefer to maintain in the chamber usually
are such that an adequate degree of homogeneity
is obtained without agitation.
Another factor which aids in establishment of
region by the pumping system (taking into ac
count that there is some diiîusion of the com
ponents in the opposite direction) should be in
l dependent of the partial pressures of other com
the required linear relationship of the mass spec
trum of the gas mixture to the mass spectra of
its pure components obtained under similar con
ponents present. ».This can be accomplished (a)
by having a pumping speed of the system em
ployed to exhaust the ionization chamber high
(as compared with the pumping speed of the ex
ditions is the maintenance of a pressure in theionization chamber such that each component is
haust port of the ionization chamber) and (b) by ‘
introducing a “bottle neck” between the pump
and the chamber, so that a high pumping speed
in the pump (as compared with the bottle neck)
is obtained. In such case, each componentiiows
through the ionization chamber at the rate it
would have ii' it alone were present.
The pressure in the ionization chamber should
ionized to the same extent in the same manner
that it would be ionized if it alone were present.
In` other words, the- pressure in the ionization
‘ region (i, e. in the space in which the ions are
- formed and through which they travel until they
mlñll another requirement, in that it should be l
such that the number of ions derived from an 1
individual component in the ionization step varies ‘
in accordance with the partial pressure of that
enter the analyzer) should be so lowthatv the
ions formed do not collide substantially with
each other or with uncharged particles. vBy
avoiding such 'collisions' in this region, inter
change of charge between particles, secondary
ionization of particles by the original ions, and `
combination of ions with other particles, are pre
vented.' In other words, .the _possibility of> col
component and independently of the partial pres
` sures of other components, intermolecular col
l lisions being minimized by the low pressure.
Again, pressure should be so low that ionization
of each component proceeds independently of "
the presence of other components.
Molecular ñow through the conduit probably
lision in the ionization chamber may be mini
mized by making the mean free path> of each and
every type of ion present greater than the dis
tance to. be travelled by these ions from their
point of formation to the point at which they
enter the analyzer.
'
'
'
Thirdly, in order-to obtain -the desired linear
relationship between the spectrum of the'mixture
and the several spectra of its individual com
l arises under these conditions because the mole
, cules in passing through the conduit into the
ionization chamber strike the walls of the con
duit to a much greater extent than they strike
each other. The molecules, upon striking the
walls of the conduit, rebound with a velocity which
ponents, collision between ions andluncharged
molecules should be avoided in the analyzer and~
this requires that the pressure prevailing in- the analyzer be so low that the mean free path of '
is controlled primarily by the character and tem- ’
' perature of the conduit surface and is independ 40 the ions at the prevailing pressure in the analyzer
is greater (and preferably much greater) than
ent of the original velocity of the molecule strik
ing the wall. Moreover, since the molecules do
the distance travelled by the ions from the point
of ionization to the point of collection. At the
same time, the space charge and interior surface
effects’ in the ionization chamber should be kept
not collide with each other to a substantial ex
` tent, if at all, in the conduit, fast-moving mole
cules have little or no opportunity to impart
ecules. The result of the collision with the con- l
low, and electron emission should be kept uni- '
form.
dependent only‘upon the temperature and nature
junction with the accompanying drawings in
their velocity characteristics to slow-moving mol
These and other features of my invention will
duit wall and the lack of collision between the
be more thoroughly understood in the light of
molecules themselves is that each kind of- mole
cule flows through the conduit at a rate which is 50 the following detailed description taken in con
which:
of the conduit surface, and upon the molecular
cule.
- ing a mass spectrometer, which may be operatedv
In any event, and whatever be the explanation,
in accordance with my invention;
_ the fact remains that under the conditions spec
Fig. 2 illustrates a modiñed form of the inlet
iñed above. each kind of molecule (i e. each
component) flows into the ionization chamber at
system of the mass spectrometer of Fig. l;
Fig. 3 represents graphically the time decay
curve of ion currents produced by the ionization
an independent rate. This phenomenon coupled
with other conditions discussed hereinafter can
be employed to establish a linear relation be
tween the mass spectrum of -the gas mixture and
the mass spectra of any of the individual com
ponents obtained under similar conditions. And
this linear relation greatly simpliñes quantitative
of a sample vof pure gas in the mass spectrom
eter of Fig. 1; and .
Figs. 4, 5 and 6 represent graphically the in
tensities of certain ion currents measured under
A Y.'w standard conditions for CO2, iso-butane, and
analysis with a mass spectrometer, especially of
l gas mixtures containing components which crack
under the conditions prevailing in the ionization
chamber.
The establishment of the required linear rela
` tionship is further aided if, during the analysis `
period, the contents of the sample chamber is
agitated with a view to maintaining it homoge
, neous throughout and so that the portion of the '
sample immediately adjacent the conduit is
j
Fig. 1 is a. schematicl diagram, partly inr cross»
section, showing a gas analysis apparatus includ
` weight and partial pressure of that kind of mole
normal butane, respectively.
Referring to Fig. 1, an unknown gas mixture
held in a sample chamber `I is admitted to‘an
ionization chamber 2 through an inlet capillary
tube 2' and a jet 1, and withdrawn from the ion
ization chamber by evacuation through an out
let port 4. Thus, the 'bore of the capillary tube
acts as an inlet port or oriñce 3.
.
`
Gas within the ionization chamber 2 -is bom
barded by electrons drawn from a helical ñla
1
2,419,930
'
5
_
,
6
.
ment type cathode l into the space within a‘grid
type anode l which is maintained at a positive _' in a co-pending patent application of Harold W.
Serial No. 513,527, filed. December 9,
potential with respect to the cathode. Positive y Washburn,
1943,- and entitled Mass
ions are formed from molecules .thus bombarded
and these ions are accelerated toward a grounded
collimator tube I 0 by virtue of- a high- positive
potential maintained at the cathode 5 `and the
spectrometry.
Gas to be analyzed is gathered ’in a detach
able container 30 and _the latter is attached> to
the sample chamber through a conduit I4. '
Prior to introduction of a gas mixture from _the
detachable container I0 to the sample chamber.
Sonie oi' the accelerated ions pass through a - stop
cocks 3|, 32
collimator slit. 9 and proceed through asecond 10 Il is kept_ closed, are opened while a stop cock
in order to- evacuate sample
collimator slit II, thereby forming a limited het
chamber I andthe connecting conduit 34. When
erogeneous positive ion beam which, when a key
the pressure within the sample >.chamber has been
K is closed, is deñected downwardly by anv elec
reduced to a suitable value, say one micron, the
trostatic field maintained between a pair of plates ' valve
y3I"is closed and some of the unknown gas
Il by' a battery I2. The stream oi' ions pro
mixture is admitted into the sample chamber by
ceeds through a gap 20 in a chamber Il where
' opening the valve 33 fora time.
pressure
said stream is bent upward bya magnetic ileld
oi' the unknown gas mixture within the sample
provided by an electromagnet I5.
_chamber I is measured by means of a pressure
Due to thefcombined eilects of the electric
gauge 35,
as a McLeod gauge. In case-t o
and magnetic ilelds and the geometry of the 20 much gas such
is
admitted
to chamber I a portion of.'
mass spectrometer, positive ions vof a predeter
said gas may be withdrawn by opening the cockmined mass-to-charge ratio vare caused to pass
II for a short time interval.
anode 6 by a high voltage battery 8.
_ _
through a narrow. exit slit I6 and -fall upon a col- «
tcßlector I'I connected in-conventional manner toa
- B’rid oi' an electrometer tube I8. The intensity of
the ion current fallingupon the collector Il is
i) A -stop cock 40 is opened to cause gas to ilow
into the ionization chamber. The rate of iiow
of a pure gas through the capillary tube 2’ is
given by the equation
measured by a galvanometer G in the output oi'
a D.-C. ampliñer‘A connected in conventional
manner to the electrometer tube. -
-
The particulaPmass-to-charge ratio of theions which fall upon the collector I'I may be
changed- by varying the current through a coil
I9 which provides the magneto-motive force for
=rate of now in c. c./sec., referred to a -unit
establishing the magnetic ñux in the .gap_2l)pressure of one dyne/cm?,
'
_
through which the ions are caused to ilow. By 35 R=radius of tube.
:length of t?be.
changing the magnetic neld, ions of different
charge-to-mass ratios are caused to fall succes
d1=density of said'puregas at a pressure yoi! one '
where:
sively upon the collector. This produces a series
of ion currents which can be measured and em
dyne/cm?,
»
,
>
a
Z=mean free path of molecules within cham- l
ployed for determining the constituents of- an 40 ber I, _
unknown gas mixture admitted into the ioniza
p1=pressure in chamber I,
tion chamber from the sample chamber._ p2=pressure in ionization chamber 2.
The space from the collimator tube II) to the
l'n most practical cases to be considered here,
electrometer tube I8 is maintained ata very low _
the pressure in the sample chamber I will be
pressure by means of vacuum pumps` connected
. large compared to the pressure in the ionization
at exhaust ports 22, 23, so that the mean free
chamber 2 so that, if the radius of the inlet port
path of molecules in said space exceeds the
3 is small compared to the mean free path of the
length of the paths traversed by the ions-in their
molecules in chamber I, Equation 1, reduces to
travel from the anode 9 to the collector I1.
In the foregoing gas analysis procedure, the ~
pressure within the ionization chamber 2 is
Mp!
maintained large enough to provide ion currents
of suitable intensity. Preferably the. pressure is
-
When the radius of the inlet port 3 is smalll
low enough for the mean free path to be large
compared to the mean free path ol.' the molecules,
compared to the dimensions of the ionization 55\jew
molecular collisions occur at that »point and
chamber. A pressure suitable for this purpose
hence the rate of iiow through the tube becomes
lies within a range of about 10 to 40 mp.- Hg. y
independent oi' the internal viscosity of the gas.
The pressure within the 4ionization chamber
Thus,
when a gas mixture is being admitted to
may be measured by means of a Knudsen gauge
24 and controlled by adjustment of a poppet 60 the ionization chamber through the inlet port 3,
the flow of molecules of one type will be substanf
valve .25 at the mouth of an exhaust tube 2B en
tially unaffected bythe ñow of molecules of any
closing the outlet port 4. A suitable Knudsen
other type present. The rate of` ilow of -each
gauge is described in articles by J. W. N. Du
component
is- governed by Equation 2 where d1
mond, and W. M. Pickles, Jr., in- the Review of
and p1 are respectively the densities and partial _
Scientiñc Instruments, volume VI, page 362
pressures corresponding to the individual com
(1936).
'
ponents.
l At the end of a poppet valve shaft .21 _oppo
site the poppet valve is a soft iron armature 28
by means of winch the position of the poppet
valve may be adjusted by the action of an exter
. nally operated
magnet (not shown). Although
it is shown in a vertical plane, the valve shai't 21
preferably is horizontal.
A detent 4I determines '
the position of maximum closure.
The outlet
system is described in more detail and claimed _
’
In another form -of my irîvention illustrated
in Fig. 2 an inlet port 3' consists of a small
oriñce in a plate 4’. In` this case also each com'
ponent oìfoagas mixture will flow through the
inlet“ port 3’ at an independent rate if the mean
free path of the molecules is large compared
with the radius of the oriiice.
‘
The rate of now of pure gas through either
inlet port 3 or inlet port 3' varies inversely as
2,412,286
the squareI root of the molecular >_weight of said
to maintain the mean free path o! molecules
within the chamber approximately equal to the'
length of the chamber. I have found, however,
While the equations of flow (l) and (2) given
that the pressures required to maintain the mean
hereinbefore are strictly applicable only tc pure
gases, I have found that in general, if I main- 5~ free path suillciently large for this purpose, lare
unnecessarily low and that we canmaintain mix
tain the mean free path of the molecules at the
tures sumciently uniform at still higher pressures.
_inlet port 3 or‘l 3’ large compared to the radius
The time constant which measures the period
R, collisions between molecules of different
during which a‘given degree of mixing occurs in
ïkinds of gas near or within the inlet portare
'
made so infrequent that molecules of different 10 a binary mixture is given by
sas.
kinds ñow through said orifice substantially un
impeded by the presence of other molecules. ‘
' It is clear that the effective radius of the
`funnel-shaped flanged end of capillary tube 2'
X2
~' .
f
“n
'
(3)
where D=diiïusion coefficient; I :length of
3 is greater than the radius of the bore of the tube 15 sample chamber.
Fora mixture ‘of hydrogen and oxygen (having
litself’. For this reason the funnel-like end of
an interdiffusion constant of 0.7 at S. T. P.) in
the tube 2’ is preferably mounted, as shown,~on
a sample -chamber 10 cm. in diameter at a pres
the low pressure side of the orifice where the
_sure of 0.10 mm. Hg, the mixing period is
mean free path is largest.
At a, suitable working pressure the mean free 20
102 0.10
,
`lpath in ionization chamber 2 will be very large
i
_T“’fîîßfmo
, compared with the radial thickness of the annu- .
«
A lar space between valve 25 and cone-shaped
valve seat 29. At 10 mu Hg and 0° C., for in
'
37.24
v
` stance, the mean free path of nitrogen molecules 25
I
have
found
that
I
can
provide
a substantial
is 650 cm. At such pressures each component of
a. mixture will ñow out of the exhaust port 4 1 ly uniform mixture in the sample chamber if the
volume of gas admitted to the ionization cham
at an independent rate inversely proportional to
sa...
.ber during the mixing period is sufllciently small
the molecular weight of said component.
Under the conditions prescribed above,v the ion4 30 compared with the volume of the sample cham
ber. Thus, for example, the quantities of hydro
, currents detected at the collector I1 will repre
sent the sums of the currents which would be
observed for the individual components if these Í
were present alone, andthe measurements of the
gen and oxygen ñowing through a simple orifice
such as the inlet port 3’ having a diameter of l
mm. during the above calculated mixing period
‘ several’ioncurrent‘s may be used to determine 35 T are 0.67 cc. and 0.16 cc. respectively.
-the constitution of the original gas mixture.
From the foregoing description it is clear that
Since
each of these quantities of gas- is very small com
pared to the volume of the sample chamber, it is
clear that the mixture in the sample chamber _is
substantially homogeneous atv any instant dur
component gas through the- ionization chamber
2 substantially independent of the presence of"40 ing the transfer of gas to the ionization chamber.
Thus the portion of gas near the oriñce is sub
other components. However, when extreme ac
stantially typical of the gas remaining in the
curacy is required, it is also desirable to provide
‘ I am able to maintain the rate of flow of each l
‘ some method for,- maintaining the gas right at
sample chamber.
„
`
By. so maintaining the gas in the sample cham
the entrance end of the inlet'port (3 or 3') sub
’ stantially typical of the entire mixture within 45 ber substantially homogeneous, complex-correc
tions that might otherwise be required due to
the sample chamber. Otherwise, the mixture
variations in sample concentration with time are
flowing into the ionization chamber 2 will be ‘
seriously affected by the rates ofinterdiiïusion Y avoided. However, the degree of homogeneity
required and hence the sample chamber pressure
of the components within the sample chamber I
and the anlysis of observations made correspond- 50 permissible depends on the degree of accuracy'
required. ‘
ingly diiiicult. The process fof obtainingv uni'
"form distributions ofthe various components is i
retarded by the collisions which occur between
unlike molecules.
I prefer to resort to stirring the mixture me-~
chanically to maintain the mixture homogeneous
when the ¿gas in sample chamber I isat too high
.
I maintain the mixture within sample chamber 55 >a pressure for interdiilusion to occur rapidly
Isubstantially homogeneousv in either of two
enough for my purpose.
s
.
By' controlling the operating conditions of a
mass
spectrometer in accordance with (the prin
rates within the sample chamber or (2) by stir-_-`
ring the mixture mechanically. - I prefer to main-J’ " ciples hereinbefore explained each component of
tain the mixture substantially uniform Ithrough- 60 a gas mixture is` caused -to ñow through the ion- ,
' ways; (l) by maintaining rapid interdiffusion
out thel sample chamber by maintaining the rates
of interdiiîusion within said sample chamberY
rapid compared to the rate at which gas is admit ' _
ted to the ionization chamber. /I achieve >this re
sult by employing a sample chamber of proper G5
shape and by maintaining the pressure within
the sample chamber low enough for the mole
cules to distribute themselves throughout that
chamber so rapidly that the mixture is main
tained substantially uniform and the mixture ad- 70
jacent the mouth of the orifice is always substan
'f tially typical of the mixture present in the
chamber.
.
_
`
One way to maintain the mixture substantial
ization chamber 2 independently of the presence
of other components; ions are derived from- each
component within theionization chamber 2_sub
stantially in direct proportion to the partial pres
sure of each component; and as a result the mass
spectrum for a mixture is a linear superposition
of the mass spectra .of the individual components
of said mixture.
'
l
Consider the conditions which exist during the
analysis of a known pure gas such as CO2 con
tained in sample chamber i. Prior to admitting
the CO2 into the ionization chamber 2, the indica
tion of the galvanometer G is zero. When the
inlet system of the ionization chamber 2 is opened
ly uniform throughout the sample chamber I, is 75 by turning the stop cock t0, the partial pressure
2,412,2se
.
9
of CO2 Within the ionization chamber 2 begins to
rise. Ions produced by electronic bombardment
It is clear that if a mixture of any of the afore
mentioned gases is admitted to the mass spec
of CO2 are formed in proportion to the partial
trometer under the operating conditions which
pressure of CO2. After a short time interval, of
the order of one or two minutes, dynamic pres- 5 I have prescribed hereinbefore, each component
of the mixture will act independently of each of
sure equilibrium is established between the sam
the other components. Accordingly, the spec
ple chamber l, the ionization chamber 2, and the
trum
observed for the mixture will be a super
exhaustpumps~ There'after the sample cham
position- of the separate spectra of the gas com- '
ber pressure decreases substantially exponen
ponentsv combined in proportion to the amounts`
tially andthe ion density in the ionization cham-y l0 of
the respective components present in the mix
ber 2 decreases in a corresponding manner. Part
of the ions formed traverse the collimators Ill-Il
and ions of a predetermined mass-to-charge ra
tio are caused to fall on the collector I1.
ture. For a mixture the intensity of a mass spec
trum line formed by ions having ' a mass-to
charge ratio of R is y
In Fig. 3, I have illustrated graphically the 15
(4)
variation of ion current with time, measured after
opening the stop cock 40. The curve represents
where KR; is the sensitivity of the mass spectrum
the collected ion` current fora given ion such as
for ions of mass-to-charge ratio R and derived
CO+ having a mass-to-charge ratio of 28 formed
from a unit amount of gas component i, and X1
by' bombardment of CO2. Abscissae represent 20 is the quantity of component :i present in the
time, and ordinates represent the logarithm of
mixture.
'
_
the reading of the galvanometer G. After the
Now assume that a mixture 4of ethane, propane,
stop cock 40 is opened, the ion current increases
and normal butane is being analyzed, and that
rapidly, shortly reaching a maximum and there
the partial spectrum for this mixture consists of
after decreasing substantially exponentially as.
indicated by the straight line portion L of the »
curve.
_
The time constant of the decaying ion current
depends on many factors, including the volume
of the sample chamber l, the dimensions of the
inlet ports (3 or 3’), and the molecular Weight of
the gas being analyzed!
For the analysis of some mixtures containing
CO2, only the CO2 ions having a mass-to-charge
ratio of 28 (C12O16+), 29 (C13O16+ and 012017),
30 (C13O17+), and 44 (C12O216+) are of interest.
The corresponding galvanometer deñections may
be measured at convenient predetermined stand
ard times of 2, 4, 6, and 12 minutes to obtain a
standard mass spectrum. A spectrum for CO2
standard time galvanometer deflections Cao-:9.9,
C44=14.8, 058:41, corresponding, respectively, to
ions having mass-to-charge ratios of 30, 44, and
58. From Equation 4 and the table itis clear that
for this case
`
,
(5)
(6)
(7)
. where X1, X2 and X3 are the quantities ói.' normal
butane, propane and ethane, respectively, in the
sample. Solving Equations 5, 6 and 7 simulta
neously, it is found that the contents of the sam
ple are, respectively:
obtained in this manner is shown in Fig. 4. In
this graphe abscissae represent mass-to-charge
ratios and ordinates represent galvanometer de
iiections per microlitre at standard temperature
The example just given shows that where thel
and pressure of CO2 originally present in the 45 number and nature of the components of a gas
sample chamber.
mixture is known, the composition of the gas may
Figs. 5 and 6, respectively, represent similar
be determined by reading the galvanometer de
standard spectra for iso-butane and normal bu
iiections corresponding to a limited number of
tane for mass-to-charge ratios of 28, 29, 30, 43,
different ions produced by electronic bombard
44, 57 and 58.
ment of the mixture. In general, the number of
50 diil’erent ion currents measured should be at least
The intensities of the ion currents measured
equal in number to the number of components
at standard times are given more exactly in the
contributing to the production of said ions. Ob
table fol~ CO2, isobutane, normal butane, pro
viously, if the number of observationsl exceeds the
pane, and ethane. The tabulated values repre
sent galvanometer deflections per /rl standard 55 number of components present the extra obser
vations may be used to check the results.
temperature pressure of the respective gases for
In case it is not known in advance of the analy
mass-to-charge ratios of 28, 29, 30, 43, 44, 57 and
sis what components are ` present, the nature
58, obtained at the standard times given in co1
umn 1.
of the components may be determined by a study
of the complete mass. spectrum of the mixture
'rattle~
Standard time
Ethanc Propane Normal
butane
'e 'e ‘ge n
60 or by supplementary methods.
butane
C02
está?
‘ses
In any case, standard spectra are determined
for gas components contributing to the presence
of particular ion currents measured for a mix
ture, and the composition of the mixture deter
, mined by comparing the mass spectrum for the
mixture with the mass spectra of the compo
nents. The calculations of the composition of
the mixture are simplified by the method be
cause of the control maintained on the rates of
70 110W.
While I prefer to obtain the standard spectra
An examination of the partial spectra repre
for pure gas from samples of the pure gas, it is
sented in the table and Figs. 4, 5 and 6, shows that
clear that standard spectra for n pure gases may
the spectra diifer widely and may be utilized in
be obtained if desired from the spectra for n dif
identifying the respective gases.
75 ferent mixtures of these pure gases. Other
2,412,236
11
modifications of the method may be made where
pure gases are unavailable.
The numerical example given above illustrates
how my method of mass spectrometry may be
utilized to determine the composition of a gas
mixture. It has particular advantages when the
two or more gas components present in the gas
sample produce ions of the same mass-to-charge
12
’ path within a suitable range in accordance with
the principles herein set forth.
_
The relative amplitudes of ther standard mass
spectral lines of .any gas `as illustrated in Figs. 4,
5 and 6 are dependent on the decay rates oi the
ion
as well as upon the conditions of
ionization. ~ For any given set of conditions, how
ever, the composition of the mixture may bev de
terminedby obtaining standard spectra of the
‘
My method of mass spectrometry is particu 10 components of a gas mixture together with a
standard spectrum for the mixture.
larly useful in the analysis of 'a gas mixture the
In the actual analysis of a gas mixture certain
components of which yield some ions of the same
steps
in addition to those already described above
' mass-to-charge ratio. And the method of analy
are desirable.
,j
sis is essential to mass'spectrometry when one
By means of the rheostat R the current in the
or more of the components yield only ions which 15 coil
I9 is adjusted to a value which produces a
are also produced by ionization of other compo
' magnetic ñeld which causes ions of a predeter
nents possibly present.
mined mass-to-charge ratio to fall on the col
Not only can the method be used in the analy
lector l1.
f
`
Y
sis of a mixture of -several hydrocarbon gases
To obtain accurate readings it is desirable to
having different molecular weights. It may also 20 - measure the background spectrum due to residual
be used to measure the concentrations of hydro
gases in the ionization chamber prior to opening
carbon mixtures made up of a plurality of struc
the
inlet system.` To do this I measure the back
turally different hydrocarbons having the same
ground ion currents corresponding to those ions
molecular weight. For example, to measure the
which I also measure from the sample. This
concentrations of iso-butane and normal butane 25' background spectrum is preferably measured just
in a mixture known’to contain only these two
before or just after a gas sample is run. The
gases, it is only necessary to measurek ion cur
background spectrum- is subtracted from the
rents corresponding to two of the common ions
4spectrum observed for the mixture, prior to com
formed. The pair of ions having mass-to-charge
puting the composition of the mixture according
ratios of 57 and 58 may be used for this purpose. 30 to Equation 4. It is to be understood, of course,
An examination of the table and Figs. 5 and 6
that the measurement or the background is not
ywill show that other pairs of ions are also suit
necessary where the background is of negligible
able.
magnitude.
From the foregoing illustrations itis clear that
When analyzing small samples of gas, such as '
the method of the invention may be utilized to 35 soil gases containing hydrocarbons or other pe
obtain rapid and accurateanalysis of gas mix
troleum indicators, observations of the intensities
tures where conventional gas analysis methods
of the ion beams Amay be made successively at
are slow, tedious and inaccurate.
'different
times and corrections applied' to the ob
The procedure described above is also -particu
servations to compensate for the loss of gas from »
larly useful where the gas sample to be analyzed 40 the sample during the observation times.
is very small. For this reason the method is ap
In case ionization currents are measured for a
> plicable to soil gas analysis for petroleum pros
mixture at times other than standard times, the
pecting purposes and leads to an accurate knowl
readings may be corrected to standard times by
edge of the minute contents of soil gases where
applying to the mixture readings, correction
45
l other methods fail to separately identify the
factors corresponding to the decay rates of the
various gases present.
gas components contributing to said ionization
In the usual method» of soil gas analysis,
currents.
In the apparatus used such corrections
groups of hydrocarbons are only roughly identi
are of the order of 1 to 5% per minute. While
ñed and measured. Individual hydrocarbon con
this correction procedure neglects differences in
` stituents of soil gases cannot be completely sep 50
decay rates for gases of diiîerent molecular
Y Y arated and identified by conventional gas analy
weights (which may contribute to a given ion
- sis procedures. By analyzing soil samples in ac
current), nevertheless`such corrections are sufii
cordance with my method, however, it is possible
ciently accurate for many commercial purposes.
Yto identify individual hydrocarbons present in
Where extreme accuracy is desired, the spectra
55
said samples.
'
. of the separate components are corrected to the
When soil gases are extracted from soil sam
times corresponding to the times at which the
ples collected in the vicinity of a petroleum de
ionization currents are determined for the mix
posit, minute quantities of hydrocarbons such as
ture.
ethanenpropane and butane are normally found.
When the gas sample to be analyzed is small,
’Such hydrocarbons, or other substances, which 60 its composition may be determined by the method
may be indicators of petroleum deposits, may be
outlined above. If the sample is large, certain
identified by my method even when non-indi
simplifications may be made in the computation
cators such` as methane CHi and ethylene C21-I4
procedure. A large sample chamber may be used
are present.
'
"
In adapting my method to soil gasr analysis I 65 to hold a large sample, the inlet port 3 may be
made smaller in diameter, and the analysis car
prefer to concentrate significant hydrocarbons
ried out without exhausting the sample during
by any conventional method, such as temperature
the run. With large samples contained in large
separation, prior to introducing the sample into
sample 'chambers and admitted slowly to the
the mass spectrometer sample chamber l. While
ionization
chamber the composition of the sample
70
it is not possible to completely separate minute
does not change appreciably during the course of
quantities of hydrocarbons from each other, yet
the readings and the time decay of the various
by concentrating them the introduction of rela
ion currents is not appreciable. Under these con
tively large amounts of petroleum indicatorsinto
ditions the standard times at which the readings
the sample chamber is facilitated while still
maintaining the total pressure and the mean free 75 areV made need not be determined accurately, if
ratio.
Y 2,412,236
13
at all. Under some conditlonsit is clear that the
decay of ion currents will not be appreciable in
the time interval during which readings are made
and that for all practical purposes the readings
may be considered as having been made simul
taneously.
I claim:
' sure therein at level
such that the number of
ionsv derived from a component varies in accord
ance with the partial pressure of that component
and independently of the partial pressuresfof .the
other components.
_
>
"
5. In a method of analyzing a gaseous mixturev
»
involving admitting the mixture from a sample
-1. In a method of analyzing a gas mixture with
chamber into an ionization zone through a pas:
a, mass spectrometer having an ionization cham
ber and a sample chamber connected thereto, the 10 sage, ionizing'components of the mixture ‘in the
zone, withdrawing the resulting ions from the
improvement which comprises ñowing the mix
z'one, and determining the`
ture from the sample chamber into the ionization
chamber while maintaining the sample chamber
amounts of with- .
drawn ions of a selected mass-to-charge ratio, the
improvement which comprises flowing the ccm
pressure and the ionization chamber pressure so
low that each component of the mixture flows 15Y ponents of the mixture simultaneously into the
ionization zone but at mutually independent rates
from the sample chamber into the ionization
by maintaining in the sample chamber a pressure
chamber at a, rate dependent on the partial pres
‘ that is higher than in the
sure of that component in the mixture and in
ionization zone but so
low that_the mean free path of molecules of the
dependent of the partial pressure of any other
component of the mixture.
_
components of the V'mixture in the sample lcham
-
2. In a, method of analyzing a gas mixture with 20 ber is at least as long as about half of the least
cross-sectional dimension of the passage, and
a mass spectrometer having an ionization cham
maintaining the pressure in the ionization zone
ber and a sample chamber connected thereto,
such that the number of ions derived from each
the improvement which comprises flowing the
component varies in accordance with the partial
mixture from the sample' chamber into the
25
pressures of that component and independently
ionization chamber rwhile maintaining the sample
of the partial 'pressures of the other components.
chamber pressure and the ionization chamber
6. In the analysis of a, gas mixture containing’
pressure so low that each component of the mix
a plurality of components with a mass spec
ture iiows from the sample chamber into the
_trometer having an ionization chamber and a
ionization chamber at a rate >dependent on the
partial pressure of that component in the 'mixture 30 sample chamber connected thereto through an
and independent of the partial pressure of any
oriñce, the improvement
other component of the mixture, and ionizing
components of the mixture in the ionization
sure iiowing the mixture through the orifice
from the sample chamber into the ionization
chamber while maintaining the pressures in the
chamber while maintaining the pressure therein
such that the number of ions derived from an in
dividual component varies in accordance with
the partial pressure of that component and in
dependently of ‘ the partial pressures of other
components.
3. In a method of analyzing a gas mixture with
a mass spectrometer having an ionization cham
ber and a sample chamber connected thereto, the
which comprises pres-
35 chambers at such values as to flow each com
ponent at the same rate with which it would flow
if it alone were present, simultaneously ionizing
leach component inthe ionization chamber while
40
improvement which comprises iiowing the mix
maintaining the pressure inthe ionization cham
ber at a value such that each component is ionized
to the same extent that it would be ionized if it
alone were present, and measuring the rate of
formation of resulting ions of a selected mass-to
charge ratio.
ture from the sample chamber into the ionization
’ chamber and simultaneously ionizing components
_ ‘
'
'
7. In analyzing a gas mixture with a mass
spectrometer involving passing the mixture from
of the mixture in the ionization chamber while
a simple region through an oriñce into an ioniza
maintaining the sample chamber pressure and
tion region, the improvement which comprises
the ionization chamber pressure so low that each
component of the mixture flows from the sample 50 ñowing each component from _the sample region
into the ionization region at a rate which varies
chamber into the- ionization chamber at a rate
directly with the partial pressure of said each
dependent on the partial pressure of that com
component and inversely as the square root of the
ponent in the mixture and independent of the
partial pressure of any ’other component of the
molecular weight thereof by maintaining the pres
mixture, the pressure in the ionization chamber
sure in the sample region greater than the pres
sure in the ionization region and at a value at
also being such that the number of ions derived
which the mean free path of molecules is suiii
ciently large for the molecules of each component
to pass through said orifice without substantial
collision with other molecules.
from an individual component varies in accord
ance with the partial pressure of that component
and independently of the partial pressure of the
other components.
4. In a method of analyzing a gaseous mixture
involving admitting'the mixture into an ioniza
tion zone from a. sample chamber, ionizing com
ponents of the mixture in the zone, withdrawing
resulting ions from the zone, and determining
the amount of withdrawn ions of a selected mass
60
8. In a method of analyzing a gas mixture with
a mass spectrometer, the improvement which
comprises diiîusing each component of a gas mix
ture into and out` of an lionization region at an
' independent rate, and maintaining the pressures
of the gas in the mass spectrometer at a value
such that the diiïusion rates of the respective
components are inversely proportional to the
flowing the components of the mixture into the
square root of the molecular weights of the re
ionization zone simultaneously and at rates de
spective -components, and measuring the relative
pendent upon the partial pressure of the respec
70 rates of formation of ions of diiîerent mass-to
tive component and independent of the partial
charge ratios produced in said region.
pressures of the other components by maintain
9. In a method of analyzing alimited quantity
ing the pressure in the sample chamber below a
of a gas mixture originally contained in a limited
speciñc level, and ionizing the components in
the ionization zone while-maintaining the pres 75 sample region of a mass spectrometer in which
only a single ion current corresponding to a single
to-charge ratio, the improvement which comprises
- -
2,412,236
16
15
ionization zone, the ionization of components of
measured at any one time while the pressure of
a component in said mixture is diminishing at a
the mixture in said zone, the withdrawal of the
resulting ions from the zone, and the determina
tion of the amounts of withdrawn ions of difier
-predetermined mass-to-charge ratio may be
substantial rate, the improvement which com
prises the steps of continuously admitting said
mixture into the ionization chamber of said mass
spectrometer While maintaining the sample region
pressure and ionization region pressure at levels
such that the respective components of the mix- .
ture iiow from the sample region into the 'ioniza 10
tion region at mutually independent rates, suc
cessively measuring ion currents corresponding
to ions of different mass-to-charge ratios, said
measurements being made at predetermined times
after initiating the ñow of said mixture into said
ent mass-to-charge ratios, the improvement
which comprises admitting the components of
the mixture simultaneously to the ionization
zone but at mutually independent rates, and
maintaining the pressure in the ionization zone
at such a low value that the distance travelled
in the zone by ions being withdrawn from the
zone is relatively short as compared with the
mean free path of molecules of the gaseous com
ponents of said zone, whereby the amounts of
ions of each mass-to-charge ratio formed inthe
ionization of the mixture are equal respectively
to the linear sum of the quantities of such ions
which would be formed if each of the compo
nents were present alone.
ionization chamber, separately admitting into said
ionization chamber known quantities of sub
stances corresponding chemically to the respective
components of said gas mixture, and measuring
13. In a method of analyzing a gas mixture,
ion currents of said mass-to-charge ratios at times 20 the improvement which comprises the steps of
corresponding to said predetermined times after
pressure iiowing the gas mixture from a high
initiating the ñow of each of »said substances into
pressure sample region into a low pressure
analysis region while maintaining the pres
composition of said mixture by comparing the ion
sures in said regions at values such that each
25
currents measured for said mixture at such times
component in the mixture ñows at a. rate which
with the ion currents measured for said sub
is independent of the presence of other compo
stances at corresponding times.
nents present in the mixture, ionizing molecules
10. In a method of mass spectrometry involv
of each component in the low pressure region
ing the ñow of diñerent components of a mixture
while maintaining the pressure in said region so
from a limited sample region into an ionization 30 low that ions are produced from the components
. said ionization chamber, and determining the
region at such different rates that the composi
tion of the mixture is changing during the analy- -
sis, the improvement which comprises 'ñowing the
`mixture into the ionization region, ionizing the `
in amounts corresponding to the partial pres
,Y sures of the respective components in said low
pressure region, and measuring the respective
rates of formation of ions of different mass-to
mixture `in the ionization region, successively 35
measuring at predetermined times the rates of
formation of ions of different mass-to-charge
ratios formed while the amount ofthe mixture
charge ratio so formed.
‘
14. In a method of analyzing a, gas mixture
with a mass spectrometer having an ionization
region, and a sample region connected thereto
in the sample region is decreasing so as to obtain
a restricted orifice, the steps which com
a mass spectrum of the mixture, similarly obtain 40 through
prise pressure iiowing diiîerent components of
ing mass spectra of substances corresponding
, the gas mixture from the sample region into the
chemically to individual components, and deter
mining the composition of the mixture by com
paring the measured rates of formation of the
respective ions in the mass spectrum of the mix
ture with those in the mass spectra ofthe sub
ionization region through the orifice at difier
ent rates whereby the-relative amounts of the
components remaining in the sample chamber
are changed during the iiow process, while main
taining the pressure in the sample region at such
stances and in this determination compensating
a value that the molecules of the respective
for changes in the composition of said mixture
components enter the oriñce from a relatively
during analysis by reducing the measurements
small
part of the sample region adjacent the
of the ion formation rates of given mass-to 50
mouth of the orifice without .any substantial pro
change ratio in the mixture and in the individual
portion of collisions with other- molecules and
substances to a common time basis by correct
mixing the components remaining in the sample
ing the measurements in accordance with the
region rapidly enough in relation to the diiïer
rates of iiow of the individual components.
ences in said ñow rates and the dimensions of
11. In a method of analyzing a gaseous mix- .Ul Cil
the
sample region to maintain the mixtureof
lture involving admission of the mixture into an
the gas sample substantially homogeneous
ionization zone, the ionization of components of
throughout the entire sample region, thereby
the mixture in said zone, the withdrawal ofthe
maintaining a typical sample in the small part
resulting ions from the zone, and the determina
tion of the amounts of withdrawn ions of differ
60 lof the sample region adjacent the mouth of the
oriñce representative of the entire sample in the
ent mass-to-charge ratios, the improvement
sample region.
which comprises admitting the components of
15. In the analysis of a gas mixture contain
the mixture simultaneously to the ionization
ing a plurality of components with a mass spec
zone but at mutually independent rates, and
having an ionization chamber, a sample
maintaining the pressurel in the ionization zone 65 trometer
chamber connected thereto through a first aper
at such a vlow value‘that ions being withdrawn
from the zone do not collide substantially with
molecules of the gaseous mixture, whereby the
amounts of- ions of each mass-to-charge ratio
ture, and a low pressure zone connected thereto
through a second aperture, the improvement
which comprises pressure flowing the mixture
through the ñrst aperture while maintaining the
formed in the ionization of the mixture are equal 70 pressures in the two chambers at such low val
respectively to the linear sum of the quantities
ues that each component flows into the ioniza
of such ions which would be formed if each of
tion chamber at the same rate with which it
the components were present alone.
would flow if it were present alone, ionizing each
12. In a method of analyzing a gaseous mix
component in the ionization chamber while the
ture involving admission of the mixture into an 75
17
2,419,230
pressure in the ionization chamber is of such
a low value that each component is ionized in
the same manner that it would be ionized it it
were present alone, withdrawing the resulting
ions of each component simultaneously from the
ionization chamber at the same rate that the ions
of each component would be withdrawn if that
nected to the intermediate chamber through a
second conduit, and a stopcock in said second
conduit.
18. In a mass spectrometer. the combination
which comprises an ionization chamber, a sam
ple chamber, an intermediate chamber, a ñrst
conduit having a restricted oriiice therein con
component were present alone and were being
nectingthe intermediate chamber to the sam
ionized, pressure flowing all of the non-ionized
ple chamber, a valve in said conduit between said
molecules from the ionization chamber into the 10 oriñce and said intermediate chamber, means
low pressure zone by maintaining the relationship
for exhausting the intermediate chamber inde
between the pressures in the ionization chamber
pendently of'the sample chamber and connected
and such zone at such values that the non
to the intermediate chamber through a second
ionized molecules of each component flow into
conduit, and a valve in said second conduit. `
the zone at the same rate at which they would 15
19. In a mass spectrometer, the combination
flow if that component were present alone, and
which comprises an ionization chamber, a sam
measuring the rate of withdrawal oi' the result
ple chamber, an intermediate chamber of iixed
ing ions oi’ a selected mass-to-charge ratio,
volume, means for admitting a portion oi’ a sam
whereby the contribution of each component to
ple from the sample chamber into the inter
the total number of such ions withdrawn is the 20 mediate chamber in gaseous form, a conduit con
same as it would be if the component were pres
ent alone.
16. In a mass spectrometer, the combination
which comprises an ionization chamber, a sam
necting said intermediate chamber and' said
ionization chamber, a valve in said conduit,
means for indicating the pressure of gas present .
in said intermediate chamber, a second'conduit
ple chamber, a conduit connectingthe sample 25 connected to the intermediate chamber, and
chamber to the ionization chamber, and a tube .
means connected to the intermediate chamber
sealed within said conduit with at least part of
through the second conduit withdrawing a por
the tube spaced from the conduit wall and ex
tion of any gas contained therein.
tending i'rom the sealed portion thereof in the
20. In a mass spectrometer, the combination
direction of the sample chamber and opening to 30 which
comprises an ionizationchamber, a sam
ward the sample chamber.
ple
chamber,
an intermediate chamber, a first
17. In a mass spectrometer, the combination
conduit having a restricted oriñce therein con
which comprises an ionization chamber, a sam
necting the intermediate chamber to the ioniza
ple chamber, an intermediate chamber of fixed
tion chamber, a valve in- said conduit between
volume, a i‘lrst lconduit containing a restricted
said
orifice and said lvintermediate chamber,
orii‘lce connecting the intermediate chamber to
evacuating means, a second conduit connecting
the sample chamber, a stopcock in said conduit.
the intermediate chamber and the evacuating
means i’or exhausting the intermediate chamber
means,y and a valve in said second conduit.
independently of the sample chamber and con
HAROLD W. WASHBURN.
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