Method of determining small concentrations of chemical compounds by plasma chromatography

An accurate method of determining by plasma chromatography the concentration of an ionizable gaseous or volatile chemical species A in air or another gaseous medium. Small, known increments of species A plus, optionally, another calibrant species are introduced in turn into the plasma chromatograph together with the unknown sample; and the respective changes of the amplitude or area of a characteristic ion peak of species A in the unknown sample are measured. The second, optical calibrant, which is different from species A, must have similar kinetic characteristics but a different ion mobility from the ion formed by A. By means of this technique, inaccurate readings caused by background concentration variations are significantly reduced.

BACKGROUND OF THE INVENTION 
This invention relates to an improved method of determining small 
concentrations of chemical compounds by means of plasma chromatography and 
to an apparatus suitable for practicing the method of this invention. 
Because of the ever growing concern with environmental problems, it is 
frequently necessary to monitor low concentrations of pollutants in 
ambient air or in industrial process gases such as, for example, smoke 
stack effluents and reactor vents. Government regulations set maximum 
permissible concentrations of many pollutants, sometimes in terms of parts 
per billion (ppb). 
Plasma Chromatography is particularly well suited for the determination of 
minute amounts of various chemical species, even in the ppb range. Most 
fundamental work in the field of plasma chromatography has been done by 
Franklin GNO Corporation, West Palm Beach, Florida. U.S. Pat. Nos. 
3,812,355 to Wernlund et al., 3,845,301 to Wernlund et al., and 3,621,239 
to Cohen are representative of the prior art. In a plasma chromatograph, a 
gas stream carrying one or more chemical substances (gases or vapors) is 
exposed to an ionization source such as, for example, a radioactive 
material. Ionizable molecules in the gas stream form ions, which are 
allowed to drift through the so-called "drift tube" of a plasma 
chromatograph between a charged shutter grid and a collector at the other 
end of the drift tube. Various ions present in the drift tube at any time 
separate according to the ion mobilities, which in turn depend, among 
others, on the ion mass, size, and shape. The amplitude of the ion current 
for any given drift time does not necessarily vary in direct proportion to 
the molecular concentration because there is competition for charge among 
all species present in the sample gas; so that the ion current amplitude 
for a given ionic species X may vary, even if the concentration of X is 
constant, because it is affected by the concentrations of other ionized 
species in the gas stream, which may not be constant. In order to improve 
the accuracy of measuring by plasma chromatography low concentrations of a 
chemical substance in a gas sample with reasonable accuracy, it is 
necessary to account for variations in concentrations of background 
species. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided an improvement in the 
method of determining the concentration, C.sub.A, of an ionizable species 
A in a gas sample by plasma chromatography by: 
(a) introducing into the plasma chromatograph a gas sample containing an 
unknown concentration of species A, and determining the size, A* 
(amplitude or area), of the characteristic peak of an ion formed by 
species A in the plasma chromatograph at its characteristic point k.sub.A 
on a plasmagram correlating ion mobility or drift time with ion current 
intensity; 
(b) while maintaining the flow of the gas sample into the plasma 
chromatograph, introducing into the plasma chromatograph an inert carrier 
gas containing a discrete amount .DELTA.C.sub.A of species A, such that 
the concentration of species A in the plasma chromatograph is increased by 
the amount .DELTA.C.sub.A, and determining the resulting logarithmic 
change, .DELTA.1nA*].sub.A, in the size of the characteristic ion peak of 
species A at point k.sub.A in the plasmagram; 
(c) while maintaining the flow of the gas sample into the plasma 
chromatograph, but cutting off the flow of carrier gas containing species 
A, introducing into the plasma chromatograph an inert carrier gas 
containing a discrete amount .DELTA.C.sub.B of species B having kinetic 
characteristics similar to those of species A but forming an ion whose 
mobility is different from that of the ion formed by A, the change in 
concentration of species B in the plasma chromatograph being 
.DELTA.C.sub.B, and determining the resulting logarithmic change of the 
size (amplitude or area) .DELTA.1nA*].sub.B of the characteristic ion peak 
of species A at point k.sub.A in the plasmagram; 
purging the plasma chromatograph with the carrier gas before one or more of 
the above steps (a) through (c) and maintaining during the entire 
operation through step (c) known flows of the gas sample and of the 
carrier gas, either alone or containing either species A or species B; 
(d) calculating the concentration C.sub.A of species A in the plasma 
chromatograph from the following equation (1): 
EQU 1/C.sub.A =K.sub.0 +K.sub.1 {.DELTA.1nA*].sub.A }+K.sub.2 
{.DELTA.1nA*].sub.B } (1) 
wherein A* is the size of the plasmagram peak of the characteristic ion 
formed by A; .DELTA.1nA*].sub.A denotes the change in 1nA* on addition of 
.DELTA.C.sub.A ; .DELTA.1nA*].sub.B denotes the change in 1nA* on addition 
of .DELTA.C.sub.B ; and K.sub.0, K.sub.1, and K.sub.2 are calibration 
constants; and 
(e) calculating the concentration, C.sub.A, of species A in the gas sample 
by means of the following equation (2) 
EQU C.sub.A =G.multidot.C.sub.A ( 2) 
wherein G is the ratio of the total gas flow through the plasma 
chromatograph in step (a), above, to the flow of the gas sample through 
the sample introduction means; 
with the proviso that when the concentrations C.sub.A and (C.sub.A 
+.DELTA.C.sub.A) are sufficiently smaller than the sum 
##EQU1## 
of all other ionizable species concentrations in the plasma chromatograph, 
so that .DELTA.1nA*].sub.A is proportional to .DELTA.C.sub.A, the above 
step (c) can be omitted, and the concentration C.sub.A of species A in the 
plasma chromatograph can be calculated from the following equation (3) 
EQU 1/C.sub.A =K.sub.0 +K.sub.1 {.DELTA.1nA*].sub.A } (3) 
wherein K.sub.0 and K.sub.1 have the same meaning as in the above equation 
(1).

DETAILED DESCRIPTION OF THE INVENTION 
It will be practical, before proceeding further, to list the principal 
symbols which are used throughout the description and the claims: 
A--ionizable chemical species whose concentration is to be determined; 
B--ionizable chemical species used as a calibrant; 
C.sub.A, C.sub.B . . . C.sub.i --concentrations (vol./vol.) of species A, B 
. . . i in the unknown sample being analyzed; 
C.sub.A, C.sub.B . . . C.sub.i --concentrations (vol./vol.) of species A, B 
. . . i in the plasma chromatograph; 
.DELTA.C.sub.A, .DELTA.C.sub.B . . . .DELTA.C.sub.i --changes in 
concentrations (vol./vol.) of species A, B . . . i in the plasma 
chromatograph; 
A*--peak size (amplitude or area) of a characteristic ion corresponding to 
species A on a plasmagram correlating ion mobility or drift time with ion 
current intensity; 
A*.sub.c --peak size of species A corrected for effects of baseline and/or 
neighboring peaks; 
.DELTA.A*--change in peak size of species A; 
.DELTA.A*.sub.c --change in corrected peak size of species A; 
(.DELTA.A*).sub.A --change in peak size of species A on addition of species 
A; 
(.DELTA.A*).sub.B --change in peak size of species A on addition of species 
B; 
.DELTA.1nA*].sub.A --change of 1nA* on addition of species A; 
.DELTA.1nA*].sub.B --change of 1nA* on addition of species B; 
C'.sub.A, C.sub.B ' . . . C.sub.i '--concentrations of species A, B, . . . 
i in a gas stream leaving a known concentration source such as an 
exponential dilution flask. 
The basic equation (1), above, can be derived from a more general equation 
(4) 
EQU A*=C.sub.A f(C.sub.A,C.sub.B . . . C.sub.i), (4) 
where f is a function of the concentrations of all species in the plasma 
chromatograph, including A, which describes their interactions and the 
resulting response to A; C.sub.A and C.sub.B are the respective 
concentrations of substances A and B; and C.sub.i are concentrations of 
all the other ion-forming chemical substances in the plasma chromatograph. 
Taking the natural logarithm of equation (4) and then the partial 
derivatives, first with respect to C.sub.A and then with respect to 
C.sub.B, one obtains 
##EQU2## 
If species B obeys ion-molecule reaction kinetics in a manner similar to 
species A, the partial derivatives 
##EQU3## 
will differ only by a constant factor, so that 
##EQU4## 
where K is related to the respective rate constants for ionization of 
species A and B in the ion-molecule reaction region of the plasma 
chromatograph. Combining equations (6), (7), and (8) gives 
##EQU5## 
For small but finite changes .DELTA.C.sub.A and C.sub.B in the 
concentrations of A and B in the plasma chromatograph, equation (9) can be 
replaced by equation (10), as follows: 
##EQU6## 
This equation relates the concentration of species A, C.sub.A, to 
measurable changes in spectral amplitude or area resulting from known 
additions .DELTA.C.sub.A and .DELTA.C.sub.B to the plasma chromatograph. 
Since .DELTA.C.sub.A and .DELTA.C.sub.B are for all practical purposes 
constant, one can convert the above equation (10) into the form given in 
equation (1), above, wherein the additive constant, K.sub.0, is a 
correction factor which is used to account for any error due to the use of 
difference values instead of differential expressions. The calibration 
constants K.sub.0, K.sub.1, and K.sub.2 can be readily determined by 
regression analysis, as shall be explained later in this disclosure. 
When C.sub.A is small compared to the sum of all other ionizable species 
concentrations, 
##EQU7## 
the response of the plasma chromatograph to .DELTA.C.sub.A additions 
frequently is linear, and the last term of equation (1), which introduces 
a linearity correction, can be ignored. In such case it is not necessary 
to additionally introduce into the plasma chromatograph calibrant B; it is 
sufficient to calibrate the instrument with substance A alone. The 
equation will then be simplified to the above equation (3). It has to be 
borne in mind that in plasma chromatography, where all components interact 
with one another and compete for electrical charge, the change of 
concentration of any of those components or introduction of another 
component affects the ionizability of all the other components. Therefore, 
the linear relationship reflected by equation (3) is valid when the 
concentration of species A is small relative to the sum of the 
concentrations of the other ion-forming species. 
In order to carry out measurements according to the method of this 
invention, it is necessary to have a sampling system suitable for 
introducing into the plasma chromatograph gas samples and calibrants, 
means for accurately determining calibrant concentrations, and means for 
displaying, storing, and/or recording spectral data. It is practical to 
use specialized electronic equipment capable of time averaging, storing, 
and recalling plasmagrams or equivalent spectral data. The basic plasma 
chromatograph can be obtained from PCP, Inc. in West Palm Beach, Florida. 
In its simplest form, a plasma chromatograph is a drift tube containing an 
ionization source, a shutter grid, and a collector. It can be combined 
with associated equipment according to the present invention, for example, 
as shown in FIG. 1. 
Line 1 conveys the sample gas to be analyzed to the first inlet port of a 
four-port junction connector 17, which can receive three gas streams for 
input into the gas inlet port 18 of the plasma chromatograph 10. Gases 
entering the plasma chromatograph pass by or through the ion source 100. 
Line 2 introduces an inert carrier gas, typically nitrogen, into an 
exponential dilution flask 3, which is used to furnish a known 
concentration C'.sub.A of molecule of interest (A) to the inlet port 18. 
This is accomplished by injecting a known concentration C'.sub.A (o) of 
standard gas A into the flask inlet 8 at an initial time t.sub.o. As 
nitrogen purges the dilution flask, the concentration C'.sub.A (t) of gas 
A supplied to the connector 17 through line 80 varies exponentially with 
time according to the following equation (11): 
EQU C'.sub.A (t)=C'.sub.A (o)e.sup.-.alpha.t (11) 
where .alpha. is the ratio (nitrogen flow rate, cc/min): (flask volume, 
cc), and t is elapsed time in minutes. 
Thus, a continuously varying but known concentration of A can be supplied 
to the plasma chromatograph 10. Dilution flask techniques are well known 
to the art. See, for example, J. J. Ritter and N. K. Adams, Anal. Chem 48, 
612 (1976). Other systems, such as regulated gas cylinder containing 
species A diluted with nitrogen, could be used but are not considered as 
practical as the exponential dilution flask, which can deliver A to the 
plasma chromatograph at a concentration which varies continuously over a 
wide range. Dynamic standards such as the exponential dilution flask also 
are inherently more accurate and reliable than static standards at very 
low concentrations. 
A regulated flow of carrier gas, such as nitrogen, is supplied through line 
4 to the first of two ports of four-way valve 5, having two inlet ports, a 
and c, and two outlet ports, b and d. Calibrant source 7 delivers a 
constant concentration, C'.sub.A, of species A to the second inlet port, 
c, of valve 5. A portion of the calibrant source output may be vented to 
exhaust (not shown), while the remainder is remixed with pure carrier gas 
to provide an adjustable concentration of A to valve 5. The specific 
calibrant source illustrated in FIG. 1 is a permeation tube, which is 
housed in a thermostatted container, 70. Permeation devices are well known 
and are generally accepted for providing stable, accurate gas standards, 
for example, for use in calibrating ambient air monitors. A discussion of 
permeation devices by F. F. Scaringelli, et al. can be found in Anal. 
Chem. 42, 871 (1970). 
The first of two outlet ports of valve 5 is connected to a vent, while the 
second outlet port communicates with the four-port connector 17. In the 
position shown in FIG. 1 in solid lines, valve 5 vents the calibrant in 
carrier gas stream from line 6 and passes the pure carrier gas stream from 
line 4 through line 9 to connector 17. In the position shown by broken 
lines, carrier gas is vented, while the calibrant stream is admitted to 
connector 17. In this way, a continuous stream of either carrier gas alone 
or carrier gas containing calibrant A flows through connector 17 to plasma 
chromatograph 10. This continuous stream is maintained at constant flow 
conditions by means of sufficient vacuum at the plasma chromatograph's 
vent 20, as well as of control valves and regulators (not shown) on lines 
4 and 6. 
A signal averager 11, such as, for example, Nicolet 1170 (Nicolet 
Instrument Company), is connected to the collector (output electrode) 16 
of the plasma chromatograph via a preamplifier, not shown. It 
time-averages, records, and displays plasmagrams of the mixture of ion 
species which reach the collector between pulsations of grid 12. FIG. 2 
shows typical ten second time-averaged waveform plasmagrams (ionic current 
versus drift time) produced by the signal averager. One can use, in 
addition, a strip chart recorder, which is operated in conjunction with an 
instrument such as, for example, a CW-1 boxcar integrator of Princeton 
Applied Research Corporation, to keep a running record of changes in ion 
current amplitude at any given ionic mobility in the plasmagram as a 
function of elapsed time (such as shown, for example, in FIG. 5). One 
should always keep in mind that amplitude changes corresponding to a given 
species do not necessarily reflect changes in that species' concentration. 
Calibration Method 
First, the concentration of the calibrant species A in the gas stream 
flowing from the calibrant source 7 into the four-way valve 5 and thence 
through line 9 into the plasma chromatograph must be determined. Valve 5 
is set as shown in FIG. 1 to admit to the plasma chromatograph 10 carrier 
gas from line 4, while venting the calibrant from line 6. Carrier gas from 
line 2 circulates through the exponential dilution flask 3 and thence to 
the four-port connector 17. Simultaneously, carrier gas also enters the 
four-port connector 17 through line 1. In neither case does the carrier 
gas carry the species of interest A, whether in a known or an unknown 
concentration. Initially, then, the plasma chromatograph will be purged of 
background impurities and the plasmagram cleared of the corresponding 
noise signals at the selected drift time characteristic of the species of 
interest A (point k.sub.A on the graphs of FIG. 2), while the volume flow 
condition at port 18 is measured. It is practical to maintain this volume 
flow constant, but this is not a requirement. When the plasmagram appears 
clear at the selected point k.sub.A (curve A in FIG. 2), valve 5 is turned 
to the dotted position to admit the calibrant gas from permeation tube 7 
into the plasma chromatograph. After the plasmagram of this species is 
obtained (curve B in FIG. 2), valve 5 is returned to its original 
position, so that the carrier gas again enters connector 17 through line 4 
while it also continues to circulate through both the exponential dilution 
flask 3 and sample line 1 into the plasma chromatograph. When the 
background plasmagram is again clear, a standard gas or solution 
containing species A but at a higher concentration than in line 9, 
C'.sub.A (o), is injected into the dilution flask at time t=o. At time t, 
when the size of the peak appearing in the plasmagram at point k.sub.A 
decreases to a level equal to that of the previously recorded calibrant 
peak, the calibrant gas concentration C'.sub.A (t) can be calculated from 
Equation (11), above, provided the gas flows through lines 9 and 80 are 
equal. This concentration naturally is the same as the calibrant A 
concentration, .DELTA.C.sub.A, so that .DELTA.C.sub.A now is known. If the 
gas flows through lines 9 and 80 are unequal, it is a simple matter to 
calculate .DELTA.C.sub.A from C'.sub.A (t) using a correction factor. 
Measurement of .DELTA.C.sub.A should be repeated periodically because of a 
long-term change of the output of the calibrant source. The carrier gas, 
respectively, from lines 4 and 2 is now allowed to purge line 9 and the 
dilution flask 3, while sample gas is admitted to line 1. The gas streams 
combined in the four-port connector 17 flow to inlet port 18 of plasma 
chromatograph 10. A plasmagram of the sample gas is recorded (curve D in 
FIG. 2), and its amplitude at point k.sub.A is determined. The four-way 
valve 5 is next turned to the dotted-line position to admit to the 
connector 17 the calibrant gas from the calibrant source 7 through line 9. 
A plasmagram of this mixture is now recorded (curve E in FIG. 2), and the 
amplitude A* of the peak at point k.sub.A is determined. The difference 
.DELTA.1nA*].sub.A between the logarithms of the amplitudes at point 
k.sub.A in curves D and E of FIG. 2 is directly related to the 
concentration C.sub.A of species A in the plasma chromatograph by means of 
equation 3. When the response is linear to species A, the calibration 
constant K.sub.0 is often small, and sufficient accuracy is obtained by 
setting K.sub.0 =0 and, by equation (10) identifying K.sub.1 
=1/.DELTA.C.sub.A. The expression for the concentration C.sub.A of species 
A in the sample gas becomes in this case 
##EQU8## 
For the small relative changes in the peak amplitude or area A*, 
.DELTA.1nA*].sub.A can by approximated by [(.DELTA.A*).sub.A /A*], and 
equation (12) becomes 
##EQU9## 
where in equations (12) and (13) .DELTA.C.sub.A is the standardized 
calibrant concentration value obtained in the earlier calibration step; 
A.sub.c * is the corrected peak size of species A on the plasmagram E in 
FIG. 2 at point k.sub.A ; (.DELTA.A*.sub.c).sub.A is the amplitude 
difference at point k.sub.A between plasmagrams E and F in FIG. 2; and G 
is the ratio of the total gas flow into the plasma chromatograph inlet 18 
to the sample gas flow in line 1. The corrected value A.sub.c * is 
obtained from the measured value A* by (1) making a baseline correction on 
the basis of curve A in FIG. 2 and (2) deconvoluting adjacent peaks. This 
deconvolution correction can be made by visual observation of graphs or by 
automatic calculation by computer. 
It is to be noted that it is not necessary to measure the gas flows in the 
course of each analysis. With the use of standard flow control equipment 
it is possible to maintain constant flows at their preset levels for 
periods as long as several months or more. Accordingly, it will usually be 
sufficient to simply occasionally check that the settings of the control 
equipment and readings of flowmeters are unchanged. 
Following the determination of the concentration of species A in a gas 
sample as described, the same operational sequence can be repeated for 
additional gas samples. Valve 5 is moved to the position indicated by 
solid lines, and a new sample is admitted to line 1. 
It can be readily seen that it is possible to use the method of the present 
invention for determining very low concentrations of different chemical 
species, A, B, C . . . I having characteristic peaks at different points 
in the plasmagram, k.sub.A, k.sub.B, k.sub.C . . . k.sub.I, corresponding 
to different drift times. Although each calibrant would have to be 
standardized, only one exponential dilution flask 3 would be necessary 
since the flask could be used sequentially to provide calibrant 
concentrations C'.sub.A, C'.sub.B, C'.sub.C . . . C'.sub.I in the manner 
described above. However, a parallel arrangement of a multiplicity of 
calibration sources 7 would be necessary, and the four-way valve 5 would 
have to be replaced by an (I+2)-way valve. 
The above-described procedure will be satisfactory for most cases, at least 
when the concentration of the species of interest A is small compared to 
the remaining impurities. In this case the response is substantially 
linear. In cases, where .DELTA.1nA*].sub.A is not sufficiently 
proportional to the concentration of A in the sample, C.sub.A, to permit 
an accurate determination of the concentration of A by means of equations 
(3), (12), and (13), above, it is recommended that a two-step technique, 
employing an additional calibrant B, be used. In this case the 
concentration of A is calculated by means of equation (1), above. 
Calibrant B should have kinetic properties similar to species A. This 
usually means that A and B should be chemically similar. In order to avoid 
overlap of the peak of B with that of A on the plasmagram, species A and B 
should have different ion mobilities. A desirable calibrant B would be a 
homologue of A; for example, if A is dimethylnitrosamine, B might be 
diethylnitrosamine. 
In practice, after the first series of operations described above for 
species A is completed, calibrant B is introduced into the plasma 
chromatograph through line 9, using a five-way valve instead of the 
four-way valve 5 shown in FIG. 1. The size of the characteristic peak of B 
on the plasmagram at its drift time point, k.sub.B, is determined (Curve C 
in FIG. 2); the apparatus is purged with carrier gas; and a known 
concentration of species B is introduced via the exponential dilution 
flask 3. Once the concentration of B in calibrant gas stream has been 
determined, as explained above for calibrant A, the instrument is again 
purged with the carrier gas. The unknown sample is then introduced and its 
plasmagram peak characteristic of A at drift time point k.sub.A is 
measured. A stream of calibrant B is introduced next. The logarithmic 
change, .DELTA.1nA*].sub.B, of the size of species A's characteristic 
plasmagram peak at k.sub.A as a result of the presence of calibrant B is 
determined (Curve F in FIG. 2). These data are used to solve equation (1). 
The concentration, C.sub.A, of species A in the sample is calculated from 
C.sub.A according to equation (2), above. The calibration constants 
K.sub.0, K.sub.1 are K.sub.2 for equations (1) and (3) are determined for 
each set of operating conditions using the exponential dilution flask as a 
source of known concentrations of species A, measuring .DELTA.1nA*].sub.A 
and .DELTA.1nA*].sub.B for different concentrations of A, (C.sub.A).sub.1, 
(C.sub.A).sub.2 . . . , and fitting these data to a function of the form 
of equation (10) by multiple linear regression. Such calculations are well 
known to a skilled engineer and can also be made routinely by computer. 
It would be obvious to one skilled in the art that the above-described 
sequence of operations, including the opening and closing of various 
valves and controlling gas flows can be entirely automatic, rather than 
manual. The four-way valve 5 can be computer-controlled or replaced by 
separate valves and associated piping, each valve being computer 
controlled according to an established program. All such automatic or 
improved alternative ways of carrying out the method of the present 
invention are within the intended scope of the claims appended hereto. 
Furthermore, it is obvious that the above-described sequence of operations 
is not critical in the sense that, for example, calibrant B may be 
introduced into the plasma chromatograph before calibrant A, rather than 
after calibrant A; or that the plasmagram of a mixture of the unknown gas 
sample with a calibrant may be obtained before the plasmagram of the 
unknown gas sample alone is obtained. 
Finally, it will be recognized that it is not strictly required that 
plasmagrams of either the unknown gas sample, or calibrants, or mixtures 
thereof be in fact either displayed or recorded, since the required 
information on the amplitudes or areas of peaks of interest can be 
directly obtained from raw data by computer calculation and displayed or 
recorded in digital form, rather than in graphic form. All such 
modifications and variations are included within this invention. 
While the above-described technique and apparatus have been developed for 
use in the field of plasma chromatography, it will be recognized that such 
an operating procedure can be likewise applied to other analytical methods 
in which a change of amplitude or area of one or more peaks of a spectrum 
occurs on addition of a calibration standard directly to the sample being 
measured. 
PREFERRED EMBODIMENT 
FIG. 3 is a schematic diagram of what is presently considered to be the 
preferred embodiment of a plasma chromatograph calibration system 
according to the present invention. Plasma chromatograph 10 has sample 
inlet port 18, a drift gas port 22, and an exhaust port 20. Sample lines 
S.sub.1, S.sub.2, S.sub.3, S.sub.4, and S.sub.5 communicate through valve 
V2 with sample line 1 and, via connector 17, with plasma chromatograph 10. 
Valve V2 also is connected through line 19 to the vent manifold 29. Valve 
5 (V1) admits to the plasma chromatograph either carrier gas from line 6 
or calibrant A from calibrant source 7A plus carrier from line 4. This 
valve also communicates with the vent via line 13. Valves V3 and V4 are 
normally operated in unison, that is, they are set so that gases either 
flow through both valves according to the solid paths or flow through both 
valves according to the dotted paths. When the solid paths are followed, 
carrier gas flows to the plasma chromatograph, and calibrant A from source 
7B flows through the exponential dilution flask 3, then, via line 15, to 
the vent manifold 29. When the dotted paths are followed, calibrant A in 
the exponential dilution flask 3 is diluted with carrier gas from 
rotameter R9 and purged into the plasma chromatograph 10, while the stream 
of calibrant from source 7B is vented through line 15. 
In this preferred embodiment calibrant B is not used. When, however, it is 
desired to also use calibrant B, the apparatus is slightly modified, as 
shown in FIG. 3A, which shows a different type of valve V1. This valve is 
connected, as before, to the vent through line 13, to valve V4 through 
line 60, to the intake manifold through line 6, and to the calibrant A 
source through line 4. In addition, this valve V1 is connected through 
line 49 to the calibrant B source (not shown). For this modification, 
valve V1 has five ports instead of four. When calibrant B is used, it is 
introduced into the exponential dilution flask 3 with a liquid or gas 
syringe through the septum port 50. Nitrogen for all instrument needs is 
supplied from the intake manifold 36, while all the exhaust streams are 
directed to the vent manifold 29. Rotameters R1 through R11 are used to 
measure gas flows in all the lines. Other means for measuring the gas 
flows, such as electronic mass flow transducers, could also be used. Gas 
flows are controlled by means of metering valves 30-35 and flow control 
valves 40-47. ASP1 and ASP2 are, respectively, plasma chromatograph and 
sample exhaust aspirators. 
While the calibrant source illustrated in FIG. 1 is a permeation tube, 
other calibrant sources, such as, for example, permeation wafers and 
diffusion tubes, can be used equally well. A good discussion of devices 
for preparing low-level gas mixtures by A. J. Martin, F. J. Debbrecht, and 
G. R. Umbreit has been published by Analytical Instrument Development, 
Inc., Route 41 and Newark Road, Avondale, Pa. 19311. 
In the preferred mode of operation of the calibration system shown in FIGS. 
3 and 3A a continuous, constant flow of gas from the four-port connector 
17 to the inlet port 18 of the plasma chromatograph 10 is insured for all 
valve settings according to the following Table 1: 
TABLE 1 
______________________________________ 
OPERATION V1 V2 V3 V4 
______________________________________ 
A. Carrier Gas Solid Position 6 
Solid Solid 
Purge of Paths to Line 1; 
Paths Paths 
Plasma Chro- Inputs 1-5 
matograph; to Line 19 
Charging 
Dilution 
Flask With 
Calibrant A 
B. Standardiza- Solid Position 6 
Dotted 
Dotted 
tion of Cali- 
Paths to Line 1; 
Paths Paths 
brant A (via Inputs 1-5 
dilution to Line 19 
flask) + 
Carrier 
C. Sample 1 + Solid Input 1 to 
Solid Solid 
Carrier Paths Line 1; Paths Paths 
Inputs 2-6 
to Line 19 
D. Sample 1 + Dotted Input 1 to 
Solid Solid 
Calibrant Paths Line 1; Paths Paths 
A + Carrier Inputs 2-6 
to Line 19 
______________________________________ 
When calibrant B is also used the sequence of operations, and the valve 
settings are shown in Table 2, below: 
TABLE 2 
______________________________________ 
OPERATION V1 V2 V3 V4 
______________________________________ 
A. Carrier Gas Input 3 to 
Input 6 Solid Solid 
Purge of Line 60; to Line 1; 
Paths Paths 
Plasma Chro- Inputs 1, 2 
Inputs 1-5 
matograph; to Line 13 
to Line 19 
Charging 
Dilution 
Flask With 
Calibrant A 
B. Standardiza- Input 3 to 
Input 6 Dotted 
Dotted 
tion of Cali- 
Line 60; to Line 1; 
Paths Paths 
brant A (via Inputs 1, 2 
Inputs 1-5 
Dilution to Line 13 
to Line 19 
Flask) + 
Carrier 
C. Standardiza- Input 3 to 
Input 6 Dotted 
Dotted 
tion of Cali- 
Line 60; to Line 1; 
Paths Paths 
brant B (via Inputs 1, 2 
Inputs 1-5 
Dilution to Line 13 
to Line 19 
Flask) + 
Carrier* 
D. Sample 1 + Input 3 to 
Input 1 to 
Solid Solid 
Carrier Line 60; Line 1; Paths Paths 
Input 1, 2 
Inputs 2-6 
to Line 13 
to Line 19 
E. Sample 1 + Input 1 to 
Input 1 to 
Solid Solid 
Calibrant Line 60; Line 1; Paths Paths 
A + Carrier Inputs 2, 3 
Inputs 2-6 
to Line 13 
to Line 19 
F. Sample 1 + Input 2 to 
Input 1 to 
Solid Solid 
Calibrant Line 60; Line 1; Paths Paths 
B + Carrier Inputs 1, 3 
Inputs 2-6 
to Line 13 
to Line 19 
______________________________________ 
*Dilution flask to be charged with calibrant B through Septum 50. 
Valves, fittings, and other associated equipment are standard parts 
obtainable from regular suppliers. Valve V1 shown in FIGS. 1 and 3 can be, 
for example, valve ASC-4-HPa, and valve V2 can be valve ASC-6-HPa, both 
manufactured by Valco Instrument Co., Houston, Tex. Valve V1 shown in FIG. 
3A is identical to valve V2, except that it has fewer inlet ports. Valves 
V3 and V4 are, for example 2X inert valves, part No. 86405, of Hamilton 
Co. of Reno, Nev. Aspirators ASP1 and ASP2 typically are vacuum flow 
transducers type AVRH-093, manufactured by Air-Vac Engineering Co. of 
Milford, Conn., and are serviced by high pressure air (typically, 
5.5.times.10.sup.5 Pa). The metering valves 30 and 31 typically are NuPro 
type SS-4MA, manufactured by NuPro Company, Cleveland, Ohio. Permeation 
tubes 7A and 7B can be purchased as complete oven arrangements from the 
manufacturer, Kin-Tek Laboratories, Texas City, Tex., catalog No. 71014. 
Metering valves 32 and 33 are NuPro type SS-2SA. 
The eight-port nitrogen intake manifold 36 (typically, a modified NuPro 
type SS-4CS-TSW-50) distributes the nitrogen supply at about 
1.4.times.10.sup.5 Pa to the outlet ports, the flow from each port being 
set and regulated by means of metering valves and flow control valves. 
Metering valves 34 and 35 are NuPro SS-2SA. Flow control valves 40 and 46 
are type 8944 #4SS made by Brooks Instrument Division of Emerson Electric 
Company, Hatfield, Pa., while flow control valves 41-45 and 47 are type 
E2P-G114ELF manufactured by Air Products and Chemical Co., Tamaqua, Pa. 
Rotameters R1-R3 and R7-R10 are steel ball Sho-Rate type R-2-15-AAA, and 
rotameters R4-R6 and R11 are glass ball Sho-Rate type R-2-15-A made by 
Brooks Instrument Division of Emerson Electric Company, Hatfield, Pa. The 
exponential dilution flask 3 is made by Glenco Scientific, Inc. of 
Houston, Tex. The flask can be adapted for either remote sample injection 
or direct syringe injection of the calibrant. 
EXAMPLE 1 
Using the equipment and flow system illustrated in FIG. 3, the 
concentration of hexafluoroacetone in air was determined according to the 
procedure of Table 1, above, as follows. The exponential dilution flask 
was maintained at 100.degree. C. All gas flows into connector 17, except 
in Step D, were maintained at 50 cc/min. Plasmagrams of ion current vs. 
ion drift time were obtained with a Nicolet 1170 signal averager and a 
model 171/2 signal digitizer. Signal amplitudes at channels k=344 and 387 
on the horizontal axis of the plasmagrams were recorded. The channel 
number is directly proportional to ion drift time and inversely 
proportional to ion mobility. The plasmagrams 4A, 4B, and 4C in FIG. 4 
cover drift times between 0 and 20 msec. 
Curve A in FIG. 4 is the plasmagram of the unknown air sample alone. Curve 
B in FIG. 4 is the plasmagram of an air sample to which a quality 
.DELTA.C.sub.A equal to 0.016 ppm of hexafluoroacetone was added 
(according to step D of Table 1 above). Curve C is the graph of the 
difference between plasmagrams A and B of FIG. 4. For better reading 
accuracy, this graph is enlarged twice. 
The corrected plasmagram peak amplitude of hexafluoroacetone, A.sub.c *, 
was obtained by subtracting from the plasmagram A in FIG. 4, at channel 
387 the baseline determined for a nitrogen blank (obtained in step A). 
Since no attempt was made to deconvolute the adjacent peaks, this 
corrected peak amplitude represents the upper limit of hexafluoroacetone 
concentration in the air sample, which was calculated from equation (13), 
after the following values had been determined: 
##EQU10## 
The upper limit of hexafluoroacetone concentration is 8 ppb. 
EXAMPLE 2 
FIG. 5 compares the results according to this invention in the 
determination of dimethylnitrosamine in air with those obtained without 
calibration. A constant stream of air containing about 0.5 ppb of 
dimethylnitrosamine was introduced into the sampling port of a plasma 
chromatograph. Various gaseous or volatile chemical compounds, which 
served as "background contaminants", were introduced sequentially through 
a parallel input via the exponential dilution flask at concentrations 
which changed continuously with time. Those compounds were as follows: (1) 
dimethylamine, (2) a mixture of ethyl benzoate, ethylenediamine, and 
bis(2-methoxyethylethyl), and (3) 1,2-dimethoxyethane. Curve A shows the 
amplitude variation of the dimethylnitrosamine peak A* on the plasmagram 
as a function of elapsed time. Curve B was obtained according to the 
method of this invention, using dimethylnitrosamine as the calibrant 
standard A. For better reading accuracy, the A*/.DELTA.A* value has been 
multiplied by 100. 
It can be seen that the amplitude of peak A* varied over a broad range as a 
result of the addition of the "background contaminants", even though the 
concentration of dimethylnitrosamine was kept constant. The points of 
addition of the above "contaminants" (1), (2), and (3) are marked in FIG. 
5 by vertical arrows along the abscissa. It is noted that the greatest 
variations of A* occurred immediately after each addition. However, the 
ratio A*/.DELTA.A* varied during the same period much more narrowly, and 
those variations occurred at random, rather than at the time of, or 
following, addition of another compound. A comparison of curves A and B 
shows the scatter of A* values to be approximate 9-10 times larger than 
the scatter of A*/.DELTA.A*.