260
Anal. Chem. 1989, 6 1 , 260-264
than 1 mm s-l to avoid an excess of the longitudinal diffusion contribution; (2) the capacity factor be kept less than five, because larger capacity factors cause higher plate height contributions of Ht and Hey Each conditions described above is also advantageous from viewpoints of analysis time and resolution (2). Finally, it is stressed that most of the discussion described above is only applicable to the experimental data of plate heights similar to the values found in this study; in other words, if the experimental plate heights are a few times larger than those described here, some other causes of band broadening should be sought, because any causes discussed above do not predict such high plate heights.
ACKNOWLEDGMENT We thank B. L. Karger for critical reading of the manuscript and S. Rokushika and H. Takayanagi for helpful suggestions and discussions about band broadening in the on-column detection.
LITERATURE CITED (1) Terabe, S.;Otsuka, K.; Ichlkawa, K.; Tsuchlya, A,; Ando, T. Anal. Chem. 1984, 56,111-113. (2)Terabe, S.;Otsuka, K.; Ando, T. Anal. Chem. 1985, 57, 834-841. (3) Mlkkers, F. E. P.; Everaerts. F. M.; Verheggen, Th. P. E. M. J. ChroMfogr. 1979, 769, 11-20, (4) Jorgenson, J. W.; Lukacs, K. D. Anal. Chem. 1981, 53, 1298-1302. (5) Mlkkers. F. E. P.; Everaerts, F. M.; Verheggen, .~ Th. P. E. M. J. ChroMfOgr. 1979, 769,1-10. (6) L a w , H. H.; McManlglll, D. TrAC, Trends Anal. Chem. 1988, 5 , 11-15. .. (7) Otsuka, K.; Terabe, S.; Ando, T. J. Chromafogr. 1985, 332,219-226. (8) Otsuka, K.; Terabe, S.; Ando, T. J. Chromatogr. 1985, 346,39-47. (9) Otsuka, K.; Terabe, S.; Ando, T. Nippon Kagaku Kalshi 1988, 950-955. (10)Cohen, A. S.;Terabe, S.; Smith, J. A.; Karger, B. L. Anal. Chem. 1987, 59,1021-1027. (11) Sepanlak, M. J.; Cole, R. 0. Anal. Chem. 1887, 59,472-476. (12)Terabe, S.;Ozaki, H.; Otsuka, K.; Ando, T. J. Chromafogr. 1885, 332,211-217.
Snyder, L. R.; Kukland, J. J. Introdoctlon to Modem UqUM Chromafography. 2nd ed.; Wlley: New York, 1979;pp 222-223. Otsuka, K.; Terabe, S.; Ando. T. J. chromefogr. 1987, 396,350-354. Scott, R. P. W. Llquld Chromatography Defector;Elsevier: Amsterdam, 1977;Chapter 3. Burton, D. E.; Sepaniak, M. J.; Maskarlnec, M. P. Chromatograph& 1988, 21, 583-586. Yang, F. J. HRC CC, J . High Resoluf. Chromafogr. Chromatogr. Commun. 1981, 4,83-85. Corti, M.; Deglorglo, V. J. fhys. Chem. 1981. 85. 711-717. Armstrong, D. W.; Ward, T. J.; Berthod, A. Anal. Chem. 1986, 58,
579-582. CRC Handbook of Chemistry and fhyslcs, 66th ed.; Weest. R. C. Ed.; CRC Press: Boca Raton, FL, 1985;p F-45. W n g s , J. C. Dynamics of Chromatography: Part 7 ; Marcel Dekker: New York, 1965 Chapters 2-4. Almgren, M.; Griesen, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 279-291. Sepaniak, M. J.; Burton, D. E.; Maskarlnec, M. P. I n OrderedMedia in Chemical Separafions; Hinze, W. L., Armstrong, D. W. Eds.; ACS Symposium Series 342; American Chemical Soclety: Washington, DC, 1987;pp 142-151. Wallingford, R. A.; Ewlng, A. 0. Anal. Chem. 1988, 60, 258-263. Rice, C. L.; Whitehead, R. J. fhys. Chem. 1985, 69,4017-4024. HJerth S.Chromafogr. Rev. 1987, 9,122-219. Hunter, R. J. Zeta fofenflslln &//old Science; Academic Press: London, 1981;Chapter 3. Martin, M.; Gulochon, G. Anal. Chem. 1984, 56, 614-620. Martin, M.; Gulochon, G.; Walbroehl, Y.; Jorgenson. J. W. Anal. Chem. 1985, 57, 559-561. Boyak, J. R.; Giddings. J. C. J. 6lol. Chem. 1980. 235, 1970-1972. Wieme, R. J. I n Chromatography: A Laborafoty Handbook of Chromafographlc and Eiecfrophcfetlc Methods, 3rd ed.; Heftmann. E., Ed.; Van Nostrand Reinhold: New York, 1975;pp 267-273. Anlasson, E. A. G.; Wall. S. N.; Almgren, M.;Hoffmann, H.; Kielmann, I.; Ulbrlcht, W.; Zana, R.; Lang, J.; Tondre, C. J. fhys. Chem. 1978,
80,905-922. Attwood, D.; Florence, A. T. Surfactanf Systems: Thek Chedstry, Pharmacy and 6lolcgy; Chapman and Hall: London, 1983;pp 85-87.
RECEIVED for review August 14,1987. Resubmitted August 15, 1988. Accepted November 7,1988. This work has been supported in part by the Grant-in-Aid for Scientific Research (No. 61790168) from the Ministry of Education, Science and Culture, Japan.
Atmospheric Pressure Ionization Tandem Mass Spectrometric System for Real-Time Detection of Low-Level Pollutants in Air S. N. Ketkar,* J. G. Dulak, W. L. Fite, J. D. Buchner, and Seksan Dheandhanoo Extrel Corporation, 240 Alpha Drive, Pittsburgh, Pennsylvania 15238
An atmospheric pressure lonlratlon tandem mass spectrometrk (API) MS/MS) system Is described that Is capable of detectlng, in real time, very low levels of contaminant (SO "C), hlgh humldlty (>90% RH), and the presence of 3% COP and>400 ppm NO,. This source Is versatlie enough to easlly replace conventional electron impact or chemical lonlzatlon sources In existing mass spectrometers. 0003-2700/69/0361-0260$01.50/0
INTRODUCTION Analysis of trace quantities of compounds in air is becoming increasingly important in air pollution studies where it is necessary to detect impurities, in real time, in the concentration range of parts per billion and less. Normal techniques used to achieve such a high sensitivity utilize separation and concentration processes that are very time consuming. Conventional electron impact sources are limited in sensitivity by the natural ionization cross sections. This problem can be somewhat circumvented by using chemical ionization with a reagent gas. However, one still encounters problems with introducing the atmospheric sample into the chemical ionization region, which is typically at a pressure of a fraction of a Torr. Atmospheric pressure chemical ionization has been shown to be extremely sensitive, and since the ionization takes place at atmospheric pressures, sample introduction becomes a trivial problem. The ionization a t atmospheric pressures can be achieved in two ways, either by the use of p particles emitted by a 63Nisource or by the use of a corona discharge 0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 3, FEBRUARY 1, 1989 TO VACUUM
I
C
400 MICRON J APERTURE
~ECLUSTERING REGION
Figure 1. Schematic of the API source.
(1-6). Corona discharge offers the possibility of obtaining higher plasma density and thus is more sensitive. In API MS use is made of ion-molecule reactions, coupled with the large air flow rates through the corona region, to achieve a rapid response. This rapid response, coupled with the sensitivity of such a system, makes it very useful in the monitoring the atmosphere for very low levels of contaminants. However, in order to achieve specificity of detection, one needs to employ tandem mass spectrometric techniques. The use of MS/MS techniques avoids the interferences that can occur if one uses a single MS detector. There is a commercially available atmospheric pressure ionization mass spectrometer system; however, it is hampered by the necessity of using bulky and costly cryogenic pumps (for example TAGA 6000 manufactured by Sciex, Inc., Thornhill, ON, Canada). The use of cyrogenic pumps also reduces the duty cycle of operation of such a system. With the widespread use of incinerators to dispose of toxic waste there is a need for rapidly detecting low-level impurities in the stack effluent of such incinerators. Stack effluents are characterized by high temperatre (>90 "C), high humidity (>W%RH), and the presence of a few percent COz and >400 ppm oxides of nitrogen and HCl, CO, SOz,etc., at ppm levels. We report here an atmospheric pressure ionization source for a triple quadrupole mass spectrometer system pumped by conventional turbomolecular pumps, to monitor the effluent of a simulated stack matrix containing trace levels of dimethyl methylphosphonate (DMMP).The characteristics of the stack effluent that were simulated in the laboratory were the high temperature, high humidity, and the presence of NO, and COP The acidic components of the stack were not simulated because they are likely to cause a corrosion problem and not directly impact on the operation of the system. The source can be easily fitted onto existing systems utilizing electron impact or chemical ionization sources.
EXPERIMENTAL SECTION The API MS/MS source is shown schematically in Figure 1. It consists of the following three main parts: the atmospheric pressure ionization source, a declustering region operating at ca. 1 Torr, and a triple quadrupole mass Spectrometer. Ionization Source. Figure 2 shows a detailed view of the API source. The corona discharge is a point to plane discharge between an iridium needle, 0.25-mmdiameter, and a stainless steel aperture plate with an aperture of 400-pm diameter. A micrometer adjustment makes it possible to accurately position the iridium needle a few millimeters from the aperture plate. A positive voltage of about 5-10 kV on the needle is used to produce the corona discharge. An 800-MQresistor in series with the highvoltage power supply serves to stabilize the discharge current. The sample air is drawn through this corona discharge region at rates of a few liters per minute by the use of a rotary vane pump. The primary ions produced in the discharge rapidly react with the neutral present in this region. The primary ions and the secondary ions produced enter the declustering region through the 400-pm aperature under the influence of the strong electric field.
2 3 4
5
261
6 7 8 91011 12 13
Figure 2. Schematic of the API MS/MS system: (1) 0.25-mm irMium needle, (2) 0.40-mm stainless steel aperture, (3) 1-mm aperture lens, (4) 0.33-mm aperture lens, (5) Simulscan ion optics, (6) delayed DC ramp, (7) Q1, (8) leaky dielectric aperture lens, (9) Q2, (10) leaky dielectric aperture lens, (11) Q3, (12) exit lens, (13) detector.
Declustering Region. The declustering region is maintained at a pressure of ca.1Torr by a Balzen UNO 16 mechanical pump. The declustering region consists of two shaped aperture plates. The first plate has an aperture of 1mm, while the second plate with an aperture of 0.33 mm separates the declustering region from the high-vacuum region of the triple quadrupole mass spectrometer. Heaters are used to maintain the temperature of these plates at about 100 O C , to prevent condensation of water. A field gradient is maintained in the declustering region to achieve collision-assisted dissociation of the clusters produced in the atmospheric pressure plasma (7). The clusters produced in the atmospheric pressure corona discharge are very weakly bound, of the van der Waals type, and are easily dissociated by collisions. The declustered ions exiting the 0.33-mm aperture have energies that are representative of the potential applied on this aperture. In typical operation the ions exiting this aperture have energies of about 60 eV. Ion Optics. The ions exiting the last aperture of the declustering region are focused by a series of cylindrical lenses onto the entrance of the fiit quadrupole of the triple quadrupole mass spectrometer. The lenses used are part of the ion optics from an EXTREL Simulscan ionizer and are shown schematically in Figure 1. The first lens is a cylindrical lens made of a coarse wire mesh. The use of a wire mesh facilitates pumping of the region around the path of the ions while at the same time shielding the ion path from the ground potential due to the grounded mount that holds the declustering region. The second lens is a thick aperture lens that incorporates a slot in its side to mount a filament. This filament can be used for diagnostic and tuning purposes by using electron impact ionization of the background gases. The other lenses are simple aperture lenses optimized to focus the ions onto the entrance aperture of the quadrupole. Triple Quadrupole Mass Spectrometer. The triple quadrupole mass spectrometer is an EXTREL Model 400-3 consisting of 19-mm-diameter quadrupoles, each with a length of 15.25cm. A delayed direct current (dc) ramp is used at the front of the f i t quadrupole (61).The use of a delayed dc ramp helps improve the transmission of Q1 by keeping the ions on a stable trajectory at the entrance of the quadrupole (8). The second quadrupole (Q2) is housed in a cylindrical collision cell. The collision cell has endplates made from a leaky dielectric material. The use of such endplates has been shown to increase the transmission of a triple quadrupole (9). A pressure of ca. 1mTorr is maintained in the collision cell to achieve collision-induced dissociation of ions selected by Q1. At the exit of the third quadrupole (Q3) an aperture lens is used to accelerate the emerging ions through the fringe field. A channeltron together with a conversion dynode serves as a detector for these ions. Single-ion counting is performed by using a preamp with a discriminator and a scaler counter. A TEKNIVENT data system together with an IBM XT is used to drive the quadrupoles to the desired mass and to acquire data. In normal operation of the triple quadrupole mass spectrometer, both Q1 and Q3 are driven by RF + DC fields, while Q2 is driven by a rf field only. The rf + dc field on Q1 is chosen so as to transmit the m J z of the ion of interest, with a unit resolution. The magnitude of the rf field on Q2 is set to half the magnitude of the rf field on Q1. The ions transmitted by Q1 undergo collisional dissociation in Q2. The rf field on Q2 serves to confine
ANALYTICAL CHEMISTRY, VOL. 61, NO.3, FEBRUARY 1, 1989
262
In order to simulate the stack effluent, COz (3%)and NO (700 ppm) are added to the air stream. The high humidity is obtained by introducing a measured amount of water to the air stream. Heaters are used to maintain the temperature of the air at about 100 “C. A heat-traced Teflon tubing is used to transport the output of the vapor generator to the API source.
RESULTS AND DISCUSSION Ionization Region. In the corona discharge region a rapid sequence of ion-molecule reactions occurs to form a series of protonated water molecular clusters of the type (H20),H+, with n 1 1 (8). These protonated clusters serve as the primary reagent ions for the ionization of the trace molecules through the following reaction:
AP I
SOURCE
-
1 R
(H,O),H+
‘5‘ -
AIR COMPRESS0
*
t
Flgure 3. Schematic of the vapor generator and the stack simulator.
+
the fragment ions on stable trajectories. The rf dc field on Q3 is c h w n so as to transmit the m/z corresponding to the fragment ion of interest. Two 450-L/s turbomolecular pumps are used to differentially pump the triple quadrupole mass spectrometer. The front chamber is maintained at a pressure of ca. 1 X lo4 Torr, while the rear chamber is maintained at a pressure of ca. 1 x Torr. Vapor Generator. In order to test the response of the system to very low levels of contaminants in air, a vapor generator system is used (the vapor generator was supplied by the office of the Program Executive Officer, Program Manager For Chemical Demilitarization, Aberdeen Proving Grounds, Edgewood, MD). Figure 3 shows the main components of such a system. The system consists of a SAGE microsyringe pump capable of driving the solution in a 1-mL gastight syringe at rates of 1-10 pL/min. The output of the syringe is passed through a 100-mmfused silica capillary to a heated nebulizer. A 50 mL/min flow of helium is maintained in this capillary to assist in the delivery of the vapor. The vapor exiting the heated nebulizer is mixed with a carefully controlled stream of air. A flow controller is used to maintain accurately the flow of air at rates up to 20 L/min. Dilute solutions of dimethyl methylphosphonate (DMMP) were made in hexane. These solutions were used in the syringe to achieve the desired fractional concentration of DMMP in air. The fractional concentration in air is given by f = [CS ( 2 2 . 4 ) ] / M F where C is the concentration of the solution used (g/pL), S is the speed of the syringe pump (pL/min), M is the molecular weight of the solute, and F is the volume flow rate of air (L/min) used to mix the solvent vapor. By the appropriate choice of C, S, and F it is possible to accurately generate concentrations at the sub-part-per-trillion level.
.-
1
12000
-
9000
-
6030
-
$ z
3000
-
1
I
I
95
115
IO5
m/z
125
15000
3000
TH+ + n(H2O) T(H,O),H+
+ (n - m)(H,O)
The trace molecular ion intensity is thus split among numerous ions of the type T(H20),H+, m 1 0. This situation produces a complex mass spectrum and is undesirable. Declustering. It is highly desirable to collapse these cluster ions into one abundant mass peak, thereby simplifying the spectrum. Kambara and Kanomata recognized that such a declustering can be achieved by the use of collision-assisted dissociation a t a pressure of ca. 1 Torr (8, IO). In the declustering region such weakly bound cluster ions, which are of the van der Waals type, are easily broken up by collisions with the residual neutral molecules present in the region. Increasing the drift field in this region makes it also possible to fragment the protonated molecular ion of the trace molecule, TH+, thus aiding in its structural identification. This is shown in Figure 4. Although such a fragmentation yields structural information, in practice the declustering region drift field is maintained so that it is strong enough to break up the weakly bound clusters but not strong enough to cause fragmentation of the protonated trace molecular ions. Mass Spectrometer. During evaluation of the atmospheric pressure ionization tandem mass spectrometric system, the first quadrupole is set to transmit m / z corresponding to the protonated trace molecular ion. Collision-assisteddissociation, with argon, is used in the second quadrupole, which is operated in a rf only mode. The third quadrupole is set to transmit m/z corresponding to the most abundant fragment of the trace molecule. The response of the API MS/MS system was tested for varying concentrations of DMMP in simulated stack air. For tests performed on DMMP, Q1 was set to transmit m / z 125 (corresponding to DMMPH+) and Q3 was set to transmit m / t 93 (the most abundant fragment ion in the DMMP CAD spectra). Tests of the system were conducted with DMMP on 4 consecutive days. Each day’s testing consisted of measuring the response of the system to six different concentrations ranging from 5.4 to 135.6 pptr. This was repeated
- 6 HEAT TRACED LINE
15000
+
+
FLOW CONTROLLER
OVEN
+T
1
1
I 95
105
115
125
m/z
Flgure 4. Collision-Induced dissociation in the declustering region. The drift fieid increases from a to c.
95
105
I15
m/z
I25
ANALYTICAL CHEMISTRY, VOL. 61, NO. 3, FEBRUARY 1, 1989
263
Table I.
series 3
series 2
series 1
RSD, %
concn, pptr
counts/s
0.0 5.4 10.9 13.55 54.2 108.5 135.6
36 83 110 131 416 843 1128
3.3 7.2 2.7 2.0 0.6 3.9 1.9
0.0 5.4 10.9 13.55 54.2 108.5 135.6
33 67 125 164 529 1044 1240
1.8 3.3 10.4 2.2 2.8 4.1 3.0
0.0 5.4 10.9 13.55 54.2 108.5 135.6
30 69 126 149 516 948 1325
20.0 9.6 3.0 4.8 2.6 1.0 2.7
0.0
27 66 109 133 452 909 1170
3.4 3.6 9.7 5.9 3.0 3.3 1.8
counts/s
RSD,%
counts/s
RSD, %
12.8 3.7 1.0 2.0 1.2 2.0 2.3
34 87 112 136 418 831 1155
17.6 1.8 6.6 0.5 2.0 4.6 0.2
1.8 6.6 1.8 3.9 7.8 0.8 4.5
34 73 123 146 539 1002 1283
2.3 6.0 4.2 3.7 1.7 0.8 2.9
11.2 4.7 7.0 4.8 3.2 1.5 1.6
29 70 120 143 522 962 1294
2.8 11.1 2.5 2.2 3.3 1.2 1.2
9.7 5.9 5.5 5.1 5.1 3.5 3.3
28 66 114 127 468 883 1185
8.7 7.1 3.1 2.2 7.3 2.8 2.4
Day 1 33 85 108 132 418 839 1118
Day 2 33 66 122 152 505 1024 1216
Day 3 29 73 129 141 503 998 1289
Day 4 5.4 10.9 13.55 54.2 108.5 135.6
three times on each day. This is referred to as series 1, series 2, and series 3 in Table I, which gives a tabular listing of the data acquired on each day. The dwell time of the counting system was set to 5 s. Nine consecutive 5-s counts were averaged to yield one data point, thus yielding three independent data points, for each concentration on each day. The high response of the system, when no DMMP was introduced through the vapor generator, is presumably coming from the DMMP adsorbed on the transfer lines and is not an indication of the system noise. In fact when the mlz transmitted by Q3 is increased by 1 amu, the system response is typically 3-4 counts/s. This is the true indication of the system noise. During each day’s testing no attempt was made to calibrate the system response. The system response to a challenge concentration of 13.55 pptr, on each day’s testing, was chosen as a calibration point to convert the system response (counts per second) to a found concentration (parts per trillion). Table I1 lists the found concentration as a function of target concentration. Following Hubaux and Vos (II), a weighted linear regression analysis of found versus target concentrations were performed to include upper and lower confidence bounds with 95% confidence intervals. From the upper and lower confidence intervals, the limit of detection as well as the decision limit was calculated. The limit of detection (LOD) is defined as the smallest true concentration that will be consistently detected. If the air stream being sampled by the system contains the trace molecule at the LOD concentration, the probability that the system will detect it is at least 95%. The decision limit (DL) is the maximum concentration resulting from the air stream containing no trace molecule of interest. The statistical parameters obtained as a result of such an analysis indicate a limit of detection of 1.15 pptr and a decision limit of 2.8 pptr. The correlation coefficient for such a regression
27 66 124 133 462 912 1166
Table 11. Found Concentration (FC) vs Target Concentration (TC) for DMMP
TC, pptr FC, pptr TC, pptr FC, pptr TC, pptr FC, PPtr 0 0 0 0 5.4 5.4 5.4 5.4 10.9 10.9 10.9 10.9 13.55 13.55 13.55 13.55 54.2 54.2 54.2 54.2 108.5 108.5 108.5 108.5 135.6 135.6 135.6 135.6
0.28 -0.05 0.1 -0.05 6.8 3.5 4.5 5.0 10.6 9.5 10.95 10.5 13.55 13.55 13.55 13.55 54.4 49.5 55.1 54.5 113.4 104.7 103.7 113.2 153.5 125.1 146.3 146.7
0
0 0 0 5.4 5.4 5.4 5.4 10.9 10.9 10.9 10.9 13.55 13.55 13.55 13.55 54.2 54.2 54.2 54.2 108.5 108.5 108.5 108.5 135.6 135.6 135.6 135.6
-0.24 -0.01 -0.05 -0.05 7.1 3.4 5.0 5.1 10.4 9.2 11.3 12.4 13.7 12.3 12.6 13.6 53.8 48.9 53.5 55.9 112.8 102.7 109.4 113.5 151.9 122.6 142.2 146.3
0
0 0 0 5.4 5.4 5.4 5.4 10.9 10.9 10.9 10.9 13.55 13.55 13.55 13.55 54.2 54.2 54.2 54.2 108.5 108.5 108.5 108.5 135.6 135.6 135.6 135.6
-0.05 0.05 0.05 0.05 7.4 4.1 4.6 5.1 10.9 9.3 10.2 11.2 14.3 11.6 12.9 12.8 53.9 52.4 55.7 56.7 111.8 100.5 105.4 109.9
157.3 129.6 142.9 148.8
analysis was 0.995. Detection of DMMP in air using a NiB3 API MS system has been reported earlier (12). This earlier study reported a detection limit of 40 pptr, obtained with an exponential dilution scheme.
284
ANALYTICAL CHEMISTRY, VOL. 61, NO. 3, FEBRUARY 1, 1989
1
, ,
50
, , ,
100
,
,
, ,
150
200
,
,
, ,
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effluent, which is being sampled by the system. As shown in Figure 5, the signal of the system decreases to the background level in less than 15 s. Figure 6 shows a series of tests performed on the system in which the system was challenged with six different concentrations of DMMP in air, with each of the challenges separated by challenges with air containing no DMMP. As can be seen from the figure, the system responds very rapidly to the changes in the concentration and does not show any lagging due to memory effects.
L!
, ,
300
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,
Registry No. DMMP, 756-79-6.
400
350
TIME (SECONDS)
LITERATURE CITED
Figure 5. Time response of the system.
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102.8 ppt
,
77.1 p p t
0
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0
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r
n
0
0
0
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I
(5) Dzidic, 1.; Carroll, D. I.; Stillwell, R. N.; Hornlng, E. C. Anal. Chem. 1976, 48, 1763. (8) Lovett, A. M.; Reid, N. M.; Buckley, J. A.; French, J. B.; Cameron, D. M. Biomed. Mass Spectrom. 1979, 6, 91. (7) Kambara, H.; Kanomata, I . Anal. Chem. 1977, 49, 270. ( 8 ) Brubaker, W. M. Advances h Mass Spectrometry; Elsevier: 1968;
0
0
n
O 0
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n
~
~
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O
l
n
O
r
(1) Hornlng, E. C.; Hornlng, M. G.; Carroll, D. I.; Dzidlc, I.; Stlllwell, R. N. Anal. Chem. 1973, 4 5 , 936. (2) McKeown, M. C.; Siegel, M. W. Am. Lab. 1975 (November), 91. (3) Siegel. M. W.; Fite, W. L. J. phvs. Chem. 1976. BO, 2871. (4) Siegel, M. W.; McKeown, M. C. J. Chromatogr. 1978. 722, 397.
( Y c U r n ~ P P l n l n u )
VOl. 4. (9) Ketkar, S. N.; Fke, W. L. Proceedings of the 35th ASMS Conference on Mass Spectrometry and Allied Toplcs, Denver, 1987; Rev. Scl. Instrum. 1980, 59, 387. (10) Kambara, H.; Kanomata, I . Int. J. Mass Spectrom. Ion Phys. 1977, 25, 129. (11) Hubaux, A.; Vos, G. Anal. Chem. 1970, 43, 849. (12) Mkchum, R. K.; Korfmacher, W. A.; Freeman, J. P. Anal. Instrum. 1986, 75, 37.
TIME (SECONDS) Figure 8. System response to varying concentrations of DMMP in air.
The time response of the system is governed by the sample flow rate and the volume of the ionization region and the associated sampling lines. The time response of the system was measured by abruptly switching off the vapor generator
RECEIVED for review February 2, 1988.
Accepted November 8, 1988. This work was supported by the U.S.Army Office of the Program Executive Officer, Program Manager For Chemical Demilitarization, under Contract No. DAAA15-86C-0107.
