Anal. Chem. 1988, 60,104-109
104
ions from the present solutions is not supported by the FAB spectra.
ACKNOWLEDGMENT We wish to thank Alan Hogg and his co-workers for the use of and help with the MS9 instrument. APPENDIX In the kinetic modeling described in this paper, the actual expansion process is ignored; i.e. [B] was assumed to be constant. In reality the gas expands and, therefore, the “effective” residence time depends on the characteristics of the gas expansion. This can be illustrated with an exponential gas expansion, described by [Bl = P I o exp(-bt) (23) where [E], is the concentration of B in the liquid. Equation 23 may be substituted into eq 8. By solving the linear first-order differential equation, we find [A+] = [A+], exp(-kdBI~/b) exp(-b[BIo exp(-W/b)
(24) which in the limit of large t simplifies to
(25) By comparison with eq 9 it is seen that, in this case, the “effective residence time” is given by t = l/b (26) In general, we expect to find that the largest contribution to the effective residence time is the time that the ions spend in a “gas” with a density quite close to that of the liquid. If this is correct, most of the ion/molecule chemistry takes place during the initial stage of the phase explosion when the density [A+] = [A+], e x p ( - M B I ~ / b )
of the “gas” is still 113 to 1/10 of that of the liquid. Registry No. G1, 56-81-5; DEA, 111-42-2.
LITERATURE CITED (1) Sigmund, P. In Sputtering by Partlcle Bombardment-I, Toplcs In App l M Physlcs; Benninghoven, A. Ed.; Springer-Vetlag: Berlln, 1981;pp 9-71. (2) Pachuta, S.J.; Cooks, R. G. in Desorption Mass Spectrometry: Are SIMS and FAB the Same?; American Chemical Society: Washington DC. 1985:I) 1. (3) Honda, F.: bncaster, G. M.; Fukuda, Y.; Rabaiais, J. W. J . Chem. Phvs. 1978. 6 9 . 4931. (4) Cdoks, R. G.;Busch, K. L. Int. J . Mass Spectrom. Ion Phys. 1983, 5 3 , 111-124. (5) Murray, P. T.; Rabalals. J. W. J . Am. Chem. SOC. 1981, 103, 1007. (6) Lanchaster. G. M.; Honda, F.; Fukuda, Y.; Rabalais, J. W. J . Am. Chem. SOC. 1978, 101, 1951. (7) Michl, J. Int. J . Mass Spectrom. Ion Phys. 1983, 5 3 , 255. ( 8 ) David, D. E.; Magnera, T. F.; Tian, R.; Stulik, D.; Michl, J. Nucl. Instrum. Methods Phys. Res., Sect. B 1988, 8 1 4 , 378-391. (9) Sunner, J. A.; Kulatunga, R.; Kebarle, P. Anal. Chem. 1986, 5 8 , 1312. (lo) Sunner, J.; Morales, A.; Kebarie, P. Anal. Chem. 1987, 5 9 , 1378. (11) Sunner, J. A.; Kulatunga, R.; Kebarle, P. Anal. Chem. 1988, 5 8 , 2009. (12) Sunner, J.; Ikonomou, M.; Kebarle, P. Int. J . Mass Spectrom. Ion Processes, In press. (13)Hogg, A. M. Int. J . Mass. Spectrom. Ion Phys. 1983, 4 9 , 25. (14) Kebarle, P. In Technlques for the Study of Gas-Phase Ion-Molecule Reactlons; Saunders, W. H., Farrar, J. M., Eds.; Wiley: New York; 1988. (15) Davidson, W. R.; Sunner, J.; Kebarle, P. J . Am. Chem. SOC. 1979, 101, 1675. (16) Bennlnghoven, A. Int. J . Mass. Spectrom. Ion Phys. 1983, 4 6 , 459-482. (17) Wllliams, D.H.; Bradley, C.; Bojesen, G.; Santlkarn, S.; Taylor. L. C. E. J . Am. Chem. SOC. 1981, 103, 5700-5704. (18) Field, F. H. J . Phys. Chem. 1982, 8 6 , 5115-5123.
RECEIVED for review May 6, 1987. Accepted September 28, 1987. The work was supported by a grant from the Canadian Natural Sciences and Engineering Research Council.
Detection of Ethylene Glycol Dinitrate Vapors by Ion Mobility Spectrometry Using Chloride Reagent Ions A. H. Lawrence* and Pave1 Neudorfl
Unsteady Aerodynamics Laboratory, National Aeronautical Establishment, National Research Council, Ottawa, Ontario K I A OR6, Canada
The detection of ethylene glycol dinltrate (EGDN) vapors by Ion mobllity spectrometry (IMS) uslng chloride reagent Ions is dlscussed. The addltlon of dichloromethane to the carrier stream leads to Increased specMcity of Ionization and to the formatlon of EGDNeCI- Ions. Two sampling and sample lntroductlon methoddogles are described. The effect of sample concentration, temperature, and matrix composltlon on the sensitivity of response of IMSICI- chemistry to EGDN vapors Is presented.
One of the most effective methods for detecting hidden bombs in either civilian or military situations is through trace analysis of ambient air for characteristic telltale vapors emanating from the explosive material (1,2). The same approach can sometimes be useful in postexplosion investigations to identify the type of explosive that has been used in a bombing incident.
Ethylene glycol dinitrate (EGDN) is the most volatile effluent in dynamite formulations and is also present as a vapor impurity in many explosive products that do not contain it as an ingredient (3). Accordingly, the detection of hidden explosives by using analytical instrumentation is often based on the analysis of ambient air for the presence of traces of EGDN vapors (2). A number of analytical techniques have been employed in recent years including gas chromatography with an electron capture detector (GC-ECD) (2),gas chromatography-mass spectrometry (GC-MS) ( 4 ) ,atmospheric pressure chemical ionization mass spectrometry (APCIMS) (5), and ion mobility spectrometry (IMS) (6-8). IMS, also known as plasma chromatography, is an atmospheric-pressure-based technique that resolves ionic species on the basis of the differences in their mobilities through a gas under an applied electrostatic field (9). Drift time (milliseconds) or ion mobility reduced to standard temperature and pressure KO(cm2V-’ s-l), much as retention time in GC, can be used to identify the trace molecules originating a peak
0003-2700/88/0360-0104$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 2, JANUARY 15, 1988
in IMS. Because of its sub-part-per-billion detection capability,,fast response time (0.1-10 s), and ease of operation, IMS is rapidly gaining acceptance as a chemical sensor for the detection of trace compounds in the ambient environment. The apparatus has been designed as a compact hand-held (10) and man-portable (11) detector. In IMS, the sample, in vapor form, is introduced into a reaction chamber by means of a carrier gas (N2, air). Ionization of reactive trace impurities in the carrier gas, such as H20 and NH3, is generally brought about by energetic electrons released from a 63Ni radioactive source with the formation of a number of positive and negative reactant ions, e.g., (HzO),H+ and (H20),0z-; the value of n depends on the concentration of the water vapor. These ions undergo a complex series of ion/molea.de reactions with the anal@, and product ions are produced. Compounds with high proton affinity form positive product ions through proton (or NH4+ and NO+) exchanges, and those containing atoms with high electron affinity, such as halogen, oxygen, and nitrogen, form negative product ions through electron attachment and other mechanisms (11). Organonitrates such as EGDN and nitroglycerin (NG) are electronegative in character, and due to the large cross section offered by these compounds to charge transfer and ion/ molecule reactions, negative IMS is particularly suited for their detection and analysis. However, EGDN and NG undergo dissociative electron capture and do not produce molecular anions in the IMS, but instead produce an ion peak with a reduced mobility of 2.46-2.48 cm2 V-' s-l corresponding to the fragment NO, ion ( 6 8 ) . This peak overlaps significantly with the peak associated with the reactant ions when air or nitrogen is used as the carrier gas, thus restricting its usefulness as an IMS marker for identification purposes. I t is well-established that the selectivity of ionization in atmospheric pressure chemical ionization as well as in conventional chemical ionization mass spectrometry is dependent upon the makeup of the reagent gas. Different ionization characteristics may be obtained by adding trace amounts of various reagent gases. Jennings et al. (12) have examined the negative chemical ionization mass spectrometry (NCIMS) of EGDN and other organonitrates by using various reagent ions including the chloride ion (Cl-). They showed that C1- attachment to EGDN parent molecule occurs readily with the formation of a new abundant ion (EGDNC1)- of mass m / z 187. The IMS behavior of EGDN with reagent ions such as C1and Br- has been reported by Todd and Proctor (13). They showed that the addition of C1- to the carrier stream led to greater selectivity of ionization and better resolution in the mobility spectrum; at a drift tube temperature of 50 "C and EGDN concentration of 100 ppb, all sample ionization resulted in one ionic species, EGDNaCl-. In an independent investigation, using negative IMS and an inlet temperature of 227 "C, Spangler et al. (14)demonstrated that the NO3peak form EGDN can be resolved from the peak associated with the reactant ions when chloride ions are added to the system. This paper presents the results of a n assessment of the sensitivity of IMS/Cl- ion chemistry to EGDN vapors. The limit of detection and the effect of temperature on the formation of the EGDN-C1- ion cluster have been characterized. Effective sampling and sample introduction methodologies have been developed. The prospective values of IMS/Cl- ion chemistry in the detection of EGDN vapors in a preconcentrated ambient air sample have also been investigated. N
EXPERIMENTAL SECTION Reagents and Chemicals. EGDN was synthesized in this laboratory according to a procedure involving nitration of ethylene
105
DIFFUSION 4 0 ccpm FLOW RESTRICTOR
1 EGDN SAMPLE,
VENl
0°C
Figure 1. Double-dilution vapor source.
glycol with an acid mixture [HNO3/H2SO4(7:3)]. Standard EGDN solutions were prepared as 4 X g/pL in hexane. Forcite 40, a commercial dynamite containing EGDN and NG, was obtained from Canadian Industries, Ltd. (CIL), Montreal, Quebec. Chromosorb 750 with 3% OV-17 (80-100 mesh) was obtained from Chromatographic Specialties (Brockville, Ontario). Linde ultrahigh-purity nitrogen was dried by Linde molecular sieve 13X and used for both carrier and drift gases. Dichloromethane and n-hexane were distilled-in-glass grade (Caredon, Georgetown, Ontario). Adsorption Tubes. The adsorption tubes used in this study were constructed of nickel tubing, 10-cm length X 0.3-cm 0.d. X 0.22-cm i.d., and contained about 30 mg of sorbent material (3% OV-17 on Chromosorb 750) held in place with two fine-mesh platinum disks. The tubes were conditioned for several minutes at 150 "C in a nitrogen stream at a flow rate of 200 mL/min. Before use an IMS trace was recorded to check the cleanliness of the adsorbent. Vapor Source. A dynamic gas blending system was used to generate known and controllable concentrations of EGDN vapor in purified air or nitrogen. The double-dilution vapor source used in this study was an upgraded version of the single-stage device reported previously from this laboratory (15, 16) and is shown schematically in Figure 1. A small stream of air (fl) was saturated with EGDN vapor by passing it through a sample of EGDN in a glass tube thermostated at 0 "C; the carrier stream was then diluted in two stages by large air streams to produce any EGDN concentration within the range 0.5-1000 pptr (parts in as required. The vapor source can deliver anywhere from 1 to 20 L/min. The actual EGDN concentration in the air stream was confirmed by GC-ECD analysis of air samples collected in adsorber tubes and based on calibration with standard solutions of EGDN (2). Reproducibility of EGDN concentrations from the vapor source was within *lo%. The measured values were in close agreement with the vapor pressure data of BGDN at 0 "C published by the National Bureau of Standards (17). In one set of experiments involving chloride ion chemistry (see below), nitrogen was used as the diluent and carrier streams, and dichloromethane was added in trace amounts to the diluting stream (Fz)by means of a diffusion tube and an independently controlled flow (Figure l),thus allowing the concentration of EGDN and that of CHzClzto be varied separately. Instrumentation. The IMS data presented in this paper were obtained with a Phemto-Chem 100 ion mobility spectrometer (PCP, Inc., West Palm Beach, FL). The instrument has a membraneless inlet and has been recently described (18). The signal from the IMS was averaged with a Nicolet 1170 signal averager and the spectra were recorded with a Hewlett-Packard 7010B X-Y recorder. The analog-to-digital converter (ADC) resolution used with the signal averager was always 12 bits and the full-scale volt setting was kept at f4 V. The ion peak amplitude was used as a quantitative measure of specific ions. Mass identification of ions giving particular mobility peaks was obtained with a Phemto-Chem MMS-160 ion mobility spectrometer/mass spectrometer (IMS/MS) (19). The experimental parameters used to operate the IMS and the IMS/MS are presented in Table I. Sample Introduction Techniques. Two sampling procedures were used in this study. In one series of experiments, the IMS was operated as a "Sniffer" and was allowed to sample a continuous nitrogen stream containing known concentrations of
106
ANALYTICAL CHEMISTRY, VOL. 60, NO. 2, JANUARY 15, 1988 PUMP ( 8 5 0 m l / m i n )
:
DRIFT REGION
1-20 SENSOR CELL HOUSING AND HEATER 600ml/min
Figure 2. Continuous detection of EGDN.
Table I. Instrument Parameters parameter
( 3 % OV-17 ON C H R O M O S O R E 7501
value
Ion Mobility Spectrometer (Phemto-Chem 100)
I1- Z O L I m i n a i r l
PUMP
(200c c l m i n )
drift length drift voltage carrier gas (purified nitrogen spiked with dichloromethane) drift gas (purified nitrogen) inlet and drift temperature pressure dwell time gate width delay time'
8 cm
-3000 V 250 mL/min
FLOWMEW
600 mL/min 75 "C
atmosphere 20 pslchannel 0.2 ms 6 ms
N2 C A R R I E R
+---
HEATED INLET OF I M S
2 -1OccImin
Ion Mobility Spectrometer-Mass Spectrometer (Phemto-Chem MMS-160)* drift length (between grid G, and IMS collector) drift voltage carrier gas (purified air spiked with dichloromethane) drift gas (purified air) inlet and drift temperature gate width dwell time delay time mass spectrometer pressure scanning speed
10 cm -3214 V 200 mL/min
41
CH2CIZ
Figure 3. Collection of EGDN vapors in adsorbent tubes.
-
KO 166 133 0 m V I IEGDNICI-
600 mL/min 15 "C 0.2 ms 25 ps/channel 6.4 ms 4X Torr 1000 amu/s
-
)\,a,,, K O = 1.66 115 8 m V i
OThe time between gate opening and start of data collection.
100 pot
* IMS-MS exDeriments were conducted at PCP Laboratories.
EGDN. A diaphragm pump (Fisher Scientific Instruments) was connected to the gas exhaust port of the instrument and suction was applied at the rate of 850 mL/min. The drift gas (N,) was supplied at the rate of 600 mL/min. When the vapor source was placed in front of the inlet of the IMS (Figure 2), 250 mL/min was drawn in the instrument and the excess spilled out to the atmosphere. In a second set of experiments, purified air was used as the carrier and diluent streams in the vapor s o m e and sampling was performed by means of a suction probe (20)remote from the IMS unit. Samples were collected from the vapor source at a sampling flow rate of 200 mL/min and the sampling time was varied over 1-4.5 min to collect various amounts of EGDN on the traps. The collector probe was subsequently inserted in the heated inlet of the IMS, which, unless indicated otherwise, was kept at 75 "C; EGDN evaporated immediately and was flushed with the carrier gas stream (nitrogen spiked with dichloromethane) to the ion reaction chamber of the instrument (Figure 3). Gas Chromatography. Gas chromatographic experiments were performed with a portable microprocessor-controlled GC which was designed and constructed in-house. The instrument is equipped with an electron capture detector, a six-port switching valve, and a two-stage thermal desorption system. The general description and operation of the GC have been described elsewhere (2).
K O = 166 I3 3 m V i
30 DP!
11 TRACES OF AIR REACTANT IONS K O = 2.67
REACTANT IONS lH201nCI-
/
I+ VERTICAL DISPLAY SCALE IVDSI
~
16K: 266 SCANS
111
6
26 16
DRIFT TIME ImUcI
Figure 4. Effect of EGDN concentration on IMS signal amplitude: (trace a) system blank, carrier gas N,, 256 scans, vertical display scale CH,CI,, 256 (VDS) = 16K;(trace b) system blank, carrier gas N, scans, VDS = 16K;(traces c-f): carrier gas N, CH,CI,, 512 scans, VDS = 4096.
+
+
Continuous-Action Preconcentrator. The continuous-action preconcentrator (CAP) was developed in this laboratory (21) and is used to enhance the concentration of gases and vapors present
ANALYTICAL CHEMISTRY, VOL. 60, NO. 2, JANUARY 15, 1988 (el
107
125°C (1000 ppt EDGN)
KO = 1.84
KO = 1.67
100°C (1000 ppt EDGNI
j
7
KO = 1.66
(EGDN.CI)REACTANT IONS
(1000 ppt EGDN) 256 SCANS (b) V D S = 3 2 K
\.
\
TRACES OF A TRA& AIR IR (a)
VDS=32K
/ J \
-
~~
6
26.48
DRIFT TIME (mrec)
Figure 5. Effect of temperature on the stability of (EGDN.CI)- ion cluster. IMS parameters: 512 scans; VDS = 4096.
in trace quantities in the atmosphere (or some other gaseous medium) to such level that they may be more readily analyzed by appropriate analytical instrumentation. The CAP is based on an adsorption-thermal desorption technique, a proven preconcentration method in trace vapor analysis. The novel feature of this device is that where, conventionally, discrete volumes of air are scrubbed of the target vapors by a stationary bed of adsorbent material, so that the process is discontinuous and time-consuming, the CAP achieves a similar concentrating effect on a continuous and nearly real-time basis; a vapor enrichment factor greater than lo3was obtained in less than 5 s when traces of EGDN vapor were presented at the inlet of the instrument (2, 21).
RESULTS AND DISCUSSION Continuous Detection of EGDN. In this series of experiments, the IMS was operated as a sniffer and was allowed to sample a continuous stream of purified nitrogen spiked with EGDN; Le., sensitivity tests in this mode of operation were limited to vapor source sampling only, since in the absence of a membrane inlet the water vapor in an ambient air sample causes the reactant ion peak [ (H20),Cl-] to vary erratically. Furthermore, in all experiments involving chloride ion chemistry, the concentration of CH2C12was increased gradually until the peak for free electrons (usually observed in negative IMS when nitrogen is used as a carrier gas) was no longer obervable and only the peak corresponding to (H20),C1- ions could be detected. Under these optimal conditions, the minimum detectable concentration at a signal amplitude of 3 times the l o noise was 30 pptr, corresponding to a mass flow of EGDN of 4.7 X g/min. The negative IMS spectrum exhibited a well-resolved major ion peak at a reduced mobility KOof 1.66 cm2 V-' s-l with a peak width a t half height of 0.4 ms, corresponding to EGDN-Cl- of mass m / z 187 (see below). As the concentration of EGDN in the test stream was increased, the amplitude of the peak corresponding to EGDN-Clincreased proportionally (Figure 4). No attempt was made to determine the linear working range. Effect of Temperature. Figure 5 illustrates the effect of temperature on the sensitivity of response of the IMS/Cl-
22 4
mu^
/ONOj
lcl
8.4
mi2 187
DRIFTTIME IrnmcI
32.0
Figure 6, Negative ion mobility spectra and mass-identified ion mobility spectral data for dynamite.
chemistry to EGDN vapors. In this experiment, the vapor source was set to deliver 1000 pptr of EGDN in nitrogen (Figure 1))and ion mobility spectra were recorded as a function of temperature over the range 50-125 "C. The spectra were acquired only after the signal became stable, and 512 scans were averaged for each spectrum. At a frequency of 41 Hz, signal averaging lasted about 10 s while the sample was introduced continuously into the IMS. As expected, increasing the temperature has the effect of increasing the mobility of the ions through the drift gas. The drift time of the EGDN-Cl- ion cluster decreases, whereas the value of KO remains constant; however, there is a significant decrease in sensitivity, e.g., at 125 OC, no peak corresponding to EGDNClis observed at an input concentration of EGDN equivalent to 1000 pptr. This could be due to the decomposition of the sample in the inlet of the IMS or to fragmentation of the EGDN-C1- cluster ion. IMS-MS Experiments. In this series of experiments, the headspace vapors from a commercial dynamite sample were
108
ANALYTICAL CHEMISTRY, VOL. 60, NO. 2, JANUARY 15, 1988
/U
ic I
1
L
I
.
L I 3U2
I
I
ION MASS ( a m u l
250
‘C 26 2
ID)
264
-
n*
(0)
L
I
EGON.CI189 r*
I
I
I
ION MASS (ornu)
I50
201.2
Figure 7. Mass spectral data of a dynamRe sample collected at small chloride ion concentrations (traces a and b) and large chloride ion concentrations (trace c).
collected on a stainless steel wire, which was subsequently inserted in the heated inlet of the instrument. One to two microliters of dichloromethane was injected directly with a syringe, which produced a strong and persistent chloride ion peak. Figure 6a illustrates the total negative IMS spectrum of a dynamite sample in the presence of chloride ions. In addition to (H,O)Cl- (m/z 53,55) and (H20),N03- ( m / z 62, 80), two intense well-resolved peaks are observed at KO1.66 and 1.47 and correspond to EGDNCI- ( m / z 187, 189; ratio 3:l) and NG-C1- ( m / z 262, 264; ratio 3:1), respectively. The ion of m / z 289 (NG.N03-) is responsible for the small peak a t KO = 1.40, the NO3- ions being formed by dissociative electron capture from the organonitrate molecules. As the concentration of the chloride ions in the system decreases, the intensity of the NG*N03-peak increases (Figures 6b and 7b). Reintroducing dichoromethane in the system has the effect of reducing the intensity of the NgN03- ion while enhancing
the intensity of the peaks corresponding to the chloride ion adducts (Figure 7c). The mass identified mobility data shown in Figure 6c clearly demonstrates that the major ion contributing to the IMS peak at KO = 1.66 cm2 V-‘ is m / z 187 corresponding to EGDN-C1-. In this mode of operation, the MS can filter a single mass while the drift time of the corresponding ion is measured. Since the detector sees only one ionic species, the mobility spectrum consists of only that peak corresponding to the ion (19). Remote Vapor Collection. In order to test the efficacy of the system in “real” air, different vapor collection and sample introduction techniques were used. “Real air” in this context was ambient air of the workplace environment with no EGDN vapor. In this mode of operation, EGDN vapors were collected from the vapor source by means of a suction probe containing an adsorbent material. Following sampling, the probe is inserted in the heated inlet of the IMS. The adsorbed chemicals are removed by heating and flushed with the carrier gas stream (nitrogen spiked with dichloromethane) to the ion reaction chamber of the instrument (Figure 3). The desorption temperature, 75 OC, was low enough to prevent decomposition of EGDN while providing a desorption time that was sufficiently short to allow for 10-20 s response time. When the above vapor collection and sample introduction techniques are used, the minimum detectable quantity at a signal amplitude of 3 times the l a noise was 30 pg; however, superimposing a 30-pg sample of EGDN from the vapor soure (source strength, 5 pptr; sampling rate, 200 mL/min; sampling time, 4.5 min) on an equivalent room air sample, or vice versa, has the effect of degrading the signal to noise ratio. In the absence of chloride ions in the IMS, autoionization and dissociative electron capture of EGDN occur and result in the formation of NO2- and NO3-, which then act as reagent ions and attach to the parent molecule to give (EGDNeNOJand (EGDN-N03)- (13).With the use of the remote sampling method and the sample introduction technique described above, the minimum detectable quantity of EGDN determined
f
I
6
/
T
VDS = 4096 1024 SCANS
/
t 6
I
I
I
I
DRIFT TIME (rnrec)
I 26.48
Flgure 8. Detection of EGDN in a complex gaseous matrix: (a) test of adsorbent cleanliness; (b) detection of 120 pg of EGDN from the CAP; CH,CI,; inlet temperatures 75 OC. (c) detection of 130 pg of EGDN from a purified alr stream. IMS parameters: carrier gas N,
+
ANALYTICAL CHEMISTRY, VOL. 60, NO. 2, JANUARY 15, 1988
as (EGDN.N03)- (KO= 1.60 cm2V-' s-l) was found to be 500 pg, as compared to 30 pg for the IMS/Cl- chemistry. The procedure involving vapor sampling by adsorption followed by thermal desorption directly in the IMS offers numerous advantages, such as elimination of the requirements for a membrane inlet thus preventing the continuous suction of unwanted chemicals into the system, higher sensitivity compared to IMS equipped with a membrane inlet (a 102-104 loss of vapor concentration through membrane systems has been reported (22)),and cleardown from overloading can be as low as 30 s. Furthermore, vapor collection by means of a detachable sampler remote from the analyzer has proven to be more useful in an actual search for hidden bombs (2). Interferences in Atmospheric Detection. A series of experiments were designed and aimed at investigating the effect of matrix composition and sample size on the response of IMS/Cl- chemistry to EGDN vapors. With the adsorbent tubes described above, a 10-mL sample was collected from the continuous action preconcentrator (CAP) and subsequently analyzed by IMS and chloride reagent ions. In these experiments, the CAP input was a 60 L/min ambient air stream spiked with 5 pptr EGDN. The CAP output was a 30 mL/min nitrogen stream with an EGDN concentration of 1700 pptr as established by GC-ECD. Figure 8b is a representative total ion mobility trace and shows a complex spectrum with a large number of broad and unresolved peaks. More importantly, the response to the ion of interestEGDN.Cl--was erratic and, in some cases, was eliminated entirely. This is probably due to competitive formation of chloride addition product ions by reaction of the chloride ions with interfering compounds present in the complex gaseous matrix of the CAP. The IMS/Cl- chemistry has been shown to be a specific, rapid, and sensitive technique for detection of EGDN vapors in a purified air sample. However, the system is very sensitive to matrix composition, and efforts are currently being made to improve the interfacing of the continuous action preconcentrator to the IMS.
ACKNOWLEDGMENT We thank Robert M. Stimac of PCP, Inc., West Palm Beach, FL, for assistance in obtaining IMS-MS data, and Dr.
109
L. Elias for helpful discussions throughout the study. Registry No. EGDN, 628-96-6; CH2C1,, 75-09-2.
LITERATURE CITED Elias, L. "Development of a Portable Gas Chromatograph Explosives Detector"; LTR-UA-57, National Research Council Canada, Jan 1981. Neudorfl, P.; Ellas, L. J. M e r g . Mater. 1986, 4 , 415. Cartrlght, N. S.,personal communication. Drew, R. C.; Stevens, C. M. "A Man Portable GC-MS for Explosives Detection"; Proceedings of the International Symposium on the Anaiysls and Detection of Explosives, Quantlco, VA, March 29-31, 1983; p 419. Davidson, W. R.; Fulford, J. E. "The Instantaneous Detection of Explosives by Tandem Mass Spectrometry"; Proceedings of the International Symposium on the Analysis and Detection of Explosives, Quantlco, VA, March 29-31, 1983; p 409. Asselln, M. "Detection of Dynamite with a Plasma Chromatograph"; Proceedings-New Concepts Symposium and Workshop on Detection and Identification of Explosives, Reston, VA, Oct 20-Nov 1, 1978; p 177. Wernlund, R. F.; Cohen, M. J.; Klndel, R. C. "The Ion Mobility Spectrometer as an Explosive or Taggant Vapor Detector"; ProceedingsNew concepts Symposium and Workshop on Detection and Identification of Explosives, Reston, VA, Oct 20-Nov 1, 1978; p 185. Spangler, G. E.; Carrlco, J. P.; Kim S. H. "Analysis of Explosives Residues with Ion Mobility Spectrometry"; Proceedings of the International Symposium on the Analysis and Detection of Explosives, Quantico, VA, March 29-31, 1983; p 267. Plasma Chromatography;Carr, T. W., Ed.; Plenum: New York, 1984. Biyth, D. A. Proceedings of the International Symposium on Protection Against Chemical Warfare Agents, Stockholm, Sweden, June 6-9, 1983; p 65. Carrico, J. P.; Davis, A. W.; Campbell, D. N.; Roehl, J. E.; Sima, G. R.; Spangler, G. E.; Vora. K. N.; White, R. J. Am. Lab. (Fairfield, Conn.) 1986, 18, 152. Bouma, W. J.; Jennlngs, K. R. Org. Mass. Spectrom. 1981, 76(8), 331. Proctor, C. J.; Todd, J. F. J. Anal. Chem. 198g35 6 , 1794. Spangier, G. E.; Carrico, J. P.; Campbell, D. N. J . Test. Eva/. 1985, 73(3), 234. Krzymien, M.; Ellas. L. J. Phys. E 1976, 9 , 584. Ellas, L., unpublished results. Pella, P. A. Anal. Chem. 1976, 4 8 , 1632. Rokushlka, S.; Hatano, H.; Hill, H. H., Jr. Anal. Chem. 1986, 58, 361. Lawrence, A. H. Anal. Chem. 1986, 5 8 , 1269. Lawrence, A. H. Forenslc. Scl. Int. 1987, 34, 73. Elias, L. U S . Patent Application No. 850 524, Nov 4, 1986. Spangler, G. E. "Membrane Technology in Trace Gas Detection" Report 2083, U.S. Army Mobility Equipment Research and Development Center, Fort Belvolr, VA, 1973.
RECEIVED for review July 20, 1987.
Accepted September 25, 1987. This paper was presented at the 69th Canadian Chemical Conference, Saskatoon, Saskatchewan, Canada, June 1-4,1986.
