Anal. Chem. 1893, 65, 299-306
Portable Hand-Held Gas Chromatography/Ion Mobility Spectrometry Device A. Peter Snyder’ and Charles S. Harden
U.S.Army Edgewood Research, Development and Engineering Center, Aberdeen Roving Ground, Maryland 21010-5423 Alan H. Brittain Graseby Ionics, Ltd., Watford,Herts, United Kingdom
Man-Goo Kim, Neil S. Arnold, and Henk L. C. Meuzelaar University of Utah, Salt Lake City, Utah 84112
The concept of a portable gas chromatographyAon mobility spectrometry (GWIMS) device Is introduced. The potential of the GC/IMS unit Is investigated for the separation and characterlzatbnof vapor mtxturesof various chemlcaiclasses. Parameters such as internal cell pressure, GC column flow rate, and column temperature were varied to determine the effects on speed and resolution for separating and characterizing mixtures. I t was generally found that by reducing both the internal IMS cell pressure and the isothermal GC column temperature, the peak widths, retention times, and peak overlap could be varied for different classesof analytes. The GCAMS system shows versatilityinthe various compound classesthat can convenientlybe analyzed by a hand-portable version. Mixtures Included phosphonates, phosphates, alkyl ketones, and chiorophenois with total separation times in the 7-s to Pmin t h e range. Positive or negative ion polarities in IMS were used depending upon the functional group.
INTRODUCTION Novel detection and monitoring applications for environmental chemistry (waste incineration site remediation, fugitive emissionscontrol),law enforcement (drug interdiction, explosives detection), and military uses (mobile engagement, stockpile reduction, treaty verification) are generating a growing demand for “field-portable” analytical instruments. Certain emerging problems in field analyses can no longer be satisfied with transportable equipment, e.g., mobile laboratories, and man-portability or (preferably) hand-portability for a fully operational instrument is required. This places strict limitations on weight, size, power consumption, ruggedness, maintainability, and user convenience. In addition, several such applications exact strenuous demands on sensitivity, specificity,and speed. Field-portable gas chromatographs meet some of these requirements, though are often inadequate in specificityand speed. Spectrometrictechniques tend to be faster and more specific than chromatographic methods, and considerable interest exists in the development of field-portable spectrometric instruments. One such promising technique is ion mobility spectrometry (IMS). Modern analytical IMS is an atmospheric pressure ionization method for detection of ~apors.l-~The primary ionization event is usually effected by a 63Ni( p radiation) ion source. A series of ion-molecule reactions ensue involving (1)Karasek, F. W.ResearchlDeuelopment 1970,21,34-37. (2)Cohen, M. J.; Karasek, F. W. J. Chromatogr. Sci. 1970,8,330-337. Keller, R. A.J.Chromatogr. Sci. 1972,10,626-628. (3)Karasek, F. W.; 0003-2700/93/0365-0299$04.00/0
primary, background, and analytevapors, and an equilibrium mixture of ions is established. The ions are electrically injected into a drift region and move under the influence of an electric field through a reverse flow of buffer gas (air or nitrogen) until collision occurs with a Faraday cup detector. Charged ionic species move primarily accordingto their mass and shape (size) and are characterized by their drift time (typically several milliseconds) or ion mobility. Since the entire process, from vapor sampling to the detection event, takes place at ambient or near-ambient pressure, highly sensitiveatmospheric pressure ionization chemistry underlies the ion formation processes. St.Louis and Hill4have provided a comprehensiveoverview of IMS in terms of theory, ionization chemistry, sample introduction device and applications, and Eiceman6 provided a comprehensive review which concentrates on IMS instrumentation. Within a few years of the inception of IMS,lp2hyphenated variations including GC/IMS were des~ribed.~~B-~O The simplicity in design and sensitivity of IMS were the primary motivations in the development of GC/IMS technology. The initial investigations with packed GC columns were soon superseded with capillary column s t u d i e ~ . ~ J ~Marked -~3 improvements in GC/IMS occurred during the last decade and involved improved designs on the IMS ion source, drift cell, and position of the GC column and gas flows in the IMS.4,11,14-16 In a recent review,16 Hill attributed the low numbers of routine applications of IMS to the ease of sample overload and to the complexity and uncertainty of ion mobility spectra from competing charge exchange chemistries. Generally, this phenomenon results in poor linearity and a limited dynamic range. However, IMS of single-component analytes can (4)St. Louis, R.H.; Hill, H. H., Jr. CRC Crit. Rev. Anal. Chem. 1990, 21,321-355. (5)Eiceman, G. A. CRC Crit. Rev. Anal. Chem. 1991,22,471-490. (6)Karasek, F. W.Znt. J. Enuiron. Anal. Chem. 1972,2,157-166. (7)Karasek, F. W.;Denney, D. W. Anal. Lett. 1973,6,993-1004. (8)Cram, S.P.;Cheder, S. N. J.Chromatogr. Sci. 1973,11,391-401. (9)Karasek, F. W.; Kim, S. H. J. Chromatogr. 1974,99,257-266. (10)Karasek, F. W.; Hill, H. H., Jr.; Kim, S. H.; Rokushika, S. J . Chromatogr. 1977,135,329-339. (11)Baim, M. A,;Hill, H. H.,Jr. Anal. Chem. 1982,54,38-43. (12)Baim, M. A.;Schuetze, F. J.; Frame, J. M.; Hill, H. H., Jr. Am. Lab. 1982,14, 59-69. (13)Baim,M. A.;Hill, H. H., Jr. J.HighRes. Chromatogr.Chromatogr. Commun. 1983,6,4-10. (14)St. Louis, R. H.; Siems, W. F.; Hill, H. H., Jr. LC-GC 1988,6, 810-814. (15)St. Louis, R.H.; Siems,W.F.;Hill, H. H., Jr.J. Chromatogr. 1989, 479, 221-231. (16)Hill, H. H., Jr.; Siems, W. F.; St. Louis, R. H.; McMinn, D. G. Anal. Chem. 1990,62,1201A-1209A. 0 1993 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 3, FEBRUARY 1, 1993
carder gar cartridge
heater p.6.
valve p.r.
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Figure 1. Overview of GCICAM modules.
produce linear quantitative information over a limited dynamic range. The recent introduction of a rapid, ambient vapor sampling (AVS) technique compatible with high-speed short GC capillary columns operating at subambient outlet pressures, e.g., in combination with mass spectrometry,17suggested the feasibility of hand-portable, combined GC/IMS instruments. This approach to field instruments (i.e. applied to handportable, hyphenated analytical instrumentation18)was demonstrated by Meuzelaar et al.lszz The inlet of a commercial hand-held IMS device was interfaced to a small isothermal GC oven with an AVS type inlet.1+z2 Full utilization of available information was made with two-dimensional data including GC retention time and ion mobility spectra. A highspeed sampling AVS inlet enhanced the performance characteristics of short-column GC through careful control of the sample injection pulse (or the amount of sample entering the IMS). The ionization region and drift cell of the IMS device were operated at 50-8096 of ambient pressure so high linear velocities for the carrier gas (He, Nz, or air) would be attained. This was necessary to achieve a short analysis time and adequate GC separation while maintaining high sensitivity and favorable detector response characteristic^.^^ K a r a ~ e kin, ~the ~ first critical review of IMS, observed that ‘...the most important development needed to make the (IMS) method practical for GC detection is to develop an instrumental package of reasonable cost and simplicity”. In this study the main principles involved in a hand-held version of GC/IMS technology are reported. Also, some of the operational characteristics in both positive and negative ion modes are explored in the analyses of mixtures of various classes of compounds of military and environmental importance.
EXPERIMENTAL SECTION Instrumentation. The main modules of the transfer line GC/IMS system (schematically depicted in Figure 1) are (1) automated vapor sampling (AVS) inlet module; (2) capillary transfer line gas chromatography (TLGC) module; (3) GC/IMS interface module; and (4) modified hand-held Airborne Vapor Monitor (AVM-GrasebyIonics, Ltd., Watford, U.K.) type IMS system. The modules are described below. (17) Arnold, N. S.; McClennen, W. H.; Meuzelaar, H. L. C. Anal. Chem. 1991,63,299-304. (18)Hirschfield, T . Anal. Chem. 1980,52, 297A-312A. (19) Snyder, A. P.; Kim, M.; Meuzelaar, H. L. C. Program, Eighteenth Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies and the Thirtieth Pacific Conference on Chemistry and Spectroscopy, October 6-11, 1991, Anaheim, CA, Paper No. 315. (20) Direct Air Sampling, Integrated GC/IMS Device, H. L. C. Meuzelaar, Patent Disclosure DAM 373-91, January 1991. (21)Development and Testingof an Integrated Gas Chromatography/ Ion Mobility Spectrometry System; H. L. C. Meuzelaar, U.S. Army Research Office contract Final Report, DAAL03-86-D-0001,November 1990. (22) Snyder, A. P.; Harden, C. S.; Brittain, A. H.; Kim, M.-G.; Arnold, N. S.; Meuzelaar, H. L. C. Am. Lab. 1992,24 (15),p 32 B-H. (23) Meuzelaar, H. L. C.; Urban, D. T.; Arnold, N. S.Proceedings of the Second International Symposium on Field Screening Methods for Hazardous Wastes and Toxic Chemicals;US. Environmental Protection Agency; Las Vegas, February 1991; p 289. (24) Karasek, F. W. Anal. Chem. 1974,46, 710A-720A.
Air Sampling Inlet Module. Design, construction, and operation of the AVS inlet module, developed at the University of Utah (U.S. patent 4,970,905) have been described previously in this journal.” For the purpose of this discussion, the AVS module may be regarded as a continuously variable, valveless flow switch (operated by pressure control) which normally keeps the inlet of the capillary column protected from the atmosphere with a small flow of helium and can be opened for time intervals continuouslyvariable from 20 ms and upward. The AVS module can be heated to several hundred degrees and presents only inert quartz surfaces to the sample flow. Capillary Gas Chromatography Module with IMS Interface. This module consistsof a heated cylindricalaluminum housing which functions as an isothermal GC oven and a 2-3m-long fused silica capillary GC column (250- or 320-rm i.d.1 coated with a suitable stationary phase. Temperatures as high as 300 “Ccan be achieved and are monitored with a thermocouple. In order to make use of the existing inlet nozzle connector design of the AVM while achieving maximum rigidity and support for the GC module, a special mechanical interface module was designed. Ion Mobility Spectrometry Unit. A modified AVM was used. As shown in Figure 2, the conventional inlet nozzle, silicone membrane, and membrane heater unit were removed. One side of the pneumatic pump unit was disconnected from the inlet and used instead as a “vacuum” pump capable of achieving subambient pressures as low as 450 Torr. The subambient pressure was needed to maintain a pressure gradient across the capillary GC column sufficient to provide an adequate carrier gas flow in the column. The need for a separate carrier gas cartridge can be eliminated by using scrubbed exhaust air. This can be achieved by removing the carrier gas supply in Figure 2 and connecting the vent outlet to V3 through a charcoal absorption cartridge. All spectra were recorded using a 286 or 386type IBM compatible PC equipped with Graseby Ionics Advanced Signal Processing (ASP) board and software. Environmental Vapor Monitor Prototype. A hand-held, portable prototype, gas chromatograph-ion mobility spectrometer, the Environmental Vapor Monitor (EVM), based on the designs and characteristics of the system illustrated in Figure 2, was constructed by Graseby Ionics, Ltd., Watford, Herts, U.K. The sample injector assembly was heated independently of the column. The 3.2-kg unit is similar in size to the AVM. Procedures. GCIIMS Design and Construction. Separate GC/IMS elution profiles were produced for each individual compound so as to identify them by their GC retention time/ion drift time pair value in mixture analyses. Analyses of both individual constituents and separations of mixtures by GC/IMS occurred under identical conditions. To illustrate the analytical improvements that can be realized by the use of the hybrid system,IMS experiments were performed on mixtures of vapors. The pulsed valveless sample inlet to the GC column was manually kept open until the ion mobility spectrum was constant and unchanging. Under this condition the GC column was in equilibrium with the vapor mixture, and the IMS yielded a spectral response to the compound vapor mixture without any chromatographic separation. In this instance, the GC served as a nonspecific transfer line. The pulsed, valveless miniature GC system was used in all studies involvingthe phosphonate, phosphate/phosphonate, and ketone mixtures. For analyses of the latter two mixtures, the GC systemwas interfacedto an AVM unit. The phosphonate mixture was analyzed with the GC system interfaced to an “exploded” version of the AVM. All operational parameters of this nonportable GC/AVM version were identical to that of the portable, hand-held GC/IMS device. The AVM system was at an ambient temperature for all experiments. Phosphonate Mixture. A mixture was prepared containing equal volumes of four liquid phosphonate compounds: dimethyl methylphosphonate (DMMP), diethyl methylphosphonate (DEMP), diisopropyl methylphosphonate (DIMP) and diethyl ethylphosphonate (DEEP). The headspace vapor was diluted in an airstream by a factor of about 250, and experimental conditions are listed in Table I. A 2-m X 320-pmi.d.DB-1column was used. IMS operating parameters were as follows: 16 waveforms integrated per spectrum;gating pulse repetition rate, 40 Hz; gating pulse width, 180 ps. Ambient pressure was 740
ANALYTICAL CHEMISTRY, VOL. 85, NO. 3,FEBRUARY 1, lQQ3 301
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microvahres. Table I. GC and IMS Experimental Conditions for Mixture Separations GC sample carrier gas flowrate, flowrate, pulse mixture temp, O C mL/min mL/min time, s phosphonates 90 60 He, 15 0.6 50 headspace Nz, 4 0.3 phosphonate/phosphate 0.04 Nz,4 ketones 40 13 air, 2.5 1.12 100 headspace polychlorophenols Torr, resulting in a pressure drop (AP)across the GC column of 280 Torr. Ammonia was used as a reagent gas providing NH4+(HnO)" reactant ions. The amounts of analytes admitted into the column were estimated using the vapor pressure of each pure compound as opposed to the resulting vapor pressure of the compound in the mixture. Estimated amounts in ppm (ng)were DMMP, 5.4 (4); DEMP, 2.7 (2.5); DIMP, 2.7 (3.0); DEEP, 1.6 (1.7). PhosphatelPhosphonate Mixture. The headspace of aliquid mixture was sampled containing four phosphoryl compounds: trimethyl phosphate (TMP), DIMP, DEEP, and triethyl phosphate (TEP). Relative volumes of each compound in the liquid mixture of TMP:DIMP:DEEP:TEP was 1:1:1:6, respectively. Experimental conditions are listed in Table I, and a 2-m X 320pm SP-5 column was used. The same IMS conditions as the phosphonate mixture was used. Ambient pressure was 650 Torr and AF' = 70 Torr. Water vapor was the reagent gas. Estimated amounts of each substance in ppm (ng) admitted into the GC column were TMP, 1500 (180); DIMP, 770 (115); DEEP, 460 (64); TEP, 150 (23). Ketone Mixture. The headspace vapors of a liquid mixture containing equal volumes of methyl ethyl ketone (MEK), diethyl ketone (DEK), methyl n-propyl ketone (MPK), and methyl isobutyl ketone (MIK) was sampled. Experimental conditions are listed in Table I, and a 2-m X 320-pm SP-5 column was used. The remaining IMS parameters were the same as for the phosphate/phosphonate mixture analysis. Ambient pressure was 650 Torr and AF' was 70 Torr. Water was the reagent gas. Estimated amounts of each substance admitted into the GC column were MEK, 15% (1pg); DEK, 9% (0.74 pg); MPK, 3% (0.3 pg); MIK, 7700 ppm (86 ng). Polychlorophenol Mixture. The headspace vapor of a liquid mixture containing equal volumes of 2-chlorophenol,2,3-dichlorophenol, 3-chlorophenol, 4-chlorophenol, and 2,4,6-trichlorophenol was satqpled. Operatingparameters are listed in Table I, and a 3-m X 320-pm BP-1 column was used. IMS parameters were the same as the aboveanalyses;AF'was85Torr. The exhaust air was recycled through a scrubber cartridge to be used again as carrier gas. Water vapor was the reagent gas. Estimated amounts of each substance in ppm (ng) admitted into the GC
IMS cell pressure, Torr 460 580 580 675
drift gas argon air air air
drift gas flow rate, mL/min 300 240 600
mode positive positive positive negative
column were 2-chlorophenol, 3000 (740); 3-chlorophenol, 400 (100); 4-chlorophenol, 280 (69); 2,4,6-trichlorophenol,41 (15). Chemicals. Chemicalswere obtained from Aldrich Chemical Corp., Milwaukee, WI (except that DMMP was purchased from Alfa Products, Danvers, MA) and were used without further purification.
RESULTS AND DISCUSSION
GC Operational Regime in a Hyphenated Hand-Held Detector. Gas chromatography necessarily relies on a number of column and flow parameters for the optimal separation of a mixture of compound vapors. A hand-held version places constraints on column performance not norm d y found in laboratory instruments. Relatively low isothermal operating temperatures and fast sample flow rates (low retention time) are desirable characteristics for a handheld GC system. However, the regimes of these parameters can compromise figuresof merit such as selectivity. Operation of a conventional GC system includes the optimization of theoretical plate height using carrier gas velocity through variation of column inlet pressure.25 Since the inlet pressure is not adjustable with the AVS sampling system, a limitation is imposed on the GC column outlet pressure, and this can affect IMS performance. The primary means of adjustment of the carrier gas velocity in a small, hand-held GC system is the column length.26 Figure 3 shows a set of theoretical curves calculated from the Golay equation2' for a peak with a capacity factor (k') of 5 for selected column diameters. Sufficient resolution (theoretical plates) and sensitivity (volumetricflow) with acceptable retention times were found (25) Cramers, C. A.; Leclerq, P. A. CRC Crit. Rev. Anal. Chem. 1988, 20, 117-147. (26)Arnold, N. S.; Kim, M A . ; McClennen, W. H.; Meuzelaar, H. L. C., manuscript in preparation. (27) Golay, M. J. In Gas Chromatography; Desty, P. M., Ed.; Butterworths: London, 1968; p 36.
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GC/IMS in~trumentation,4+~'-'~ and pressures are near 1 atm. However, in the hand-held GC/IMS, internal cell pressures are usually 100-300 Torr below atmospheric pressure and ion drift times vary for a given analyte. For example, Figure 4 shows the effect of pressure on the reactant ion peak (RIP), protonated water clusters, in the positive ion mode. Under the instrumental conditions of the hand-held ion mobility spectrometer unit (Table I), the RIP position at close to atmospheric pressure (740Torr) was at a drift time of 5.1 ms. When the internal pressure was decreased to 560 Torr by increasing the pumping speed of the internal pumps, an RIP drift time of 3.8 ms was observed (Figure 4). To our knowledge, this is the first presentation of a systematic analysis of subambient pressure on an ion mobility system. However, reduced mobilities (KO) of ion species are normalized for pressure and temperature through eq 1
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compoundwlthcapacityfactor(k)= 5.0. Nitrogencarrlergas(dlffusbn coefficient = 0.1 cm2/s and viscosity = 200 ppoise) and a column outlet pressure of 585 Torr (Inlet at 760 Torr) were used. The area enclosed by the interaction of the four dotted lines in each graph identifies the desired operating reglons for the TLW/IMS system.
within the intersection of the dashed lines in each graph in Figure 3. The retention time, volumetric flow, and theoretical plates were governed by column length and diameter. The selection of one independent and one dependent variable (e.g., diameter and column flow) or the selection of three dependent variables will uniquely determine the other parameters. For example, the selection of 8 s for a retention time, 8 mL/min for a flow rate (at the inlet), and 1400theoretical plates should be attained with a 2-m X 320-pm i.d. column. The optimal regime in the parameter blueprint for the GC portion of a hand-held GC/IMS (Figure 3) was tested with a series of compound mixtures for analysis speed,compound separation, and data interpretation. The parameters that govern GC performance also affect IMS behavior and are described below. Parameters Affecting IMS Analyte Response. The IMS response for a compound is strongly dependent on temperature, pressure, analyte concentration/vaporpressure, and proton affinity (or electronheagent affinity). Pressure mainly affects the ion drift time, and spectral profiles are governed by concentration and ionization properties of the analyte. Complex interactions among analytes in a mixture can yield an ambiguous number of peaks (less, equal to, or greater number of peaks than analytes) with unpredictable relative intensities. Interpretive and predictive models in IMS can be severely vulnerable to either matrix or sample complexity. Incomplete separationof a mixtureof compounds can result in similar uncertainty, and examples of this phenomenon are included in the main discussion for a handportable GC/IMS system. Ion mobility spectrometers are usually operated at fixed pressure for stand-alone spectrometers including laboratory
where K is the observed ion mobility, T and P are the ion mobility cell temperature and pressure, respectively, and 2'0 and POare the STP standard values.16 The variation of drift cell pressure and the AP across the GC column affected the retention times of the analytes. With respect to selected parameter regimes, the operation and application of a handheld GC/IMS system are explored in the next section. GC/IMS Mixture Analyses. Phosphonate Mixture. Mixtures of compounds were chosen to access different scenarios for detection and identification with GC/IMS. Phosphonates and phosphates represent model compound classes and are relevant for incinerator stack effluentsF8 chemical warfare agents29930 and their hydrolysis produde,30 and residual pesticides in agricultural matrices.31 Currently, the determination of the presence and identification of these materials occur with laboratoryGC and GC/MS systems. The availabilityof a portable GC/IMS device could offer relatively rapid monitoring and screening benefits with respect to decision-making and economic planning. In Figure 5, GC/IMS results are shown for the separation of a mixture of four phosphonate compounds in the positiveion mode. The first IMS trace was taken 0.46 s after the AVS inlet pulse of the analyte vapor mixture. IMS spectra were taken subsequently at 0.46-6intervals during the chromatographic analysis. As etated in the Experimental Section, ammonia was used as the reagent gas. The first two scans show a low-intensity, short drift time ion, most likely representing an amine impurity, since exposure to alcohols and ketones (oxygen-containing solvents) gave no response under ammonia reagent ion conditions. Ketones, aldehydes, alcohols, and organic acids have proton affinities well below those of a m i n e ~ ~and ~ 9did ~ ~ not form product ions. The appropriate choice of reagent can, thus, add another dimension to analyses by GC/IMS.34 The third scan ehows the elution of DMMP as a "monomer" ion, NH4+ (DMMP), (drift time of 4.72 ms) and a "dimer" ion, NH4+(DMMP)2,(6.28 me) (monomerand dimer refer to the number of analyte molecules in an ionic species, e.g., NH4+(DMMP)and H+(DMMP) are both monomers. The impurity is still present, suggesting (28) Ketkar, S. N.; Dulak, J. G.; Fite, W. L.;; Buchner, . . J Dheandhanoo, S. Anal. Chem. 1989,61, 260-264. (29) Ketkar, S. N.; Penn, S. M.; Fite, W. L. Anal. Chem. 1991, 63,
457-459. (30) DAgostino, P. A.; Provost, L. R. J . Chromatogr. 1992,689,287294. (31) Witkiewicz, Z.; Mazurek, M.; Szulc, J. J. Chromatogr. 1990,503, 293-357. (32) Gas Phase Zon Chemistry; Bowers, M. T., Ed.; Academic Preas: New York, 1979; Vol. 2. (33) Bartmesa, J. E. Mass Spectrom. Rev. 1989,8, 297-343. (34) Proctor, C. J.; Todd, J. F. J. Org. Mass Spectrom. 1983,18,509-
516.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 3, FEBRUARY 1. lOQ3
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Flgure 4. Display of the positlve mode water (background)reagent ion. The dritt time of the reagent ion peak Is shown as a function of internal
ion mobility cell pressure.
similar or greater proton or reagent ion affinities than DMMP and/or favorable vapor concentrations/vapor pressures. At scan8 the NHd+(DEMP)monomer (5.11 ms) and dimer (7.22 ms) appear and completely elute from the GC column by scan 14. A residual amount of DMMP is still present as well as the short drift time ion impurity, indicating that ammonia, DMMP, and the impurity all differentially compete for the available charge in such a way that all are observed in the 8-12 scan GC elution time frame. In addition to the presence of DEMP monomer and dimer in scan 8,another peak with a drift time (6.72 ms) intermediate between that of the DMMP and DEMP dimer peaks was also observed and is labeled by an asterisk. This peak is postulated as a mixed dimer that includes a molecule of DMMP and DEMP. Further elution of DMMP (scan 9) decreased concentrations such that the DMMP dimer ion disappeared. However, enough can be present to interact with DEMP and cause the generation of a low-intensity mixed dimer peak. The peak attributed to the mixed dimer disappears in scans 12-13 and occurs at the same time as when the DEMP concentration, in its elution profile, is too small to support a dimer ion. At scan number 13, the NHd+(DIMP)monomer appears (5.60 ms) and in scan 14, the corresponding dimer ion (8.00 ms) was observed along with the NHd+(DEEP) dimer ion (7.55 ms). In scan 15, the NHd+(DEEP)monomer (5.33 ms) appears, and then DIMP monomer and dimer ions are also observed. In this instance, the difference in mobility spectra for compounds of comparable proton affinities allowed detection of both compounds even though chromatographic resolution was poor. A similar phenomenon as with DMMP and DEMP also is observed with DIMP and DEEP in scan 15. A mixed dimer consisting of a molecule of DIMP and DEEP is strongly suggested by the appearance of a peak at 7.78 ms which occurs between the DIMP and DEEP dimer peaks (Figure 5). Scan number 16 clearly resolves these three peaks along with the DIMP and DEEP monomer peaks. It is interesting that even though DEEP (molecular weight 166)has a shorter drift time than DIMP (molecular weight 180), the former has a longer GC retention time than DIMP. It is worthwhile to note that during the elution of DIMP and DEEP, the impurity at the short drift time and the residual DMMP species are virtually absent. As before, this
can be rationalized by the magnitudes of the vapor pressure and charge affinities as well as the relative concentrations in the 15-18 scan number elution zone. Scans 19-22 exhibit a clearance of DIMP and DEEP as well as the reappearance of the impurity at the short drift time and the DMMP residual species. The bottom ion mobility spectrum in Figure 5 illustrates the results where the ion mobility spectrometer receives the four-component mixture without any GC preseparation. Nota the slight appearance of the water ion peak at 3.55 ms which effectively indicates the onset of ammonia clearance. The DMMP monomer is present at a verv low intensitv while the DEMP and DIMP monomer ions &e resolved. h e DEEP monomer is just barely resolved and is observed as a shoulder on the tailing edge of the DEMP monomer peak. For the dimer portion of the ion mobility spectrum, the DEMP, the DIMP, the DEEP, and both mixed dimer peaks essentially coalesce into a continuum. The DMMP dimer peak is absent. While it is possible to distinguish visually the monomer peaks, this task is very difficult when overlayed by dimer ions. However, it might be possible to resolve the dimer envelope with the aid of computar data reduction packages baaed on the second and/or sixth derivative algorithms36@and fiiite impulse response digital filters.37 It should be kept in mind that since all phosphonates are competing for the charge in this situation, the peak amplitudesdo not represent accurately the quantity of each phosphonate in the mixture. Deconvolution methods cannot, therefore, be relied upon for quantitation, and identification of the components could become confused for lack of expected peaks or additional mixed dimer species. An instrumental analysis of the ions making up the dimer envelope can be addressed with IMS/ MS t e c h n ~ l o g y . ~ ~ Despite complicating factors such as continued analyte bleed, impurities and mixed dimer species, a mixture of four phosphonateanalytes can be successfullyresolved with a small GC/IMS device (”exploded”version in the present analyeis) in under 8 s. (35) Lawrence, A. H.; Barbour, R. J.; Sutcliffe, R. Anal. Chem. 1991, 63,1217-1221. (36) Goubran, R. A.; Lawrence, A. H. Zntl. J . Mass Spectrom. Zon Processes 1991, 104, 163-178. (37) Davis, D. M.;Kroutil, R. T.Anal. Chim. Acta 1990,232,261-266. (38) Spangler, G. E.; Carrico, J. P. Zntl. J. Mass Spectrom. Zon Phys. 1983,52, 267-287.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 3, FEBRUARY 1, 1993
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Flgure 5. Successive IMS spectra of a GC analytical separatlon of a mixture of the phosphonate compounds DMMP, DEMP, DIMP, and DEEP. A miniaturized GC interfaced to an “exploded”handheld IMS device was used. The bottom ion mobility spectrum represents a headspace challenge of the four phosphonate compounds without a GC separation,and the gain has been increased to show salient details. Retention times and scan number appearance of the compounds are labeled on the ordinate.
Phosphate/PhosphonateMixture. The GC/IMS separation of a mixture of two phosphate and two phosphonate species with water-based ionization in the ion mobility spectrometer is presented in Figure 6. Product ions are protonated forms of the analytes. At scan 5 the TMP analyte is observed as monomer (7.71 ms) and dimer (8.76 ms) peaks. A peak at 9.71 ms is tentatively assigned as a trimer peak pending IMS/MS confirmation. The TMP analyte eluted from the GC column at approximately 17 s and the experimental conditions differed from those for the phosphonates (Figure 5) with a reduced column temperature and nearambient (atmospheric) IMS pressure. The relative settings of these parameters necessarily increase the GC retention and ion drift times. At scan number 21, DIMP is observed as monomer (8.76 ms) and dimer (10.76ms) peaks. Note that the TMP dimer, containing six carbon atoms, has a similar drift time as the seven-carbon-containing DIMP monomer species. Under these instrumental conditions, the DIMP analyte is better resolved than DEEP, the latter of which is observed at scan number 27 as a monomer (8.38 ms) and dimer (10.10 ms) peak, Once again as in Figure 5, a peak attributed to a mixed dimer (10.38 ms) is observed between the dimer peaks of DIMP and DEEP. This occurs first as a shoulder on the leading edge of the DIMP dimer at scan 25 and ends as a shoulder on the trailing edge of the DEEP dimer in scan 30, Under mild temperatures and pressures,
6.0
8 0 10.0
hin The (mrec) Flgure 6. Successive IMS spectra of a GC analytical separatlon of a mixture of phosphate (TMP, TEP) and phosphonate (DIMP, DEEP) components. A miniaturized GC Interfaced to a handheld IMS unit was used. Retentbn times and scan number appearance of the compounds are labeled on the ordinate.
the separation and analysis of the mixed dimer phenomenon was possible. A t scan number 33, a TEP monomer (8.38 ms) and dimer (10.38 ms) peak are observed. The TEP monomer has the same drift time as the DEEP monomer while the TEP dimer does not match the drift time of the DEEP dimer. Instead, the TEP dimer peak has the same ion flight or drift time as that of the mixed DIMP-DEEP dimer peak. Both TEP and DEEP monomers present a very similar cross section to the IMS drift gas with respect to their three linear ethyl subunits; however, the extra oxygen atom in TEP provides a polarity differential with respect to the GC column. Thus, the ultrashort GC column adequately separates all four of these species. With the exception of TMP, the analytes display broadened monomer ion peaks. The reason for this is not readily apparent. The bottom panel of Figure 6 portrays the IMS response of the hand-held unit with respect to the phosphatephosphonate compound vapor mixture when no chromatography occurred. The TEP dimer ion dominates the spectrum as the major component with very little visual guidance on the presence/absence of the other components. A small TMP monomer ion signal is observed at 7.70 ms, and the feature a t 8.57 ms could have contributions from the TMP dimer ion as well as the monomer ions of the other three compounds. More information on the composition of this peak could be obtained by IMS/MS procedures or spectral computation technique^.^^-^^
ANALYTICAL CHEMISTRY, VOL. 65, NO. 3, FEBRUARY 1, 1993
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9.20
7.40
11.00
12.5
TIME (msec)
MIK
16.3
3.80
5.60
7.40
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Figure 7. Successive IMS spectra of a GC analytical separation of a mixture of the ketones MEK, M K , MPK, and MIK. A miniaturized GC interfaced to a hand-held IMS unit was used. Retention times of
the compounds are labeled on the ordinate.
Ketone Mixture. Aseries of four ketones were investigated for their relatively rapid separation by the portable GC/IMS configuration. Alkyl ketones, along with compounds such as methanol, ethanol, and acetic acid, represent organic solvents used in the manufacture and purification of illegal drug substances. This class of compound is also found as ground water contamination in municipal sewage39and as an air pollutant.4o The detectionand continuous monitoring of these solvents by rapid and convenient means in real time can provide significant advantages to appropriate regulatory authorities. Each of the four ketone compounds displayed mobility spectra with two peaks (monomer and dimer). For the pure compounds, the retention time in seconds and drift times in milliseconds are MEK, 8.86,6.18,and 7.18;DEK, 12.5,6.45, and 7.77;MPK, 12.5,6.49,and 7.86;MIK, 15.4, 6.84,and 8.52. The DEK and MPK isomers have a similar retention time, and their IMS signatures displayvery similar drift times for both peaks. The similarity in drift times is due not only to the same molecular weight but also to similar cross sections of these molecules. The linear cross section between these two isomers provides a similar resistance or drag when they travel through the nitrogen drift gas in the IMS drift tube. The relative polarity and interaction of DEK and MPK with the GC stationary phase appear very similar because of their identical GC retention times. A separation of headspace vapors above an equal volume liquid mixture of the four ketones by GC/IMS is shown in Figure 7. The ketones were relatively low in concentration and mobility spectra displayed Gaussian peak shapes. High (39) Hutchina, S.R.;Tomson, M. B.; Bedient, P. B.; Ward, C. H. CRC Crit. Reu. Enuiron. Control 1985, 15, 355-416. (40) Welch, D.I.; Watts, C. D. Int. J. Enuiron. Anal. Chem. 1990,38, 185-198.
Flgure 8. Individual ion mobility spectra of (a) 2thlorophenoi, (b) 2,3dlchlorophenoi,(c)4chiorophenoi, (d)3-chlorophenoi,and (e)2,4,6 trichlorophenoi. Spectra were recorded on a hand-held, Integrated QClIMS device (EVM).
concentrations of the same ketones produced broad, poorly resolved peaks with considerable spectral overlap in the retention time dimension (not shown). Retention times of the ketones from the mixture are similar to those of the pure compounds, within the 1-s resolution of the IMS scan collection procedure. These values in seconds are 9.6 for MEK, 12.5 for DEK and MPK, and 16.3 for MIK. The ion drift times provide a good match in a further corroboration of the identities of the peaks in the IMS separation of the ketones. The values for MEK are 6.16 and 7.14 ms and for MIK are 6.84 and 8.50 ms. The drift times for the isomer pair ion mobility peaks are 6.48 and 7.82 ms. For these coeluting isomers, mixed species, which are almost certainly formed, are indistinguishable from dimer ions. Verification of these assignmentscan be obtained by an IMS/MS system. Polychlorophenol Mixture. Chlorophenols are ubiquitous contaminants in water and wastewater and can originate from paper and pulp mill effluents,4lindustrial wastest2and coking plant and brown coal liquefaction wastewater^.^^ Thus, stream, surface, sea, and potable waters as well as soil can become ~ o n t a m i n a t e d A . ~hand-held ~ ~ ~ ~ GC/IMS, the EVM device, was used to investigate its qualitative performancein the separation and analysis of various polychlorophenol compounds. Figure 8 presents the ion mobility spectra of the five pure chloro- and polychlorophenol compounds and Table I1 provides the numerical details of the spectra. The IMS analysis was conducted in the negative ion mode. The GC retention time in Table I1 is that time where the most intense IMS analyte intensities are found and where the reactant ion peak is at a minimum. The dominant, fast drift time peak in each spectrum is the reactant ion peak and includes a low-intensity shoulder on the leading edge. This low intensity peak is composed of chloride ions while the ~~
(41) van Loon, W. M. G. N.; Pouwels, A. D.; Veenendaal, P.; Boon, J. J. Znt. J. Enuiron. Anal. Chem. 1990, 38, 255-264. (42) Gaitonde, C.D.;Pathak, P. V. J. Chromatogr. 1990,614,389-393. (43) Frank, D.;Engelhardt, H. Fresenius' Z . Anal. Chem. 1989,333, 720-722. (44) Torazzo, A.; Zelano, V.; Ostacoli, G. Int.J.Enuiron. Anal. Chem. 1990,38, 599-605.
(45) Knuutinen, J.; Palm, H.; Hakala, H.; Haimi, J.; Huhta, V.; Salminen, J. Chemosphere 1990,20, 609-623.
906
ANALYTICAL CHEMISTRY, VOL. 85, NO. 3, FEBRUARY 1, 1993
\\
44 50
62 114
24,brriCI
120
130
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Flguro 9. Successtve IMS spectra of a GC analytical separation of a mlxture of five polychlorophenols. IMS spectra of the GC eluate were recorded at 2-5 Intervals. Numbers on the ordinate Indicate the GCretentbntlmeInseconds. Noanatytepeakswereobservedbetween the 28-40- and 62-1 1 4 s Intervals and are not shown In the flgwe. Spectra were recorded on a hand-held, Integrated GC/IMS devlce (EVM).
Table 11. Characterization of Pure Polychlorophenols compound GC retention time: s drift times, ma 2-chlorophenol 18 6.12,6.98,”8.56 2,3-dichlorophenol 45 6.64,1.06,9.32 4-chlorophenol 51 6.12,1.14,8.62-9.20c 3-chlorophenol 52 6.10,1.16,8.62 2,4,6-trichlorophenol 118 1.40,1.82,9.86 a GC elution time yielding a maximal response and is equivalent to the ion mobility spectrum containing the most intense peak signals as well as a minimum in reactant ion peak intensity. * Drift time of the shoulder on the dominant peak. c The time interval indicatesthe span in milliseconds of the slow, broad ion mobility peak.
tation of monomerldimer efficacy for the three remaining compounds over their GC elution proved problematic due to the complexity of the IMS spectra. Details as to the identities of the various IMS peaks and the role of the reagent ion plays in these various spectra will be provided elsewhere. Note that the retention time for the 3- and 4-chlorophenols is virtually identical and that it differs considerably from 2-chlorophenol. These observations are to be expected when a consideration of vapor pressures a t 25 OC is taken into account: 2-, 3-, and 4-chlorophenol are 2.30, 0.27, and 0.19 Torr, respectively, making the differentiation of 3- and 4-chlorophenol more difficult in their ion mobility spectra. Except for the broadened nature of the peak with the slowest drift time in the ion mobility spectrum for 4-chlorophenol, their distinction is not trivial. A mixture of the five polychlorophenols was subjected to analytical separation with the hand-held GC/IMS or EVM device. Figure 9 presenta the GC and IMS portions of the mixture analysis and an ion mobility spectrum of the GC eluate was taken every 2 8. Table I11 presenta a compilation of the retention times, IMS peaks, and compound identities. For 2-chlorophenol, 2,3-dichlorophenol, and 2,4,6-trichlorophenol, the GC elution and IMS drift times provide almost identical matches with that of the pure compounds (compare Table I1 and I11 and Figure 8a,b,e). The GC elution times of the compoundsallow for a relatively poor 2-8 resolution to adequately separate the three species. This 2-5 resolution also allows for the distinction of the 2,3dichlorophenol and 4chlorophenol compounds (Figure 9 and Table 111). Because of the similar elution times of 3- and 4-chlorophenol (Table11),one would expect a composite IMS spectrum of Figure 8c,d. However, Figure 9 shows that at a GC elution time of 50 s,the ion mobility spectrum is compoeed of, among other peaks, a slow, broad peak that is more suggestive of the 4-chlorophenol (Figure 8c) than the 3-chlorophenol (Figure 8d) spectrum. Given their similar vapor pressures (similarheadspace concentration) and elution times, it can be postulated that charge exchange from the negative reagent ion to neutral 4-chlorophenol is more efficient than to that of 3-chlorophenol. This phenomenon would allow for the appearance of a 4-chlorophenol reactant ion adduct at the expense of 3-chlorophenol suggested in Figure 9 a t the 50-5 GC elution time.
CONCLUSIONS
Drift time of the shoulder of the dominant peak. The slow, broad peak appears in the spectrum (Figure 8c) as opposed to the relatively sharper peak at 8.62 ma for 3-chlorophenol (Figure 8d).
The efficacy of a portable, hand-held GC/IMS unit was introduced, and ita potential as a device for a two-dimensional separation of a mixture of analytss was presented. Despite ita stringent, small physical dimension with respect to routine laboratory GC and IMS devices, separations for most of the compounds were feasible. Separation speed and resolution of the individual substances, with the exception of isomer compounds, could be conveniently varied depending on changes in major operating parameters. The performance of the hand-held GC/IMS unit appears to lend itself to indoor and outdoor monitoring scenarios for a host of compounds and classes of compounds.
predominant species in the main peak is negatively charged molecular oxygen and 02-adducta.46 A single, sharp peak for each chlorophenol analyte is not observed. Instead, fairly complex spectra result from the ion-molecule interactions of this compound class with the negative reagent ions. The 2-chlorophenol peaks (6.72 and 8.56 ms) in Figure 8a appear to be composedof a monomerldimer-type constitution, where over the course of elution, the slower peak disappears prior to the faster 6.72-ms peak (data not shown). Likewise, the 7.06- and 9.32-ms peaks of 2,3-dichlorophenol (Figure 8b) exhibit this effect (data not shown). A visual interpre-
The authors would like to thank Stephen Taylor, Colin Cumming, and Graham Crouch for the construction, commissioning and testing of the prototype EVM, Jacek Dzworanski for assistance in the development of the GC/IMS concept unit, Professor Gary A. Eiceman for helpful discussions and a critical reading of the manuscript, Dennis M. Davis for data reduction and computer display software, and Ms. Linda G. Jarvis for the preparation and editing of the manuscript.
Table 111. Characterization of a Mixture of Five PolvchloroDhenols peak identified as GC retention time, s drift times, ma %-chlorophenol 18 6.12, l.OO,a 8.58 2,3-dichlorophenol 44 6.62,1.08,9.34 4-chlorophenolb 50 6.12,1.14,8.64-8.98 2,4,6-trichlorophenol 120 1.40,1.84,9.86
(46)Eiceman, G. A.; Shoff,D. B.; Harden,C. S.;Snyder, A. P.;Martinez, P. M.; Fleiacher, M. E.;Watkins, M. L. Anal. Chem. 1989,61,1093-1099.
ACKNOWLEDGMENT
RECEIVED for review April 6, 1992. Accepted October 5, 1992.
