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(14) Perchelski, R. J.; Yost, R. A.; Wilder, B. J. Anal. Chem. 1983, 5 5 , 2002. (15) Colter, R. J. Anal. Chem. 1980. 52, 1770. (16) M e C F d , R. D. ACC. chem.Res. 1982, 75. 288. (17) MacFarlane, R. D.; Vemura, D.; Veda, K.; Hkata, Y. J . Am. Chem. Soc. 1980, 102, 075. (18) Contemlus, R. J.; Capellen, J. M. Znt. J . Mess Spectrom. ion Phys. 1980. 34, 197. (19) Jurgclas, H.; Danlgel, H.; Schmldt, L.; Dellbrijgge, J. Org. Mess Spectom. 1982, 77, 499. (20) Dab, S.; Taylor, E. H. J . Chem. Wys. 1958, 2 5 , 389. (21) Carroll, 0. J.; Wernlund, R. F.; Cchen, M. J. U.S.Patent 3639757 (Feb 1, 1972). (22) Hornlng, E. C.; Hornlng, M. G.; Carroll, D. 1.; Dzldic, I.; Stillwell, R. N. Anal. Chem. 1973, 45, 938. (23) Sherman, M. Q.; Kingsby, J. R.; Hemminger, J. C.; McIver, R. T., Jr. Anal. Chem. Acta 1985, 178, 79. (24) Tembreull, R.; Lubman, D. M. Anal. Chem. 1988, 58, 1299-1300. (25) Kolaitis, L.; Lubman, D. M. Anal. Chem. 1986, 58. 1993-2001. (26) Smer, L. 8bChernkby W. H. Freeman: San Franclsco, CA, 1975. (27) Fales, H.M.; Mllne, G. W. A.; Wlnkler, H. V.; Beckey. H. D.; Danlce, J.
N.; Barron, R. Anal. Chem. 1975, 47, 207. (28) Kambara. H. Anal. Chem. 1982, 54, 143. (29) Kambara. H. Mass Spectrom. 1983, 37, 87. (30) Irlbarne, J. V.; Dziedzlc, P. J.; Thomson, B. A. Znt. J . Mess Spectrom. Zon Phys. 1983, 50. 331.
RECEIVEDfor review March 7,1986. Accepted April 28,1986. We gratefully acknowledge initial financial support of this research by a Cottrell Research Grant and the donors of the Petroleum Research Fund, administered by the American Chemical Society. We also acknowledge support of this work from the Army Research Office under Grant DAAG29-85-K1005 and also the Department of Defense through the U.S. Army Research Office under Grant No: DAAG29-85-G-0018 for purchase of the equipment used in this work. We also acknowledge support under NSF Grant CHE83-19383.
Photoionization in Air with Ion Mobility Spectrometry Using a Hydrogen Discharge Lamp C. S Leasure, M. E. Fleischer, G. K. Anderson, and G. A. Eiceman* Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
An lon moMHy spectromeier (I=) W a b k for operation In alr at atmospheric pressure udng &eel gaseous photdonlzatlon (PI) was demomtrnted by udng an on-8xk mounted 1 0 . 2 4 hydrogen discharge lamp (Lx 121.5 nm). Penetration of UV photons Into ak In PI-IMS was determlned by udng product Ion Intedty and analyte location as 15 mm tromthebmpwhdow. s p e c t r a w e r e ~ f o r t e n o r I y n l c compwnds and were rknple with one to three peaks. Detectlon W r In PI-IMS ranged from 0.1 to 50 ppb dkectly wlthout any sample precommtratlon and worklng ranges were 1000-1OOOO. Dotedon llmlk were sLnllsr to those for conventbnal chemlcal lonlzatlon but worklng ranges were Improved 100- to looo-ldd. Oporatlon of PI-IMS In nltrogen wlth on-axb placement of the dircharge lamp was complicated by penetration of photons >2 cm through the Bradbury-Nkhon shutter Ieadlng to analyte lonlzation In the drlft regbn. Use of nitrogen as a drift gas was posdMe wlth the lamp placed perpendicularto gas flow. However, Senritivlty and #mns d daectlon were b6tter wlth the on-axls deIn alr since lonlzath volumes were 7.1 cms for the on-axis mount but only 4.0 cm’ for the rlde-mount.
Ion mobility spectrometry (IMS) has been used in the separation and detection of organic (1-5) and inorganic (6, 7) compounds in the gas phase at atmospheric pressure. Separation is achieved through differences in gaseous mobility of molecular or pseudomolecular ions under the influence of an applied electric field and against the flow of a neutral drift gas. Ions are formed from analyte indirectly through ion/ molecule collisions with reagent ions (chemical ionization) or directly through photoionization. Attractive features for use of IMS in the detection of atmospheric pollutants include high sensitivity, operation at atmosphericpressure, relatively simple instrumentation, and moderate selectivity. However, extreme memory effects, overloading of response, and, more impor-
tantly here, a narrow working range have been associated with and restricted the utility of IMS. Each of these limitations may be attributed to either flow characteristics or principles of ionization in IMS rather than limitations in mobility-based separations. Historically, the most common ionization method in IMS has been chemical ionization (CI) initiated with reactant ions formed from /3 emission. While CI provided high sensitivity (subpart-per-billion levels) and some selectivity in ionization processes, working ranges of analyte response vs. concentration were only 10-100. Selectivity of ionization in mixtures was quantitatively shown recently to be governed in CI-IMS by proton affiiities in the positive mode (5) and electron affinities in the negative mode (6). Unfortunately, selectivity of ionization was controlled mostly by molecular parameters, rather than by the analyst,except for the choice of reagent gas. While lasers have been used in IMS for controllable selectivity through multiphoton ionization (8),the high cost and typically large size of lasers presently restrict convenient and economical application of laser-IMS as a routine laboratory tool and especially as an atmospheric sensor. However, a Kr vacuum (vac)-ultraviolet (UV) discharge lamp was proposed (9)as an inexpensive and practical alternative to lasers for photoionization (PI) in IMS. While the P I discharge lamp in prior PI-IMS measurements showed sensitivity only 0.14.01 of that obtained in CI-IMS, reactant ions that interfere in detection of analytes with short drift times were not observed. Thus, PI-IMS may have promise as a direct atmospheric sensor for contaminant in air. A serious limitation with vac-UV discharge lamps is the presumed need for a nitrogen atmosphere due to oxygen quenching of photons (10). Although this was not important in PI-IMS use as a gas chromatographic detector with nitrogen carrier gas (9),direct IMS sensing of atmospheric samples would be facilitated with air as the drift gas. In exploratory studies in this laboratory, an existing PI-IMS design (11) showed satisfactory response for aromatic hydrocarbons in nitrogen. However, response for these compounds with the
000~2700/86/0358-2142$01.50/0 0 1986 American Chemical Society
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Table I. Reduced Mobilities for Selected Compound Determined in Air Using PI-IMS I
'.
J
analyte benzene p-xylene naphthalene acetone 2-butanone acetophenone 2-propanol
l-butanol aniline
KO,cm2/(V s) 2.23 (C&3')" 2.04 (C~HGNO')" 2.01 (CSHlo') 1.83 (CSHioNO+) 1.92 (CloHS') 1.80 (CloHsNO+) 2.19 (C3HeO') 2.05 (CdHSO')
1.82 1.87 1.97 2.13 2.46 1.88
1.73 1.97 2.09 1.98 1.54
n-butylamine (SC)SHUTTER CONTROLLER
Flgure 1. Schematic of ion mobility spectrometer with hydrogen dlscharge lamp for photoionization in air. Values for resistors and capacitors are as follows: R,, 59 kQ; R2, 1.8 MQ; R3, 220 kQ; R4, 240 kfl; R,, 470 kQ; Rg, 18 kfl; R,, 15 kfl; R,, 20 kO; Rg, 110 kQ; C1, 0.0047 pF, 6 kV; CB, 0.002 pF, 6 kV; C3, 0.005 FF, 4 kV; C4, 0.47 pF, 600 V; C5, 0.1 mF, 500 V; Cg, 4 mF, 450 V; C,, 0.047 pF.
same IMS design in air was weak and irregular. In work reported here, a vac-UV hydrogen discharge lamp was incorporated into an PI-IMS with a reconfigured ionization chamber for detection of organic compounds directly in air with high sensitivity. Our objectives were to explore response characteristics of this new PI-IMS design for comparison to existing ionization sources and to describe the PI behavior of selected organic compounds in air at atmospheric pressure. The goal of this study was development of IMS instrumentation that will be suitable for direct atmospheric sensing of hazardous organic compounds without membrane interfaces or radioactive sources.
EXPERIMENTAL SECTION Instrumentation. An ion mobility spectrometer described previously (12) was modified by replacing the 63Niionization source ring and repeller with a 10.2-eV hydrogen discharge lamp (HNU Systems, Boston, MA) as shown in Figure 1. A second design was assembled with the lamp mounted perpendicular to flow as described by Hill et al. (9). In both arrangements, the drift region was 9.0 cm and the reaction region was 2.0 cm. the IMS drift gas was typically breathing air and was used in unidirectional flow at 300 mL/min. Conditions of operation were as follows (unless otherwise noted): tube temperature, 149 "C; reaction region field, 221 V/cm; drift region field, 244 V/cm; ion shutter pulse width, 0.2 ms; repetition rate, 28 Hz;amplification, lO9-lO" V/A; and barometric pressure, 662.0-668.1 mmHg. Sample was introduced to the IMS using headspace vapors over pure solutes in a small flow (1-10 mL/min) of nitrogen gas as previously described (5, 7, 12).
1.95 1.69
% re1 abundance
20 100 100
20 100 20 100 100 100 100
20 20 10
100 50 25 100
25 10 100 100
Mass identified, ref 13. Reagents. The following compounds were acquired from J. T. Baker ChemicalCo. (Phillipsburg,NJ): 2-propanol, 1-butanol, p-xylene (all reagent grade), and acetone (HPLC grade). Pesticide grade benzene and reagent grade aniline were acquired from Fisher Scientific Co. (Fair Lawn, NJ); HPLC grade 2-butanone and 99+% naphthalene were acquired from Aldrich Chemical Co. (Milwaukee,WI). All reagents were used without further purification. Procedures. Characteristics of operation and features of response for PI-IMS in air were determined by use of several studies. These studies were designed generally to measure flow effects, electric field effects, and particulars in the use of air as a drift gas in PI-IMS. Comparisons were made to PI-IMS operation in nitrogen drift gas. A . Response Curves for On-Axis Mounting. Response of the IMS to concentration for pure compounds was measured in air as ion current for peaks vs. gaseous concentration of neutral a n a l 9 in the drift gas. Compounds of environmental importance from several chemical classes were selected for study as shown in Table I. B. Zon Shutter Field Study. At concentrations of 15,177, or 1500 ppb for p-xylene in air drift gas, dc offset was measured in PI-IMS vs. shutter field with the ion shutter operated in either static open or static closed modes. Identical measurements were made in nitrogen drift gas. Peak heights were also measured for pulsed shutter operation in air at p-xylene concentrations of 170-1500 ppb. Measurements were obtained at eight settings of ion shutter field from 260 to 3000 V/cm. C. Response Curves for Side Mount. Response curves of peak height vs. concentration for benzene, p-xylene, and aniline were collected in air drift gas. The PI-IMS ionization chamber was assembled by use of an existing PI source design (9). Response curves were also collected with this design using nitrogen drift gas. D. Measurements of Penetration of Photons in Air. The PI-IMS is on-axis configuration with undirectional flow was altered so that air drift gas was vented from the side of the drift tube. Spacers made from 6 mm i.d., 1 mm thick Teflon O-rings were then used to locate the vent at a fixed distance from the discharge lamp. In ths particular design, clean air was introduced into the IMS at the repeller at 2 L/min and was used simply to oppose drift gas flow for displacing neutrals from the lamp window to the gas vent. Analyte was introduced to the IMS through the ring adjacent (and down-flow)to the shutter. The vent was set at distances of -0,0.5, 1.0, and 1.5 cm from the lamp window. In this experimental design, the distance from lamp face to available neutral analyte could be partially controlled to measure distances photons traveled. Response at each distance was
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Flgure 3. Plots of dc offset vs. Ion shutter field for p-xylene at static open and statk closed ion shutter modes in air for (0)15 ppb, (A)177 ppb, and (0)1500 ppb.
DRIFT T I M E ( M 5 1
Figure 2. Ion mobility spectra for (A) benzene, (B) p-xylene, (C) naphthalene, (D) acetone, (E) 2-butanone, (F)acetophenone, (G) 2propanol, (H) 1-butanol, (I) aniline, (J) n-butylamine.
measured by using ion currents for peak heights with p-xylene at 150 ppb in the drift gas.
RESULTS AND DISCUSSION Ion mobility spectra in air using the on-axis mounting for the hydrogen discharge lamp are shown for ten compounds in Figure 2 and were characterized as simple spectra with limited or no fragmentation. As expected, reactant ions were not observed and thus presented no interference in spectra. Reduced mobilities for these same ten compounds are listed in Table I along with ion identities, some of which were mass identified while others were only posulated. The IMS spectrum for benzene consisted of two peaks at reduced mobilities (KO) of 2.04 and 2.23 cm2/(V s) while literature values were 2.04 and 2.25 cm2/(V s), respectively, (13). The smaller peak at KOof 2.23 cm2/(V s) was mass identified previously as the molecular ion and the larger peak at KOof 2.04 cm2/(V s) was C6H6NO+(13).Similar behavior was also seen in spectra for other aromatic compounds including p-xylene and naphthalene. Spectra for ketones showed one product ion which may have been the molecular ion but which was not maes identified. The trend in drift times for acetophenone, 1-butanone, and acetone reflected a size difference consistent with and indicative of molecular ions for each compound. Alcohols typically formed a major ion with three or more minor ions as shown in Table I. One major peak with a reduced mobility of 2.09 cm2/(V s) (literature value 2.01 cm2/(Vs) (14)) and two minor peaks with reduced mobilities of 1.98 and 1.54 cm2/(Vs) were found in the PI-IMS spectrum of aniline. The widths of product ions from molecules with -OH, -NH,and C=O moieties were occasionally 2- to 3-fold larger than those for aromatic compounds. This may have been due to formation of multiple cluster ions or ion-ion repulsions in the PI source. Indeed ion currents in PI-IMS were 100-fold larger than ion currents in CI-IMS. Apart from improvements in peak broadening, a larger ion shutter field for optimum control of ion leakage was anticipated from their greater ion densities. Thus, effects from shutter field on sensitivity and response in PI-IMS were determined. Nonetheless, preliminary results showed that on-axis operation of a PI-IMS was suitable for
I880 2080 ION SHUTTER F I E L D t V * C M - '
1
Flguro 4. Plots of ion current vs. ion shutter field for p-xylene for pulsed ion shutter modes: (0)177 ppb, (A)1500 ppb.
air sensing of compounds with a range of moieties and that spectra were simple as described in earlier PI-IMS instruments (9). In Figure 3, a plot of dc offset vs. shutter field in air for three concentrations of p-xylene in air (15,177, and 1500 ppb) are shown for static testa of the ion shutter in fully open and in fully closed modes. With the shutter in the fully open mode, ions passed continuously through the shutter to the drift region while ions were deflected from the drift region with the shutter closed and only ion leakage (or background current) was measured. Ions penetrated through the shutter almost completely at the lowest field for all three concentrations as shown in Figure 3. The difference in offset between the open and closed modes increased initially with higher shutter fields and then became constant and at concentrations of 177 and 1500 ppb the effect from increasing ion shutter fields from 260 to IO00 V/cm was particularly pronounced. On the basis of these results the shutter field of greatest strength should provide highest sensitivity with a low background. However, at increasing shutter fields in the pulsed shutter (normal) operation, the peak heights for p-xylene increased rapidly, reached a maximum between 600 to 1000 Vjcm, and decreased slightly a t higher fields as shown in Figure 4; the maximum was concentration-dependent. This shutter behavior was described previously in CI-IMS and was rationalized by using transverse vs. axial field arguments (15).
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