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Anal. Chem. 1988, 58, 1690-1695
ACKNOWLEDGMENT We thank W. R. Summers, M. U. D. Beug, and L. M. Divis for their personal experimental assistance and the Cyclotron Operations personnel for their cooperation. We also thank the Electron Microscopy Center personnel for their assistance with the electron microprobe scans. Registry No. CgI, 7789-17-5;RbCI, 7791-11-9;KCl, 7447-40-7; NaC1, 7647-14-5. LITERATURE CITED (1) Mactarlane, R. D. Acc. Chem. Res. 1982, 75, 268-275. (2) Hakansson, P. Doctoral Dissertation, University of Uppsala. (3) Fllpusluyckx, P. E. FkD. Dissertation, Texas A&M Unhrersity, C o w Station, TX, 1985.
(4) Cookson, J. A. Nucl. Instrum. Methods 1979, 765, 477-508. (5) Applications for Merochannel Plates; Varian Associates Inc., Light Sensing and Emission Divlslon: Palo Alto. CA, 1981. (6) Wiza, J. L. Nucl. Instrum. Methods 1979, 762, 587-601. (7) Adler. L.; Blando, F. Cyclotron Institute, Texas A&M University, private communication. (8) Tabk of Isotopes, 7th ed.; Lederer, C. M.,Shirley, V. S., Ed.; Wiley: New York, 1978. (9) Macfarlane, R. D.; Torgerson, D. F. Science (Washington, D . C . ) 1978, 797, 920-925. (IO) Guthier, W.; Becker, 0.;Della Negra, S.; Knippelberg, W.; LeBeyec, Y.; Wenheft, U.; Wlen. K.; Wleser, P.; Wurster, R. Int. J. Mass Spectrom. Ion Fhys. 1983, 53, 185-200.
RECEIVED for review December 24,1985. Accepted March 10, 1986. Support for this work by NSF Grant CHE-8310783 is gratefully acknowledged.
Effects of Laser Beam Parameters in Laser Ion Mobility Spectrometry G . A. Eiceman,* V. J. Vandiver, and C. S. Leasure Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
G. K. Anderson,' Joe J. Tiee, and Wayne C. Danen Chemistry Division, Los Alamos National Laboratories, Los Alarnos, New Mexico 87545
Influences of beam parameters were detennlned for laser lonlzatbn of benzene, toluene, and naphthalene at atmospheric pressure In kn moMyty spectrometry (IMS). moMllty spectra In nltrogen for low fluences and at a wavelength of 266 nm were eknple with only a single peak (M'). Reduced mobiles (cm2/V*s) were for benzene, 2.22; toluene, 2.10; and naphthalene, 1.89. At fixed concenbatkno d 10-100 ppb for naphthalene In the IMS, tncreases In laser beam tluence and cross-sectional area caused Increases In both peak heights and bandwidths. Increases In peak broadening were attributed to ion-ion repulsionsfrom creatbn of kns In a small lonlzatbn volume. Quantitatlve accuracy In laser-IMS with a pulsed ND-YAG laser was llmited by lnhomogenelty of the beam cross section, variations In beam energy between pulses, and ionization characteristics Including undescrlbed flow effects of the IMS. Location of the sample Inlet tube relatlve to the reactlon rlng affected sensltlvlty and slgnal stablity. Thls phenomenon seemed related to a beam SampIlng error from streamlng of analyte In the drw gas and has not been reported for chemical ionkatbn IMS. Response curves were linear In reglons with slopes (pA/ppb) from a hlgh of 917 for naphthalene to a low of 0.84 for benzene. Linear ranges and slopes were dependent on hence or energy at afbted beam gcKnnetry. SensWky hcremed In order to naphthalene > toluene > benzene and dbnated lhdts of detection (parts per bllbn) were 0.3, IO, and 95, respectively.
Ionization processes at atmoepheric pressure in ion mobility spectrometry (IMS) have been based largely on use of chemical ionization through proton affinity (1,2)or electron affinity (3)between analyte and reactant ions or electrons. Differences Present address: Deparment of Chemistry, New Mexico State University, Las Cruces, NM 88003. 0003-2700/86/0358-1690$01.50/0
in these chemical properties for molecules with various structures led to different response factors, limits of detection, and selectivity in IMS detection. Although such distinctions were appreciated during early years of IMS development (4, 5), lack of application of these principles prompted conclusions that IMS was unsuitable for mixture analysis and response was unpredictable (6, 7). Recently, response characteristics for binary and ternary mixtures for both negative and positive ion IMS have been described quantitatively (8-10) using a segmented, unidirectional flow design with low residence times and memory effects for analytes (11). Response of IMS is consistent and can be predictable when governing chemical ionization principles are known. Although certain advantages in selectivity may be obtained by use of reactant ions other than those normally employed (12), in practice, existing chemical ionization methods are unpromising generally due to limited working range (and extremely narrow linear range) and the influence from matrices when unknown in composition. A more serious practical limitation is that selectivity is governed mostly by molecular properties rather than the analyst. Photoionization of organic compounds at atmospheric pressure with a discharge lamp (13) and a laser (14-16) has been demonstrated for IMS and might offer increased flexibility, simplified IMS spectra, and new opportunities for detection in IMS. Such improvements may complement other IMS advantages such as operation at atmospheric pressure and simplicity of instrumentatioin as an atmospheric sensor. Direct photoionization of gaseous compounds using lasers in IMS was fist reported in 1982 (14)and was shown to produce molecular ions (M+) or pseudo molecular ions (MH+) with limited fragmentation a t the beam energies reported. An additional advantage of photoionization was the absence, in the IMS spectra, of reactant ions commonly seen in chemical ionization IMS. For molecules with molecular weights of 30 to 100, these reactant ions appear at comparable drift times and can be major interferences in IMS spectra. Although some 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 58, NO. 8, JULY 1986 SRHPLE ,n
\
IN
wm DRIFT SRE OU1
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IN5ULRTOR R I N S
HERTING TRPE DETECTOR PLRTE DRIFT 6R5 I N
1. Diagram of ion mobility spectrwneter UJed for laser ionizadon
at atmpheric pressure. Approach of the laser beam Is perpsndicular to the diagram. Components are labeled.
selectivity of ionization was possible for aromatic hydrocarbons using a fixed wavelength (266 nm) from a quadrupled NdYAG pulsed laser, selectivity in ionization was better accomplished with a tunable dye laser (15, 16). Alternatively, an ArF excimer laser emitting at 193 nm was proposed as a general ionization source. Regardless of laser employed, guidelines for operation of lasers and effects on response characteristics from various heam parameters were not reported. Certain laser beam parameters including geometry, energy, and duration have been cited as variables in ionization (14) hut were not further described particularly as effecting IMS performance. For example, peak bandwidth for product ions in laser-IMS were reported as 1-3 ms at base line (14)while widths of only 500 ps are observed for the same compounds in chemical ionization IMS. However, since peak widths for ions created by 0 ionization and by photoionization (with a low intensity discharge lamp) were similar (13),factors responsible for hand excessive hand broadening in laser-IMS may have been beam parameters. This study was prompted by advantages of lasers fur ionizatioin in IMS and by the need for a detailed description of laser beam parameters in IMS response. Although gaseous photoionization has already been successfully incorporated into other analytical instrutments such as gas chromatographs since the mid-19708, a detailed description of beam parameters in laser-IMS is necessary for both evaluation and refinement of existing instrumental designs. A secondary objective was to determine laser suitability with unidirectional flow designs for quantitative IMS.
EXPERIMENTAL SECTION Instrumentation. An ion mobility spectrometer was constructed at NMSU using already reported designs and dimensions (11)with slight modifications. In this IMS, the Bradbury-Nielson shutter was removed and the @-sourcering was replaced hy Teflon inserts and Suprasil quartz windows to permit laser beam entry into the IMS as shown in Figure 1. The beam was directed completely through the IMS and precautions were taken to enswe that the laser beam did not strike metal or insulating surfaces. Headspace vapors over pure samples contained in 2-mL miniviats (for liquids) or in a 10 in. long X 6 mm 0.d. glass tuhe (for solids) were swept with nitrogen gas into the IMS continously using
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flexible fused silica capillary tubing. The tubing was attached to the repeller and the effluent end was positioned near the IMS detector to control extent of mixing of analyte with drift gas. Concentration of analyte delivered to the IMS was controlled by dilution of headspace vapors and temperature of the sample container (8-10). The IMS was attached to a supporting base plate and was placed vertically in a cylindrial that was wrapped with heating tape to control temperature. The glass cover was insulated at the base with glass pack and at the top with a Teflon cap to reduce thermal gradients in the IMS to zt5 "C. Resistors (240 kR) were attached directly to the drift rings and a 255-kR resistor was used to connect the aperture grid to ground. The detector plate was held at virtual ground. The tuhe and some of the associated IMS eleetronie were placed in a walk-in Faraday cage. The high-voltage source was a Model HV-1556 from Power Designs, Inc. (Westbury, NY).A Tektronix, Inc. (Beaverton, OR), Model Am 502 differential amplifier was used to amplify signal at 5 X lo7VIA. The amplified signal was directed for digital signal averaging (DSA) into a Nicolet Instrument Corp. (Madison, WI) Model 1074 instrument computer equipped with Model SD-72A ADC and Model SW-71B wide-range sweep control. The instrument computer was synchronized from laser pulses with an optical trigger rather than from a Bradhury-Nielson shutter as in conventional IMS instnunentation. Spectra were obtained from 1024 individual scans. Spectra were displayed on a Tektronix, Inc., Model 5110 ascilloseope and printed with a Hewlett-Pachd (Palo Alto, Ca) Model 7046-A X-Y recorder. Other conditions for analysis were drift gas (N,) flow, 600 mL/min, tube temperature, 120-140"C, tube voltage, 3 kV (for a drift field gradient of 256 V/cm), and atmospheric pressure, 595-600 torr. The laser was Quanta Ray (Mountain View, CA) Model DCR-1 Nd-YAG equipped with a Model HG-1 harmonic generator and Model HS-1 prism harmonic separator. The wavelength used was the fourth harmonic (266 nm) with a pulse width of 5 ns at 10 Hz and Characteristics of the laser beam were varied as described below. AIL laser experiments were conducted in the Chemistry Division as Los Alamos National Laboratories. Procedures. Five studies on laser beam parameters were completed using benzene, toluene, and naphthalene. A. Effect of Fluence. A t three beam geometries (2 mm and 4 mm round and 2 x 8 mm rectangular),fluence was varied using laser output energies from 55 to 1250 rJ per pulse. Spectra were collected at 10 to 23 fluence settings. Beam energies were measured prior to and following entry into the IMS using a Model RK 3232 energy ratiometer from Laser Precision Corp. (Utica, NY). The concentration of naphthalene was -4 pph in the drift gas flow. B. Effect of Beam Geometry. The laser fluence was fixed at four setting between 0.28 and 320 rJ/mm2 and spectra were acquired for naphthalene at 4 pph at beam diameters of 1,2,4, or 10 mm for a round beam. C. Effect of Flow Mixing. With a 16 mm2 rectangular beam (2 X 8 mm) at laser energy of 1250 rJ per pulse, the position of the fused silica (inlet) tuhe was adjusted to six distances from the beam location in the IMS. Distances were 9.8, 7.3,4.8, 3.8, and 2.3 cm. D. Limits of Detection. With a 2 X 8 mm2rectangular beam geometry, three fluences from 5.3 to 206 pJ/mm2 were used to collect spectra for benzene, toluene, and naphthalene at concentrations between 0.70 and 1900 pph. E. Influence of Voltage. For benzene, toluene, and naphthalene at fixed concentrations of 640,200,and 4 ppb, respectively, tube voltage was varied from 500 to 4000 V to determine any effect of field on the reduced mobilities. F. Effect of Quartz Windows. The quartz entrance and exit windows were removed and response curves for benzene and toluene were determined. The laser beam energy was fixed at 1mJ per pulse with a 2 x 8 mm redangular beam. Concentrations were varied from 24 to 300 ppb for toluene and 22 to 240 pph for benzene. G . Background Noise. Signal was acquired with no sample, with the beam completely hlocked, with the IMS voltage off, and with the tube oven heater off.
RESULTS AND DISCUSSION Preliminary Studies. Ion mobility spectra for benzene,
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
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DRIFT TIME ( M 5 ) Flgure 2. Ion mobility spectra for (A) naphthalene, (e) toluene, and (C) benzene from laser ionition of gaseous sample diluted In nitrogen gas. Concentrations (ppb) and fluences (pJ/cm*)were as follows: (A) 1, 20; (B) 2, 40, and (C) 3, 10, respectively.
toluene, and naphthalene in nitrogen at 125 O C and a t atmospheric pressure are shown in Figure 2 for laser ionization at 266 nm. Spectra were distinguished by single intense ions, no reactant ions as seen in chemical ionization IMS, and no detectable fragment ions. Although peak widths were narrower than in prior laser-IMS studies (14-16),general appearances of spectra were similar. Reduced mobilities, KO (cm2/V.s), and peak widths at half-height, wll2 ( ~ s ) ,from spectra in Figure 1 were as follows: (a) naphthalene, 1.86,440; (b) toluene, 2.11, 388; and (c) benzene, 2.22, 380. These KO values compared favorably to reported (14) KOvalues of 1.87, 2.11, and 2.27, respectively, for ions postulated to be M+. While assignment of ion identity as M+ is reasonable pending mass analysis, identification is tentative since KOvalues for M+ and MH+ for these compounds were virtually identical (8). Values for w I l 2 in , other laser-IMS studies were a factor of 2 to 3 larger than those in Figure 1 and were controlled by laser beam parameters as described below. Multiple peaks in benzene and toluene mobility spectra at KOvalues (greater molecular size and longer drift times) lower than those for M+ or MH+ may have been dimers or contaminants. Dimers were unlikely since incremental differences (cm2/V.s) were slight at 0.18-0.26 for benzene and 0.19 for toluene. Peaks for molecular ions for all compounds showed slight tailing with skew ratios of 0.40 to 0.60. The relationship between drift time and tube voltage according to the Nernst-Einstein relationship was used to measure mobility trends. These measurements were consistent with product ion formation and regular mobility principles. Pulse Energy, Beam Fluence, and Beam Geometry. In Figure 3, ion current and peak widths at half-height are plotted vs. beam energy for two sizes of circular beams and for a rectangular beam. In this and later studies, the ion current was determined by using peak height at the maximum. With each curve, increases in energy per pulse resulted in greater peak heights. As the laser energy was increased from 250 to 1000 wJ/pulse, effects from beam geometry and size became noticeable. Total response for beams of different sizes (and cross sectional areas) increased in the order 2 mm round (3 mm2) > 4 mm round (12 mm2) > 2 X 8 mm rectangle (16 mm2),but response factors for energylfluence were unaffected by size or shape. Although these results demonstrated a dependence of total response on beam size and energy, observations on reproducibility of response made during these studies had greater practical importance in laser-IMS. The
I m
a
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Figure 3. Plots of ion current vs. pulse energy for naphthalene in laser-IMS for peak heights (A) and peak widths at half-height (B). Beam diameters were 2 mm round, 4 mm round, and 2 mm X 8 mm
rectangular. Error bars are shown In lower right corners. error bars shown in Figure 3 include only variance from energy deviations pulse to pulse (f50 pJ) and so are minimum values for error particularly in terms of day-to-day variations. Other effects on reproducibility and indeed overall laser-IMS behavior were inhomogeneous cross sections of laser beams and location of the beam in the IMS ionization chamber. For example, in Figure 4, inhomogeneity of beam cross sections is shown to demonstrate the importance of beam alignment and of selection of portions for use in reproducibility of ionization. Only portions of the beam were used since a focused beam caused gaseous breakdown in air or nitrogen. The importance of alignment of doubling crystals can be seen in comparison of frames A and B in Figure 4. Since alignment was temperature dependent and since temperature varied as a function of time and energy, long-term drift of signal was particularly severe. Drift of over 200 to 400 mJ was observed with the NdYAG over a 30-min period and regular attention to tuning was necessary particularly after laser power was set to another value. The position of the laser beam inside the IMS was also important from two perspectives. Grazing of surfaces inside the IMS caused irregular and often enhanced response, which was initially unpredicatable and was avoided in further studies. Also significant was the possibility of inhomogeneity in the distribution of neutrals in the flowing gas drift inside the IMS. This aspect could have a magnified affect in variance since only a small portion of the drift gas was sampled by the small laser beam as described in better detail in the next section. The increase in bandwidth for peaks was also attributed to the small size of the laser beam and the creation of ion density greater than that seen with chemcial ionization IMS. Based on ion currents and beam volumes inside the reaction ring, ion densities increased from 840 ions/mm3 at 100 PUJl pulse to 1.45 X lo4 ion/mm3 at 800 wJ/pulse or a factor of
ANALYTICAL CHEMISTRY. VOL. 58. NO. 8. JULY 1986
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Fluwe 5. M o t bn anem vs. (Lancaat tardbmters for circular cross-sBc1im for laser beam. Dlamters were 1 mm. 2 mm. 4 mm. and 10 mm. Error bars are shown in lower right corners. Frame A is for peak haiiht and h a m B is for peak widm at half-habht. F l p v 4. F?mtogapcrS of beam cross secflon han M V A G bVrm doubling crystal detuned (A) and tuned (6). Energy vas near 100 rJ/pdse.
17 increase in ion density. This led presumable to ion-ion repulsions and a 2.5-fold increase in bandwidth. These densities were much greater than I@ ions/mm3 as calculated for chemical ionization IMS (18) hut much lower than I@ ions/mm'reported from a prior laser-IMS study ( 1 4 ) . Lower pulse energies in laser-IMS should favor improved resolution for mixture analysis although S I N ration will also be lowered. Regardless of complications cited here, energy of the laser beam did effect IMS response, although heam fluence (energytarea) may be more descriptive than energy per pulse. Results from measurement of response vs. fluence (and beam size)are shown in Figure 5. Since the crms-spction area of the IMS window intm the reaction region was 4 mm X 8 mm, IMS response for beams with diameters greater than 4 mm should he lower than expected due to simple physical obstruction of the beam. Ion currents from laser beams with diameters of 1 to 4 mm showed a linear rise with increased fluence inanmuch as the number of ions produced should be proportional to P (141. Curvature was found in plots for a 10 m m diameter beam and presumably was due to simple deflection losses of up to 50% from the beam cross section. However, dramatic leveling in response vs. fluence was not ohserved with the I and 2 mm diameter beams. Thus. IMS response was proportional not only to a laser beam fluence but also to cross sectional area and both parameters should be controlled in laser-IMS instrumentation. As with pulse energy studies described above, bands were broadened with creation of pealer ion densities as shown in frame B of Figure 5. Peak broadening was also linear w. fluence and no leveling in either ion current or bandwidth was o k r v e d . In summary, increaqes in pdsp energy or fluence will yield greater sensitivity with attendent increases in bandwidth. This approach to
n
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om ccm FlpmE. pld of km curent vs. dsifmm of hlet Me horn hser beam. Cuves IC? naphthalsn, were show for peak hei#ll (A)and peak wkRh at halfheight (6). E m f bars are shown in lower r@Mcorners. S
DIE~INCC
improved S I N may be suitable for simple mixtures where losses in resolution might be tolerated but would be unacceptable for analysis of mixtures containing similar KOvalues for components. For example, resolutions between benzene and toluene were estimated by using conventional separation definitions as 0.75 at 25 rJ/mm2. 0.65 at 75 rJ/mm2. and onlv 0.2 at 150 rJ/mm2. Effects from Location of Samale Introduction. In unidirectional flow IMS, a8 introduced by Hill (17). drift gas is used to carry analyte into the reaction region from a point
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CMS)
Figure 8. Effect of hi@ fluence on spectra for benzene. Spectra were selected from low, medium, and high concentrations of benzene with beam fluence of 206 pJ/mm2.
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Figure 7. Response curves for aromatic hydrocarbons in laser-IMS at differentbeam ftuences. Curves are for (A) naphthalene, (6)toluene, and (C) benzene. Fluences (pJ/mm2)were A (0)94, (A)56, (H) 5; B (0)156, (A)56, (H) 26; and C (0)206, (A)125, (U) 22. Fluence X 16 mm2 equals the energy/pulse of the laser beam. Estimated error
bars are shown in lower right corners.
located upstream (down gradient electrically)from 63Nisource (I7). Unidirectional design has been characterized by reduced residence time and low memory effects and control over concentrations in the reaction region was suitable for completion of quantitative proton exchange studies (8,11). This configuration was also chosen for a laser-IMS as shown in Figure 1. However, response was irregular and irreproducible during preliminary operations until location and alignment of the sample inlet tube were indentified as the cause for this behavior. The position or distance of the inlet in the IMS tube up-flow from the laser beam affected IMS response as shown in Figure 6 for a constant rate of naphthalene doping into drift gas. This rate was 50 ng J s into a flowing drift gas at 600 mL/min. When the sample was introduced close (2-4 cm) to the laser beam, response was weak, but response increased rapidly by 3.2-fold over a distance of 4-7 cm displaced from the ionization chamber. Since only a part of the total reaction ring volume was sampled by the laser beam, streaming of analyte through the IMS tube without extensive mixing
in the drift gas was likely responsible for the observed behavior. At a location near the reaction ring, this effect should be more pronounced, while at increased displacement, mixing of analyte and drift gas should be more complete and the effect should be less noticeable. The drop in peak current from 7.5 to 10 cm was within statistical deviations. Although these flow characteristics have never been reported in a IMS tube of any design, rationalization of these results was limited by uncharacterized flow patterns inside closed-tube unidirectional flow tubes. Response Curves. Plots of ion current vs. concentration are shown in Figure 7 for naphthalene, toluene, and benzene with a 2 mm X 8 mm rectangular beam at high, medium, and low fluences. Generally, response curves showed proportional relationship to concentration and curves were proportioned to concentration if not especially linear. The sensitivity and LOD for benzene were improved with higher fluences (pulse energies) as seen in slopes of 0.8 pA/ppb a t 22 pJ/mm2 and 2 pA/ppb a t 50 125 pJ/mm2. While comparable behavior was also observed with toluene at fluences below 56 pJ/mm2 (900 d/pulse), the slope for naphthalene response was nearly lo3 greater at near 917 pA/ppb for similar fluences. Values (and t) were as follows: benzene, 204 nm (7900); for A,, toluene, 207 nm (7000); and naphthalene, 286 nm (9300). Response curves seemed sensitive to spectral profiles. Differences in curves were outside statistical deviation, as demonstrated by the estimated error bars shown in these figures, and major differences should not be observed for curves that differ by less than 200-300 J per pulse or by 12-19 pJ/mm2. Clearly, the laser-IMS was sensitive to operating parameters of the instrumentation that were not completely controlled and this limited quantitative precision. As seen with effects of energy, IMS spectra were concentration-dependent, particularly at high fluences. The effect of larger fluence on IMS response is shown in Figures 8-10 where peaks in spectra were broadened and more complex at higher concentrations at fluences above 75 pJ/mm2 (2 &/pulse). Although band broadening was observed for all compounds, only naphthalene did not exhibit extensive dimer formation and fragmentation. Presumably fragmentation of naphthalene may occur at energies greater than 1.5 mJ/pulse, but this speculation was not confirmed. As above, ion-ion repulsion was believed responsible for peak shape, although ion-molecule associations in the drift region must be con-
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Figure 11. Noise in unaveraged signal for laser-IMS under conditions of (A) no sample, (8)blocked laser beam, (C) no tube voltage applied, and (D) no power to oven heater.
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Figure 9. Effect of high fluence of spectra for toluene. Fluence was 156 p.l/mm2 and concentrations are shown.
I
I
Since the tube was already shielded in a walk-in Faraday cage, effects directly from the laser are unknown. However, when the cage door was open, no increase in noise was observed. No improvement in S I N through reduction in noise seemed immediately possible from these studies. Removing the quartz windows improved LODs slightly with about a 1.5-fold increase in the response. ACKNOWLEDGMENT The positive influence of J. Rein, formerly of Los Alamos National Laboratories, in promoting this joint project is gratefully acknowledged. The IMS components were constructed by John Tobin and Hans Bishof at NMSU. Aid in installation of the laser-IMS and in collection of data at Los Alamos National Laboratory by Monte Ferris is greatly appreciated. Registry No. Benzene, 71-43-2;toluene, 108-88-3;naphthalene, 91-20-3.
LITERATURE CITED S
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Flgure 10. Effect of high fiuence of spectra for naphthalene. Fluence was 94 A/mm2 and concentrations are shown.
sidered another source of band broadening. However, in other IMS studies, with the same aromatic hydrocarbons and in which neutrals were added to the drift gas, no band broadening was observed (19). Other molecules may be susceptible to ion-molecule reactions leading to increased bandwidths as observed in previous studies (20). While such behavior seems to be pronounced with hydrogen-bonding molecules which form cluster ions, no similar properties have been reported for these aromatic hydrocarbons and ion repulsions seem most likely for observed bandwidths. N o i s e F e a t u r e s of L a s e r - I M S . Noise observed in the laser-IMS a t first seemed to exceed that for chemical ionization IMS and several parameters were tested to determine contributions to noise. Results from successively removing sample, beam, tube voltage, and heating jacket voltage are shown in Figure 11. No major contribution to noise was seen from the laser beam, sample, or tube voltage and noise was: mostly due to amplifier electronics and live transmission.
(1) Kim, S. H.; Betty, K. R.; Karasek, F. W. Anal. Chem. 1978, 50, 2006. (2) Kim, S. H.; Karasek, F. W.; Rokushika, S. Anal. Chem. 1978, 5 0 , 152. (3) Carr, T. W. Anal. Chem. 1977, 49, 828. (4) Cohen, M. J.; Karasek, F. W. J . Chromatogr. Sci. 1970, 8 , 330. (5) Benezra, S. A. J . Chromatogr. Scl. 1976, 14, 122. (6) Metro, M. M.; Kelier, R. A. J . Chromatogr. 1973, 1 1 , 520. (7) Metro, M. M.; Keller, R. A. Sep. Sci. 1974, 9(6), 521. (8) Vandiver, V. J.; Leasure, C. S.; Eiceman, G. A. I n t . J . Mass. Spectrom. Ion Processes 1985, 66, 223. (9) Leasure, C. S.; Eiceman, G. A. Anal. Chem. 1985, 57, 1890. (10) Eiceman, G. A.; Leasure, C. S.; Vandiver, V. J. Anal. Chem. 1988, 58, 76. (11) Eiceman, G. A.; Leasure, C. L.; Vandiver, V. J.; Rico, G. Anal. Chim. Acta 1985, 175, 135. (12) Proctor, J.; Todd, J. F. J. Anal. Chem. 1984, 56, 1794. (13) Baim, M. A.; Eatherton, R. L.; Hili, H. H., Jr. Anal. Chem. 1983, 55, 1761. (14) L u b k n , D. M.; Kronick, M. N. Anal. Chem. 1982. 54, 1546. (15) Lubrnan, D. M.; Kronick, M. N. Anal. Chem. 1983, 55, 867. (16) Lubman, D. M.; Kronick, M. N. Anal. Chem. 1983, 55, 1486. (17) Baim, M. A.; Hili, H. H., Jr. Anal. Chem. 1982, 54, 38. (18) Spangler, G. E.; Collins, C. 1. Anal. Chem. 1975, 47, 403. (19) Eiceman, G. A.; Vandiver, V. J. Anal. Chem., in press. (20) Leasure, C. S.; Fleischer, M. E.; Eiceman, G. A , , submitted for publication.
RECEIVED for review October 28, 1985. Resubmitted April 7, 1986. Accepted April 7, 1986. This work was supported for G.A.E., C.S.L., and V.J.V. through the U S . Department of Energy, Contract No DE-AS04-83ER60184.
