Anal. Chem. 1907, 59, 2283-2288
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Supercritical Fluid Chromatography Interface for a Differentially Pumped Dual-Cell Fourier Transform Mass Spectrometer David A. Laude, Jr., S t e p h e n L. Pentoney, Jr., Peter R. Griffiths, a n d Charles L. Wilkins*
Department of Chemistry, University of California, Riverside, California 92521
A Fourier transform m a s spectrometer used as a detector and ldentifler for supercritical fluid chromatography demonstrates low nanogram detection limits and a pressure-limited resolution In excess of 10 000 at m / r 128 for the molecular b n of naphthalene. Supercritical carbon dioxide Is introduced directly #It0 the source of a dual differentially pumped trapped ion ceU generating pressures In the low lo4 Torr range with sample detection occurrlng In the analyzer cell at pressures a factor of 100 lower. Ionization is accomplished by charge exchange with carbon dioxide to yield spectra exhibiting 70eV-llke fragmentation patterns. Supercritical fluid chromatography/Fourler transform mass spectrometry data for three test samples, a six-component mixture of substttuted aromatk compounds and poiyaromatk hydrocarbons, a flvecomponent barbiturate mixture, and a seven-component pesticlde mlxture, are presented.
Incompatibility between the high gas load imposed by some types of mass spectrometric sources and the low-pressure analyzer requirements of Fourier transform mass spectrometry (FTMS) in its original single-cell configuration has hindered the analytical development of the technique (1). In particular, the union of FTMS with gas chromatography (GC) and liquid chromatography (LC) interfaces is inferior to other separation/characterization techniques which use quadrupole and sector mass spectrometers despite demonstrated FTMS advantages including rapid scanning (21,high resolution (31, and accurate mass measurement in a dynamic measurement mode (4). Among the several demonstrated solutions to the problem of mismatch between source and analyzer pressure requirements are pulsed introduction of the sample (5) and two distinct ionization/detection configurations, one with a source cell placed in the magnetic field and adjacent to the analyzer cell (a dual-cell design) (6) and the other with the source located in the fringe magnetic field where conventional MS sources may be employed (external source design) (7, 8). Although the external source appears most promising for high mass analysis (9),the dual cell designed by Nicolet Analytical Instruments appears to be readily amenable to both direct introduction GC and supercritical fluid chromatography (SFC). The renaissance in supercritical fluid chromatography has been facilitated, in part, by successful extension of open tubular capillary column SFC to information-rich mass and infrared detectors as alternatives to technically more difficult liquid chromatography interfaces (10-12). Especially for mass spectrometry (MS), the analysis of nonvolatile compounds by using capillary SFC for sample introduction is superior to LC because of reduced gas loads and ease of solvent elimination, with potentially improved MS sensitivity. Smith and coworkers have developed an SFC/MS system with quadrupole MS and report low-picogram detection limits for chemical ionization (CI) mass spectra (13). The decreased gas load generated by SFC is especially appealing for FTMS applications which require analyzer cell pressures of lo-' Torr and
lower. Henion and Cody recently demonstrated the feasibility of dual cell SFC/FTMS in initial experiments which, even with the dual cell design, approach the limited gas load capabilities of FTMS (14). LC/FTMS will likely require still further alterations in source, detector, and vacuum system design. In the present work an SFC/FTMS interface was constructed which utilizes the pressure differential between source and analyzer cells to facilitate high-pressure ionization followed by selective transfer of sample ions to the analyzer cell for detection. Although high-pressure conditions reduce the effectiveness of electron ionization (EI) and chemical ionization (CI), use of COB as a charge-transfer reagent permitted the acquisition of electron impactlike spectra for mixtures of substituted aromatics and polyaromatic hydrocarbons (PAH), pesticides, and barbiturates. EXPERIMENTAL S E C T I O N Supercritical carbon dioxide was pumped by a Varian Model 4300 syringe pump through a Hewlett-Packard Model 5890A gas chromatograph with a Commodore 64 microcomputer used to pressure program the separations. The Fourier transform mass spectrometer was a Nicolet FTMS-1000 operating at 3.0 T, upgraded to accommodate a Nicolet-designed cubic 17/8-in.dual differentially pumped cell of 80% transmissive stainless steel, with a source/analyzer pressure differential of a factor of 100 for COz. Alcatel diffusion pumps, with pumping speeds of 700 L/s and 300 L/s on sources and analyzer vacuum chambers, respectively, maintained background pressures of less than 1 X lo4 Torr with the COPflow off. A probe-mounted filament was inserted through a gate valve on the analyzer side of the FTMS and positioned 20 cm from the cell to provide a source of electrons. Nicoletdeveloped GC/FTMS software executed on a Nicolet 1280 computer was used to acquire and process the SFC/FTMS data. SFC/FTMS Interface Design. Schematic diagrams of the interface are presented in Figure 1. The effluent from the SFC column was transferred through a 2 m X 156 pm i.d. uncoated fused silica capillary, heated to 45 "C,into the FTMS source side vacuum chamber. A 10 cm X 5 pm i.d. length of fused silica, connected to the transfer line with a 1/32-in.Valco zero dead volume union, served as the pressure/flow restrictor. Inside the vacuum system the transfer line and restrictor were sheathed in thin-wall stainless steel tubing. The vacuum seal was made by using a Swagelok reducing union and a Vespel/graphite ferrule as shown in Figure lb. The stainless steel tubing facilitated heating of the transfer h e , as it was wrapped with thermocouple wire which was connected by feedthroughs to an external power supply. The temperature at the restrictor was monitored by thermocouple wire which was threaded into the '/gin. tubing up to the vacuum seal. With the temperature of the vacuum can maintained at 160 "C and a restrictor temperature in excess of 250 "C, sample clustering was not observed for the compounds analyzed. Chromatography. Carbon dioxide, with a P,of 1072 psi and T,of 32 "C, was used as the mobile phase for all experiments. Separations were pressure programmed from 1250 to 4000 psi at rates of 25 or 30 psi/min following initial isobaric delays of 10-40 min. Initial linear velocities between 1and 2 cm/s were obtained for the restrictor used, the actual value depending upon the restrictor temperature and the initial pressure. These linear velocities were higher than the value required to achieve the highest chromatographic resolution but permitted run times of
0003-2700/87/0359-2283$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987
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TRAP PLATES SOURCE COND LIMIT ANALYZER + 0 Q U E N C H L S r n s
a) 01
o n gauge
gauge
I
+
0
+
+
+
+
E X C I T E L I 2 rns,0-2 6MHz
+
+
+
A C Q U I R E 0 2 6 MHz, 32K data
BEAM -25
ms
EJECT 52-
in4
Figure 2. Experiment pulse sequence used during the SFCIFTMS
diffusion
experiments. Potentials applied to the trap plates and conductance limit of the dual cell to facilitate transfer and trapping of positive ions are included.
diffusion
pump
pump
b) i o cm x
STAINLESSSTEELTUBING
sum ia ILlCA RESTRICTOR
I SEALTO VACUUM
I
CiLL
Flgure 1. (a) Schematic diagram of upgraded differentially pumped
dual cell FTMS system used as a detector for SFC. (b) Expanded view of SFC/FTMS transfer line and interface. Table I. Dual-Cell Pressure Differentials in the FTMS from Direct Introduction of Supercritical COz ratio column head pressure (psi) 1100 1200 1412 1775 2140 2500 3010
3530 4010
source ion gauge, Torr 3.7 x 4.3 x 6.0 x 9.5 x 1.3 x 1.8 x 2.5 x 3.1 x 3.5 x
10-5 10-5
10-5 10-5 10-4 10-4 10-4 10-4 10-4
analyzer ion gauge, Torr 3.8 x 4.4 x 6.0 x 9.6 x 1.3 X 1.8 x 2.5 X 3.3 x 4.2 X
10-7 10-7 10-7 10-7
10" 104 lo+
10" 10"
(:source/ana-
lyzer) 97 98 100 99 100 100 100 94 83
about 1h. Samples were dissolved in suitable low-boiling solvents and injected directly without splitting in 60-nL quantities onto a 20 m X 100 wm i.d. DB-5 (5% phenylmethyl-silicone) column, highly cross-linked for SFC applications (J&W Scientific). Isothermal temperatures between 100 and 150 "C were maintained in the SFC oven. Specific chromatographic conditions for all separations are included in the figure captions of SFC/ FTMS reconstructured chromatograms. FTMS Instrumental a n d Software Parameters. SFC effluent introduced directly into the source-side vacuum chamber within 2 cm of the cell yielded source-side ion gauge readings of between 3 X and 3 X lo4 Torr as indicated in Table I (actual pressures at the cell were probably 3-5 times higher). Under molecular flow conditions which were present at source ion gauge readings up to about 2 X lo4 Torr, the conductance limit provided an analyzer cell pressure a factor of 100 lower. As column head pressurs exceeded 3500 psi, transition flow conditions predominated and the source/analyzer pressure-differential decreased. Conventional modes of ionization were not practical because of the high excess of CO,; for example 70-eV E1 spectra demonstrated very low sensitivity because of the dynamic range limitations of the cell, even when the COP ion with mlz 44 was continuously ejected. The addition of reagent gases for chemical ionization only exacerbated the problems a t high pressure (although self-CI is an alternative (14)). Ultimately, satisfactory ionization of the analyte was achieved by using a 13-eV electron beam which selectively ionizes the majority of organic molecules
(IP = 8-10 eV) while minimizing C02 ionization (IP = 13.6 eV). Optimized FTMS ionization conditions included a 20-25-ms electron beam of 13 eV to maximize the duty cycle, a 1.024 MHz ejection pulse applied continuously during the beam to eject COP ions at mlz 44, and trap voltages of 1.5-2.0 V. The optimum emission current was established just prior to an SFC separation by leaking 1 X lo-? Torr of o-dichlorobenzene into the source through the volatile inlet in the presence of 1 X Torr of COP and maximizing the molecular ion resonance (mlz 146). SFC/FTMS measurements with ncminal unit mass resolution up to mass 500 provided maximum analyte signal-to-noise ratio when 32K transients were acquired over a 2.6-MHz bandwidth. One hundred co-added spectra were stored on disk every 4 s, which was sufficient to define the minimum SFC peak widths of 30-60 s. Data processing of all spectra shown included a single zero-fill, base-line correction, and sine-bell apodization prior to magnitude mode Fourier transformation. High-resolution spectra obtained under pressure-limited, rather than data-point-limited, conditions required a minimization of the number of ions in the cell, with a concomitant decrease in spectral signal-to-noiseratio. Altered conditions for high-resolution measurements included a 0.7-V trap voltage and decreased emission current. Data points (64K) were collected over a 400-kHz bandwidth in direct mode and were processed as described above. Heterodyne measurements over smaller bandwidths did not yield increased resolution measurements, which indicated the resolution was pressure-limited. Observation of analyte spectra was achieved only because the dual cell design permitted selective transfer of the ions from the source cell to the analyzer cell maintained at pressures in the low lo4 TORregion. Figure 2 presents a diagram of the discrete events and their associated trap plate potentials for the FTMS pulse sequence. Ions were continuously transferred during the beam between source and analyzer cells by grounding the conductance limit plate. The electron beam passing through the two cells served to focus the positive ions created, and transfer efficiencies approaching the optimum value of 50% were achieved. Because the beam time of 20-25 ms was far in excess of the average transfer time for the low mass ions created, no mass discrimination effects were observed. This is in contrast to alternative uses of the dual cell in which ions created on the source side are selectively transferred after ionization ( 1 5 ) . Separations. Table I1 lists the pertinent information for the three model mixtures which were successfully analyzed by SFC/FTMS. The mixture of aromatic molecules was employed to determine system capabilities such as detection limits, mass resolution at increasing pressures, and effectivenessof the interface for increasingly nonvolatile compounds. The barbiturate and pesticide mixtures provided further evidence regarding the generality of the interface.
RESULTS AND DISCUSSION Selection of an appropriate SFC restrictor limits the flow rate and the source pressure which ultimately determines the mass resolution. It was determined empirically that a 10 cm x 5 wm restrictor yielded a maximum source pressure in the mid Torr range for C 0 2 column pressures of 4000 psi. Thus, given the pressure differential factor of 100,spectra from SFC separations would be acquired at background pressures not exceeding 3 X lo4 Torr. In this pressure regime, spectra with pressure-limited resolution due to collisional damping
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Table 11. Mixture Components and Elution Pressure Data for the Three Samples Analyzed by SFC/FTMS elution order separation 1
o-dichlorobenzene naphthalene 1-bromonaphthalene fluorene anthracene pyrene barbital amobarbital aprobarbital mephobarbital phenobarbital lindane aldrin DDE dieldrin DDD DDT methoxychlor
1
2 3 4 5 6 separation 2
1
2 3
4 5 separation 3
compound
1
2 3
4 5 6 7
elution mol pressure, wt psi 146 128 207 166 178 202 184 226 210 246 232 288 362 316 381 318 352 344
1715 1850 2190 2270 2500 2750 2120 2390 2420 2540 2660 2510 2750 2900 2960 2990 3050 3140
d I
'S',
I
25
30
35
40 RUN 1 I M E
45 50 lh M I N U T E S
55
I
62
Flgure 4. Segment of reconstructed SFC/FTMS chromatogram (150-230 amu integration window) from a 56 ng per component injection of the six-component substituted aromatic mixture. Numbered peaks reefer to the elution order in Table 11. SFC conditions were as follows: oven temperature 100 OC, pressure programmed from 1250 to 3000 psi at 30 psi/min after an initial 10-min isobaric period.
D 1
Figure 3. FTMS spectra from 6 ng injected per component of (a) 1-bromonaphthalene, (b) o dichlorobenzene, and (c) pyrene for an SFC/FTMS separation of the six-component mixture of aromatics. Each spectrum is from a single stored file of 100 scans collected over 4 s.
may be acquired in direct mode. High source pressures of COz hindered ionization of the sample; for example even with the continuous ejection of m / z 44 C 0 2 ions, 70-eV E1 detection limits were in the low microgram range because of the dynamic range limitation of the cell. However, the combination of a 13-eV electron beam in the presence of COz with continuous ejection of COz ions provided surprisingly good sensitivity for analyte spectra. FTMS spectra of three compounds from the mixture of aromatic molecules with 6 ng injected per component are shown in Figure 3. Although sensitivity decreased with increasing source pressure due to the larger percentage of COz ions in the cell, the effect was not significant at SFC column pressures below 3000 psi which corresponded to source pressures of 1 X Torr. The spectra suggest detection limits in the
midpicogram range for this interface and ionization method. The focusing of signal intensity into the relatively few fragment ions produced by these compounds, however, undoubtedly provides near-ideal detection limits. Nevertheless, these results indicate that detection limits in the 1-ng range should be routinely achieved for low molecular weight organic analytes. To test the viability of the interface for the increasingly less volatile compounds which elute a t higher COz pressures, polyaromatic hydrocarbons of higher mass were added to the test mixture. Figure 4 presents the SFC/FTMS reconstruction from a six-component mixture with pyrene eluting at 2750 psi. Other PAHs which were successfully analyzed at higher pressures include benzo[a]pyrene a t 3000 psi and perylene at 4100 psi. However, at these pressures with the present interface sensitivity decreased 10-fold,and spectral resolution and line shape were distorted by space charge effects. Even with an electron beam of 13 eV, the 140 V,, power of the excitation amplifier was insufficient to effectively eject COz+ from the dual 17/*-in.cubic cells. The sensitivity performance described above and illustrated in Figure 3 was obtained at the expense of resolution by setting bandwidth and number of data points equal to about 3 times the lifetime of the ions, T , and increasing the emission current
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Table 111. Pressure-Limited Resolution Data for FTMS Spectra of Aromatic Compounds"
column pressure, compound
m/z
psi
o-dichlorobenzene naphthalene bromonaphthalene fluorene anthracene pyrene
146 128 207 166 178 202
1715 1850 2190 2270 2500 2750
source pressure, torr
resolution (fwhh)
6.0 X 7.0 X 9.6 X
9853 10682 6878 7985 8108 6026
1.0 X 1.2 X low4 1.4 X lo4
"FTMS conditions: 200 ng per component, 64K data over 400 = 0.7 V, decreased emission current.
kHz bandwidth with 64K zeroes, trap a)
SCC RW TIME
C 0
lmin)
1 , 750
0 0 1
-1
35
40
is SfC Ru(
50 TIME (minl
i5
Figure 6. Segments of reconstructed SFC/FTMS chromatograms for (a) 60 ng injected per component of a five-component barbiturate mixture, arid (b) 200 ng per component of a sevencomponent pesticide mixture. Numbered peaks refer to the elution order in Table 11. SFC conditions for the barbiturates were as follows: oven temperature 100 'C, pressure programmed from 1250 to 4000 psi at 25 psi/min after an initial 25-min isobaric period. The integration window was 150-230 amu. SFC conditions for the pesticide mixture were as follows: oven temperature 150 'C, pressure programmed from 2000 to 4000 psi at 30 psi/min after an initial 20-min isobaric period. The integration window was 140-300 amu.
Flgure 5. FTMS spectra of (a) naphthalene and (b) l-bromonaphthalene molecular ion regions with data acquisition parameters chosen to obtain pressure-limited resolution values.
and trapping voltages to maximize the ion population in the cell. Operating under these conditions, data-point limited mass spectra with a t least unit mass resolution up to mass 500 were obtained. In contrast, pressure-limited spectra displaying the maximum possible resolution were acquired by increasing the acquisition time to 5 to 10 times T and minimizing space charge in the cell. With a systematic decrease in emission current and trap voltage, resolution was increased a t the expense of sensitivity until pressure-limited conditions with minimal space charge perturbation were obtained. Table I11 presents pressure-limited resolution values for the aromatic mixture with 200 ng injected per component. Representative spectra of the molecular ion region for naphthalene and 1-bromonaphthalene are presented in Figure 5. Resolution values for the dynamic SFC/FTMS measurement were equivalent to those achieved after tuning the instrument in the static batch mode. The product of mass and resolution did not vary appreciably as a function of pressure, given the
relatively small differences in pressures generated by the C02 pressure ramp. To test the generality of the interface and experiment parameters, seveal additional test samples, including a fivecomponent barbiturate and a seven-component pesticide mixture, were analyzed by SFC/FTMS. FTMS reconstructions of the two separations are presented in Figure 6 along with representative mass spectra in Figures 7 and 8. . For both separations, the conditions established for the high sensitivity SFC/FTMS analysis yielded satisfactory spectra, despite the observation that only a very select set of conditions would yield mass spectra at these high pressures. The reconstructions presented in Figures 3 and 6 were generated from a summation of ion intensities over wide mass ranges which significantly degraded the signal-to-noise ratio; for a mass range of 100, typical chromatographic detection limits are in the low nanogram range. Using the equivalent of a single ion monitoring procedure when processing the data yields reconstruction sensitivities approaching the detection limits for the mass spectrum. The time resolution of 4 s for these separations was adequate for the half-widths of typical peaks for a 100 pm i.d. column but could easily be reduced to less than 1 s if the chromatography had been performed at higher resolution.
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E
3
G
Flgwe 7. FTMS spectra from five-component barbiturate sample with 60 ng injected per component of (a) barbital, (b) aprobarbital, and (c) amobarbital. The peak at m l z 161 in part c is a noise spike.
Total signal response for the pesticide and barbiturate mixtures is similar to that for the aromatic compounds and PAHs, although overall spectral signal-to-noise ratio decreases proportionally as fragmentation increases. Thus although the barbiturate spectra, for which the signal is concentrated in a few ions, exhibit excellent spectral signal-to-noise ratio, several of the pesticides exhibit decreased sensitivity due to extensive fragmentation. This can be observed in Figure 8 by comparing the spectra of aldrin and dieldrin, which exhibit considerable fragmentation, to that of methoxychlor, for which the molecular ion predominates. The mode of ionization for these SFC/FTMS measurements appears to involve charge exchange with COz, as an excess of 3-4 eV in energy corresponding to differences in ionization potential between the COz and the analyte is available for fragmentation. A comparison of SFC spectra for the 18 compounds listed in Table I1 with 70-eV spectra from the NBS mass spectral library indicates a similarity in
Flgure 8. FTMS spectra from seven-component pesticide sample with 200 ng injected per component of (a) aldrin, (b) dieldrin, (c) methoxychlor, and (d) lindane.
mass-intensity information at higher masses, generally above 100 amu. Low mass fragmentation, especially for the highly chlorinated pesticides, is present in the SFC/FTMS spectra but a t substantially reduced intensities. The capability of generating 70-eV-like spectra by this mode of ionization is important given the lack of success with LC
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987
interfaces in generating E1 spectra with sensitivities approaching those for CI spectra. LC/MS detection limits for while Smith and Co-workers E1 spectra are around 1pg (16), have devised an interface for SFC/MS with reported E1 spectra from 100-pg injections (17). The present efforts with FTMS are promising not only because spectral identification by library search techniques is feasible a t the low nanogram level but also because if the enhanced sensitivity demonstrated for CI spectra with other SFC and LC/MS interfaces is retained, then low picogram detection limits for SFC/FTMS could be achieved. Any significant improvements in SFC/FTMS require that analyzer pressures be reduced by 1 to 2 orders of magnitude, not only to accommodate materials eluting a t higher SFC pressures but also to enhance FTMS resolution and spectral signal-to-noise ratio. Within the constraints of the present MS pumping system, significant decreases in pressure could be achieved by increasing the flow restriction, which would result in a decreased linear velocity of the mobile phase. Although longer analysis times for separations effected by using a 100 pm i.d. column would ensue, the chromatographic efficiency would be improved. An obvious improvement in the SFC/FTMS interface, given the sensitivity of the current design, would be to reduce the gas load by employing 50 pm i.d. capillary columns a t the same linear velocities. In this manner, the gas load could be reduced 4-fold for the same analysis time, with the benefit of higher chromatographic efficiency but the disadvantage of decreased capacity. Among the improvements to be realized at reduced source pressures are increased mass resolution, increased sensitivity derived from a reduction in mobile phase ions generated, and alternative ionization methods including chemical ionization and 70-eV electron ionization. The significantly improved operating conditions at lower pressures will also permit low parts per million mass measurement accuracy over the entire mass spectrum to be achieved and, more importantly, facilitate the analysis of larger molecular weight compounds. Given the intolerance of FTMS to high gas loads, SFC/FTMS should provide an attractive alternative to liquid chromatographic
methods for the analysis of nonvolatile and thermally labile compounds.
ACKNOWLEDGMENT We thank Carl Ijames for building the probe-mounted filament.
LITERATURE CITED (1) Laude, D. A., Jr.; Johlman, C. L.; Brown, R. S.;Weil. D. A,; Wilkins, C . L. Mass Spectrom. Rev. 1988, 5+ 107-166. (2) White, R. L.; Wilkins, C. L. Anal. Chem. 1982, 5 4 , 2443-2447. (3) Sack, T. M.; McCrery, D. A.; Gross, M. L. Anal. Chem. 1985, 5 7 ,
1290-1296. (4) Johlman, C. L.; Laude. D. A., Jr.; Wilkins, C. L. Anal. Chem. 1985, 57, 1040-1044. (5) Sack, T. M.; Gross, M. L. Anal. Chem. 1983, 55, 2419-2421. (6) Nicolet Analytical Instrument Guide; Nicolet Analytical Instruments: Madison, WI, 1985. (7) Kofel, P.; Allemann, M.; Kellerhalds, H.; Wanczek, K. P. Int. J . Mass Spectrom. Ion Processes 1985, 6 5 , 97-103. (8) Hunt, D. F.; Shabanowitz, J.; McIver, R. T., Jr.; Hunter, R . L.; Syka, J. E. P. Anal. Chem. 1985, 5 7 , 765-768. (9) Hunt, D. F.; Shabanowitz, J.; Yates. J. R.; Mclver, R. T., Jr.; Hunter, R. L.; Syka, J. E. P.; Amy, J. Anal. Chem. 1985, 5 7 , 2728-2733. (IO) Pentoney, S. L., Jr.; Shafer, K. H.; Griffiths, P. R. J . Chromatogr. Sci. 1988, 24,230-235. (11) Shafer. K. H.; Pentoney, S. L., Jr.; Griffiths, P. R . Anal Chem. 1988, 58,58-64. (12) Wright, 6. W.; Kalinoski, H. T.; Udseth, H. R.; Smith, R. D. HRC CC, Chromatogr. Commun. J . High Resolut. Chromatogr. 1988, 9 , 145-153. (13)Smith, R. D.; Kalinoski, H. T.; Udseth, H. R.; Wright, 6. W. Anal. Chem. 1984, 5 6 , 2476-2480. (14)Lee, E. D.; Henion, J. D.; Cody, R. 6.;Kissinger, J. A. Anal. Chem. 1987, 59, 1309-1312. (15) Giancaspro, C.; Verdun, F. R. Anal. Chem. 1986, 58, 2099-2101. (16) Covey, T. R.; Lee, E. D.; Andries, P. 6.;Henion, J. D. Anal. Chem. 1988, 58, 1451A-1461A. (17) Smith, R. D.; Udseth, H. R.; Kalinoski, H. T. Anal. Chem. 1984, 5 6 , 2973-2974.
RECEIVED for review April 16,1987. Accepted June 17, 1987. D.A.L. and C.L.W. acknowledge support from the National Science Foundation under Grant CHE-85-19087. S.L.P.and P.R.G. acknowledge partial support from the U.S. Environmental Protection Agency through Cooperative Agreement CR-812258-02to the University of California, Riverside, and from the University of California Toxic Substances Program.
