1170
Anal. Chem. 1989, 61, 1178-1181 B
C
other plasma sources since the dc plasma can tolerate high levels of salts without adversely affecting the plasma. Detection limits for As, Ge, Sb, Sn, and P b determinations were 230, 12, 200, 20, and 140 pg mL-'. Further investigation of the optimum conditions for the determination of As and Sb should lead to improvements in the detection limits for these two elements.
LITERATURE CITED
Comparison of signals of blank and germanium in the presence of 1.0 M KCI and distilled water: A, blank, with introduction of 1.0 M KCI; B, 0.50 ng mL-' Ge, with introduction of 1.0 M KCI; C, 0.50 ng mL-' Ge, with introduction of distilled water. Flgure 2.
determinations of 2.0 ng mL-' lead gave RSD of 6.0% in the absence of EIE and 4.6% in the presence of 1.0 M CsC1. We can also see from Figure 2 that there was no signal from the germanium blank even in the presence of 1.0 M KCl. The introduction of EIE reagents, with analytical grade purity, to the dc plasma did not increase the blank value because the concentrations of the analytes in these reagents are negligible compared to their detection limits by conventional DCP-AES method. Calibration. With EIEs, reproducible and enhanced signals were achieved, which is promising in improving sensitivity for trace analysis. In order to estimate the analytical applicability of signal enhancement by EIE, selective calibration and linear regression were performed. In the cases of lead, for example, correlation coefficients of two curves were 0.997 for water aspiration and 0.998 for 1.0 M CsCl aspiration over the range of 0-80.0 ng mL-'. The slopes of the line were 1.14 and 2.17 for the introduction of distilled water and 1.0 M CsC1, respectively. Detection Limits. Detection limits, defined as 3 times the noise, for the determination of arsenic, antimony, germanium, and lead, were 230,200,12, and 140 pg mL-', respectively, with the introduction of 1.0 M EIE solution (KC1 or CsCl), whereas they were 360, 360, 20, and 250 pg mL-', respectively, in the absence of EIE. The detection limit for the determination of tin was limited by the reagent blank to a level of 20 pg mL-' as previously reported, although the introduction of EIE further enhanced the signal.
CONCLUSION Emission enhancement by EIEs has shown advantages in improving sensitivities for the determination of As, Sb, Ge, Sn, and P b by hydride generation. Alkali elements are preferred to alkaline-earth elements since the latter gave much higher background. The application of EIE signal enhancement seems to be more practical with dc plasma than with
Miller, M. H.; Eastwood, D.; Hendrick, M. S.Spectrochlm.Acta, Part8 1984, 398, 13-56. Szivek, J.; Jones, C.; Paulson, E. J.; Valberg. L. S. Appl. Spctrosc. 1968, 22. 195-197. Nygaard, D. D.; Gilbert, T. R. Appl. Spectrosc. 1981. 35,52-56. Johnson, G. W.; Taylor, H. E.; Skogerboe, R. K. Anal. Chem. 1979, 51. 2403-2405. Fox, R. L. Appl. Spectrosc. 1984, 38, 644-647. Cantillo, A. Y.; Sinex. S. A.; Helz, G. R. Anal. Chem. 1984, 5 6 , 33-37. Biggs, W. R.; Fetzer, J. C.; Brown, R. J. Anal. Chem. 1987, 59, 2798-2802. Johnson, G. W.; Taylor, H. E.; Skogerboe, R. K. Appl. Spectrosc. 1980. 3.4 . 19-24. ..., Nygaard, D. D.-Anal. Chem. 1979, 5 1 , 881-884. Skogerboe, R. K.; Urasa, I. T. Appl. Spectrosc. 1978, 32, 527-532. Frank, A.; Petersson, L. R. Spectrochim. Acta, Pari 8 1983, 388, 207-220. Urasa, I . T. Anal. Chem. 1984, 56, 904-908. Goiightiy, D. W.; Harris, J. T. Appl. Spectrosc. 1975, 29, 233-240. Bankston, D. C.; Humphris, S. E.; Thompson, G. Anal. Chem. 1979, 51. 1218-1225. Lajunen. L. H. J.; Kurikka, A.; Ojaniemi, E. At. Spectrosc. 1987, 8 , 142- 144. Eastwood, D.; Hendrick, M. S.; Miller, M. H. Spectrochim.Acta, Pari 8 1982. 378. 293-302. Johnson, G. W.; Taylor, H. E.; Skogerboe, R. K. Spectrochim. Acta, Part E 1979, 3 4 8 , 197-212. Williams, R. R.; Coleman. G. N. Appl. Spectrosc. 1981, 35,312-317. Felkel, H. L., Jr.; Pardue, H. L. Anal. Chem. 1978, 50,602-610. Blades, M. W.; Lee, N. Spectrochlm. Acta, Pari B 1984, 398, 879-890 - . Zander, A. T.; Miller, M. H. Spectrochlm. Acta, Part 8 1985, 4 0 8 , 1023- 1037. Miller, M. H.; Keating, E.; Eastwood, D.; Hendrick, M. S. Spectrochim. Acta. Part E 1985. 408. 593-616. Hendrick, M. S.; Seltzer. M. D.; Michei, R. G. Spectrochim, Acta, Part 8 1986, 4 1 8 , 335-348. Robbins, W. B.; Caruso, J. A. Anal. Chem. 1979, 5 1 , 889A-899A. Sparkes, S.;Ebdon, L. ICP I n f . News/. 1986, 12, 1-6. Brindle, I. D.; Le, X-c. Ana/ysf (London) 1988, 113, 1377-1381. Boampong, C.; Brindle, I. D.; Ceccarelli Ponzoni, C. M. J . Anal. At. Spectrom. 1987, 2 , 197-200. Brindle, I. D.; Le, X-C.; Li, X-f. Presented at the 4th Biennial National Atomic Spectroscopy Symposium, York, UK, June 29-July 1, 1988. J. Anal. At. Spectrom. 1989, 4 , 227-232. Spectraspan V Emission Spectrometer, Operator's Manual: Spectrametrics: Andover, MA, 1983. Boampong, C.; Brindle, I . D.; Le, X-c.; Pidwerbesky. L.; Ceccarelli Ponzoni, C. M. Anal. Chem. 1988, 60, 1185-1188. Jin, K.; Taga. M. Anal. Chlm. Acta 1982, 143, 229-236.
RECEIVED for review October 24,1988. Accepted January 31, 1989. The authors gratefully acknowledge receipt of a grant from the Ontario Government BILD program for the purchase of the Spectraspan V dc plasma atomic emission spectrometer. The authors also thank the Air Resources Branch of the Ontario Ministry of the Environment for funding this research (project 360 G ) .
Back-Pressure Regulated Restrictor for Flow Control in Capillary Supercritical Fluid Chromatography Douglas E. Raynie, Karin E. Markides, Milton L. Lee, and Steven R. Goates* Department of Chemistry, Brigham Young University, Prouo, Utah 84602
INTRODUCTION In chromatography, control of mobile phase flow rates near the optimal linear velocity preserves both efficiency and resolution. With gaseous mobile phases, especially in con0003-2700/89/0361-1178$01.50/0
junction with capillary columns, flow rate changes during temperature programming are commonly experienced but are not severe. In liquid chromatography, pumps are operated synchronously to deliver a controlled flow rate. Pressure 1969 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 10, MAY 15, 1989
(density) programming of the mobile phase from gaslike to liquid-like densities is used routinely in SFC for the control of solute elution. Since flow rates are a function of pressure, a means for flow control in SFC is desirable. Linear velocity variations in SFC can have significant effects on chromatographic resolution because of large mass transfer effects when nonoptimal conditions are used. In capillary SFC, efficiency, as a function of linear velocity, is described by the Golay equation (1). The effect of linear velocity on efficiency is pronounced in capillary SFC because linear velocities several times the optimum (-0.2 cm s-l for COz) are used, whereas in packed column SFC, operational linear velocities approach the optimum. Slopes of plate height versus linear velocity at velocities greater than the optimum increase sharply as larger diameter capillary columns are used (2). Linear velocity changes during pressure (density) programming significantly affect chromatographic efficiency; better efficiencies are obtained at low pressures (densities) and high temperatures (3). In packed column SFC, efficiency variations with linear velocity are more difficult to predict ( 4 , 5 ) . As the linear velocity increases in packed column SFC, the pressure drop along the column increases, resulting in a changing density gradient in the column. This makes the elution process difficult to describe. While density is the main factor affecting retention, the average flow rate is the most important parameter affecting resolution (4, t?), and optimum resolution occurs at the lowest ratio of pressure drop to total change in flow rate along the column (4). Klesper and Schmitz have reviewed the effects of linear velocity (flow) gradients in packed column SFC (7). Two methods of pressure programming have been used in SFC (8): (a) programming the inlet pressure while the outlet pressure is controlled by a restrictor or needle valve, and (b) delivery of a constant flow of mobile phase while controlling the outlet pressure by adjusting the stem of a needle valve. In capillary SFC, only the first approach has been applied. Pressure restrictors for capillary SFC have been evaluated (9-17). Flow control has been reported in only one study (18). Various flow control methods in packed column SFC have been reported. One of the earliest efforts involved splitting the column effluent, with part going to a flame ionization detector and part being pressure regulated by using a motor-driven automatic expansion valve (19). Gere et al. (20) used a manually adjustable back-pressure regulator as a restrictor, allowing the column to operate isobarically a t pressures up to 430 atm (425 bar), while independently varying the linear velocity. A similar back-pressure valve design (21) showed a retention time reproducibility of 0.40% relative standard deviation. Another system used two pumps and two restrictors (22),while a similar design with only one restrictor (8)was capable of programming both inlet and outlet pressures independently. A variable flow rate restrictor has been developed that does not employ adjustable valves (9). With this restrictor, a crimped stainless steel capillary is placed on a heated metal block, and restriction is controlled by thermal expansion of the restrictor. Back-pressure regulation via valves or regulators appears to be the most prevalent and satisfactory method for use in packed column SFC. A method is presented here that incorporates back-pressure regulation with a sheath-flow nozzle, previously developed for supersonic jet spectroscopy (23),for use at high pressures and for linear velocity control in capillary SFC. This pressurecontrolled restrictor was evaluated for control of linear velocity. The effect on chromatographic resolution of solute mixtures over the full operational pressure range was also studied. EXPERIMENTAL SECTION Design a n d Construction of t h e Restrictor. Figure 1 shows the high-pressure sheath flow nozzle in which make-up gas acts to partially restrict and control flow out of the column.
ORAPHITIZED
MAKE-UP G A S
VESPEL GASKETS
I
\
I
1179
LASER-DRILLED ORIFICE
II
n v)
0;:
CAPILARY - 1 - 1 CAP
COL co
:g
f
0 CAPILLARY
Figure 1. Schematic drawing of the high-pressure sheath-flow nozzle used as a controlled back-pressure restrictor.
The nozzle is composed of a T-connector, purchased from SGE (Scientific Glass Engineering, Austin, TX), and a tapered outlet cap assembly, made in-house. In the original design of the nozzle, which was used in this study, the tapered section was made to be an integral part of the T-section. A secondgeneration nozzle with a removable tapered section, depicted in Figure 1, facilitates cleaning and allows for more precise positioning of the column in the sheath cone, which is important for transfer of the sample out of the sheath region (23). In this study, however, postrestrictor detection was not employed, and the positioning of the column in the sheath was not important. Since the cone of the taper cannot be machined to a fine enough point for flow restriction, a laser-drilled pinhole in a 1/8 in. stainless steel disk was used as the orifice. Pinholes, ranging from 1to 50 pm, in 3/8 in. disks were purchased from Optimation, Inc. (Windham, NH), or Lennox Laser (Phoenix, MD) and the 1/8 in. diameter disks were cut from these stock disks. Graphitized Vespel gaskets (SGE) are used to seal the three parts together. The pinhole is placed directly on the cone tip and sealed from the back side to eliminate a region of turbulent mixing between the pinhole and cone tip; Teflon tape may be used to complete the seal on the sides of the retaining cap. Although some gas may bleed into the cap region, it is stagnant and does not disrupt the nozzle flow dynamics. Initial results from supersonic jet spectroscopic investigations (24) have shown that the sample tends to move through the nozzle as a plug, and mass transfer problems have not been observed. The capillary column runs through a larger (ca. 250 Fm), more rigid capillary into the sheath cone in order to keep it centered in the gas flow. We have demonstrated that the back pressure of the make-up gas can be used to provide sufficient flow restriction without any other restriction at the end of the column; however, to facilitate comparison of runs with and without back pressure control in this study, a porous ceramic frit restrictor (Lee Scientific, Salt Lake City, UT) or a -20-cm length of 5 pm i.d. fused silica tubing was attached at the end of the column. High pressure (6000 psi) argon gas (Union Carbide Corp., Linde Division, Danbury, CT) was employed as the make-up gas. Experimental Evaluation. Two SFC systems were used for the chromatographic evaluation of the nozzle restrictor. The systems employed either an Isco Model 314 liquid chromatographic syringe pump (Isco, Lincoln, NE) controlled through a pressure feedback using an Apple IIe microcomputer (Apple Computers, Cupertino, CA) or a Lee Scientific Model 501 SFC pumping system and a Hewlett-Packard Model 5710 or Model 5890 gas chromatographic oven (Hewlett-Packard, Avondale, PA). Detection was accomplished with either a Hitachi Model FlOOO fluorescence spectrophotometer (Hitachi, Tokyo, Japan) modified to accommodate a fused silica flow cell, similar to that previously described (25),or a Chi-
1180
ANALYTICAL CHEMISTRY, VOL. 61, NO. 10, MAY 15, 1989
A
c
I
I
io0
200
I 3°C
APPLIED BACK-P9ESSURE ( A I M )
Figure 2. Plot of linear velocity as a function of applied back pressure at constant pump pressures of (A) 100 atm, (E) 200 atm, and (C) 300 atm. The average percent relative standard deviation velocity measurements was 0.012.
for the linear
ratech Model 203 UV-visible spectrometer (now produced by Linear Instruments, Reno, NV) modified as previously described (26). Prerestrictor detection was employed in the evaluation of the sheath-flow nozzle to avoid problems with sample transfer and band-broadening in the sheath region. However, the restrictor may be adaptable to postrestrictor detection, as demonstrated in investigations of this highpressure sheath-flow nozzle for supersonic jet spectroscopy (23,24). SFC grade carbon dioxide (Scott Specialty Gases, Plumsteadville, PA) was delivered to the chromatographic column using the syringe pump. For linear velocity measurements, fused silica capillary tubing, 10 m X 50 pm i.d. (Polymicro Technologies, Phoenix, AZ), statically coated with poly(methy1-n-octylsiloxane) (27) and cross-linked using azo-tert-butane (28) was used. For efficiency and resolution measurements, either a 4 m X 50 pm i.d. SB-Phenyl-5 or a 2 m X 50 pm i.d. SB-Biphenyl-30 fused silica column (Lee Scientific) was used. Samples were introduced into the chromatographic column by split injection with a 0.2-pL internal sample volume valve (Valco Instruments, Houston, TX). Evaluation of chromatographic performance was done by evaluating chromatograms of a partially hydrolyzed biphenylmethylsiloxane polymer or a 5% phenyl-substituted methylpolysiloxane polymer (OV-17, Applied Science Laboratories, State College, PA). In all cases, the linear velocity was determined by using benzene as an unretained solute.
RESULTS AND DISCUSSION Initially, measurements were made to determine linear velocity as a function of applied back pressure a t several constant column pressures. Triplicate measurements, a t a variety of applied back pressures and with a pinhole orifice of 25-pm diameter, were taken at column pressures of 100, 200, and 300 atm. The results, plotted in Figure 2, indicate that linear velocity is a second-order function of applied back pressure. Next, the applied back pressure needed to maintain a constant linear velocity as a function of column pressure was measured. These data yielded a linear relationship (correlation coefficient = 0.9974) over the range of column pressures from 100 to 300 atm for a constant linear velocity of 3.00 cm s-l. Applied restrictor back pressures necessary to maintain this linear velocity ranged from -50 to -150 atm. A partially hydrolyzed biphenylmethylsiloxane sample was analyzed to show the effects of linear velocity control during pressure programming. Duplicate analyses of the sample were run under identical conditions, except that in the first case, no argon back pressure was applied, while in the second case, the applied back pressure was manually adjusted to maintain a constant linear velocity. A nominal velocity of 0.75 cm s-l
3;O
I
75
PRESSURE [ATMI
1
B
0
4do
I(
i
10
I
75
30
20 100
40
I
zoo
k L __ so l
70 TIME [MHUTES)
60
,
300
l
i
400 PRESSURE (ATMI
Figure 3. SFC chromatograms of a partially hydrolyzed biphenylmethylsiloxane. Peak 1 is the cyclic tetramer. Conditions were as follows: carbon dioxide at 80 OC; 4 m X 50 p m i.d. SEPhenyl-5 fused silica column; pressure programmed from 75 to 400 atm at 5 atm min-' ,, = after an initial 10-min isobaric period, fluorescence detection, A 255 nm, A,, = 315 nm. (A) Chromatogram with no applied back pressure. Trennzahl between peaks 2 and 3 is 11.9. (E) Chromatogram with constant linear velocity. Trennzahl between peaks 2 and 3 is 16.9.
was maintained during the controlled linear velocity run with a 25-pm pinhole orifice. The resulting chromatograms are shown in Figure 3. Although the selectivity was unchanged
ANALYTICAL CHEMISTRY, VOL. 61, NO. 10, MAY 15, 1989
600
400
200
0
0
100
200
300
APPLIED BACK.PRESSURE (ATM)
Figure 4. Plot of volumetric flow rate from the sheath-flow nozzle as a function of applied back pressure for pinhole orifice diameters of (A) 25 pm, (8)5 pm, and (C) 1 pm.
during these runs, the Trennzahl value (TZ,separation number), a measure of apparent resolution during programmed (i.e., nonisobaric) analyses, increased from a value of 11.9, using peaks 2 and 3, when no back pressure was applied, to 16.9 in the constant linear velocity run, a gain of 42%. This value is not unreasonable in light of theoretical efficiency calculations based on the Golay equation (using the chromatographically measured capacity ratios, the average linear velocity during the run, and reasonable diffusion coefficients (29)), which yield increases in TZ of 46%. This 40-45% improvement in TZ was also observed over the pressure range up to 400 atm during the pressure programmed SFC analysis of an OV-17 sample. Control of linear velocity during pressure programming will, however, make solutes elute later than without linear velocity control. In the OV-17 analysis, 13 oligomers were eluted before an isobaric period at 400 atm, and 15 oligomers were eluted in a 2.5-h analysis time without linear velocity control (COP at 45 "C; 90 atm for 15 min, then 3 atm min-' to 400 atm; 2 m X 50 pm i.d. SB-Biphenyl-30 column). Under identical conditions except for a constant linear velocity of 0.28 cm s-l (5-pm pinhole orifice), only 11 oligomers were eluted before 400 atm was reached and 13 oligomers were eluted in an analysis time of 2.5 h. The discrepancy in analysis time can be addressed by appropriate adjustment of the programming rate. I t is known (3) that as density increases (e.g., during a pressure-programmed run) the diffusion coefficient of a solute in the mobile phase decreases, resulting in an increased plate height. This loss in chromatographic efficiency (increased plate height) could offset some of the potential gains in resolution obtained by maintaining a constant linear velocity. With the OV-17 sample, no striking deleterious effects due to efficiency loss were observed until the final isobaric period a t 400 atm was reached. Short columns were used in these experiments to avoid prohibitively long analysis times a t the linear velocities used. The effect on resolving power would be much more pronounced at linear velocities further from the optimum, with longer columns (more theoretical plates), or with less efficient, wide-bore columns, especially for samples requiring high resolution for the separation of closely eluting compounds. The current design of the controlled back-pressure apparatus w8s developed for detectors that are independent of mass flow (e.g., such as fluorescence or supersonic jet spectroscopy (24,30)). However, with the possibility of interfacing to mass flow-dependent detectors, such as the flame ionization detector or mass spectrometer, it was of interest to measure the volumetric flow rate of the effluent from the restrictor. The volumetric flow rate of the argon make-up gas was measured with a soap bubble flowmeter for a variety of pinhole orifice
1181
diameters over the range of applied back-pressures from 25 to 275 atm. As shown in Figure 4, volumetric flow rate as a function of applied back-pressure is linear (correlation coefficient > 0.9994) for each pinhole diameter tested. Although the flow rate is about 700 mL min-' a t a 275-atm applied back-pressure with a 25-pm pinhole, the range of flow rates extends up to 500 mL min-' at typically applied back pressures. At a 275-atm applied back pressure, the volumetric flow rate was about 46 mL min-' for a 2-pm pinhole and 26 mL min-' for a 1-pm pinhole. Since this applied back-pressure (275 atm) is higher than what would be typical to maintain a practical operating linear velocity, sheath-flow nozzles with pinholes in the 1-2 pm region can be considered for interfacing flow-controlled capillary SFC to mass spectrometry or flame ionization detection if potential plugging problems associated with these small orifice diameters can be resolved.
ACKNOWLEDGMENT The authors thank John K. Simons and Brent J. Allen for assistance in constructing and initial testing of the sheath-flow nozzle. LITERATURE CITED Peaden, P. A.; Lee, M. L. J. Chromatogr. 1983, 259, 1-16. Fields, S . M.: Kong, R. C.; Fjeidsted, J. C.; Lee, M. L.; Peaden, P. A. HRC CC , J . High Resolut . Chromatogr Chromatogr . Commun 1984, 7 , 312-318. Fields, S . M.; Lee, M. L. J. Chromatogr. 1985, 349, 305-316. Graham, J. A.; Rogers, L. B. J. Chromatogr. Sci. 1980, 18, 75-84. Mourier, P. A.; Caude. M. H.; Rosset, R. H. Chromatographia 1987, 2 3 , 21-25. Klesper, E.; Hartmann, W. Eur. Polym. J. 1978, 14, 77-88. Klesper, E.; Schmitz, F. P. J. Supercr. Fluids 1988, 1 , 45-49. Hirata, Y.; Nakata, F.; Kawasaki, M. HRC C C , J. High Resolut. Chromatogr. Chromatogr. Commun. 1986, 9 , 633-637. Greibrokk,T.; Berg, E. E.; Blilie, A. L.; Doehl, J.; Farbrot, A,; Lundanes, E. J. Chromatogr. 1987, 394, 429-441. Smith, R. D.; Fulton, J. L.; Petersen, R. C.; Kopriva. A. J.; Wright, B. W. Anal. Chem. 1986, 5 8 , 2057-2064. Bally, R. W.; Cramers, C. A. HRC C C , J. High Resolut. Chromatogr. Chromatogr. Common. 1986, 9 I 626-632. Richter, B. E. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, 1986; Paper No. 514. Guthrie, E. J.: Schwartz, H. E. J. Chromatogr. Sci. 1986, 2 4 , 236-241. Koehler. J.; Rose, A.; Schomburg. G. HRC C C , J. High Resolut. Chromatogr. Chromatogr. Commun. 1988, 1 1 , 191-197. White, C. M.; Gere, D. R.; Boyer, D.; Pacholec, F.; Wong, L. K. HRC C C , J. High Resolut. Chromatogr. Chromatogr. Commun, 1988, 1 1 , 94-98. Raynor, M. W.; Bartle, K. D.; Davies, I . L.; Clifford, A. A.; Williams, A. HRC CC. J. High Resolut. Chromatogr.Chromatogr. Commun. 1988, 1 1 , 289-29 1. Green, S.;Bertsch, W. HRC C C , J. High Resolut. Chromatogr. Chromatogr. Common. 1988, 1 1 , 414-415. Fjeldsted, J. C.; Jackson, W. P.; Peaden, P. A,; Lee, M. L. J. Chromatogr. Sci. 1983, 2 1 , 222-225. Bartmann. D. Ber. Bunsen-Ges. Phys. Chem. 1972, 7 6 , 336-339. Gere. D. R.; Board, R.; McManigill, D. Anal. Chem. 1983, 5 4 , 736-740. Jahn, K. R.; Wenclawiak, B. W. Anal. Chem. 1987, 5 9 , 382-384. Hirata, Y.; Nakata, F. Chromatographia 1986, 2 1 , 627-630. Sin, C. H.; Simons, J. K.; Markides, K. E.; Lee, M. L.: Goates, S. R., in preparation. Goates, S. R.; Sin, C. H., in preparation. Fjeldsted, J. C.; Richter, B. E.; Jackson, W. P.; Lee, M. L. J. Chromatogr. 1983. 279, 423-430. Fields, S . M.; Markides, K. E.; Lee, M. L. Anal. Chem. 1988, 6 0 , 802-806. Kuei, J. C.; Tarbet, B. J.; Jackson, W. P.; Bradshaw, J. S.:Lee, M. L. Chromatographia 1985, 2 0 , 25-30. Kong, R. C.; Fields, S. M.; Jackson, W. P.; Lee, M. L. J. Chromatogr. 1984, 289, 105-116. Fields, S. M. Ph.D. Dissertation, Brigham Young University, Provo, UT, 1987. Goates, S. R.; Barker, A. J.; Zakharia, H. S.; Khoobehi, B.; Sheen, C. W. Appl. Spectrosc. 1987, 41, 1392-1397.
.
.
RECEIVED for review December 27, 1988. Accepted February 21,1989. This work was carried out with the support of the U S . Department of Energy, under Grant No. DE-FG2286PC90534. Any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of DOE.
