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Reduction of Contamination Levels in On-Line Supercritical Fluid Extraction Systems Jeffrey C. Wallace, Mark S. Krieger, and Ronald A. Hites' School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
INTRODUCTION Supercritical fluid extraction (SFE) has become an important analytical technique. SFE experiments are carried out in one of two modes: (a) In the "off-line" mode, the extraction fluid is passed through a cell containing the sample, through a pressure restrictor, and into a small volume of solvent or onto a small bed of an adsorbent (which is then extracted). Small aliquots of the resulting extract are then analyzed by gas, liquid, or supercritical fluid chromatography. (b) In the "on-line" mode, the extraction fluid is passed through the sample cell, through the restrictor, and directly into the chromatographic column, where the analytes of interest are retained. The latter approachgives a considerable increase in sensitivity, since all of the extracted analytes are transferred to the chromatographic column. Unfortunately, this expected increase in sensitivity has not been fully realized, due (at least in part) to contamination from the supercritical fluid delivery system. In the on-line mode, the contaminants are all transferred into the chromatographic system. If this contamination is severe, blank levels become high, and the advantages of on-line SFE vanish. These problems are particularly apparent when an electron capture gas chromatography (GC) detector is used. Several researchers have noted that the purity of the supercritical fluid ultimately limits the sensitivity of on-line SFE-GC.1-3 WHile this is true, other sources of contamination exist in the SFE system that can far exceed the supercritical fluid itself. These other sources include the seals in the pump and in the valves. Most pumps, either reciprocating or syringe types, have a series of seals that ensure leak-free operation. The valves that isolate the pump from the rest of the system also have several seals. The extraction fluid must flow over all these seals in normal operation, and any contamination or degradation of the seals will be solvated by the extraction fluid, carried through the system, and deposited in the GC column. The seals in an SFE pump must be rigorous enough to withstand the high operating pressures of SFE yet pliable enough to contain the extraction fluid. Typical seals contain a high-density polymer such as Teflon, polyethylene, or polyether-etherketone (PEEK), depending on the manufacturer. The raw polymer used to fabricate the seals is a distribution of oligomers of varying molecular weights, as well as any contamination or byproducts that result from the polymerization process. Since the seals in an SFE system are polymeric and since SFE is an excellent technique for the analysis of it is reasonable to expect that some solvation will occur as the extraction fluid contacts the seals and contamination will result. This paper suggests a method (1) Nielen, M. W. F.; StPb, J. A.; Lingeman, H.; Brinkman, U. A. Th. Chromatographia 1991,32, 543-545. (2) Nielen, M. W. F.; Sanderson, J. T.; Frei, R. W.; Brinkman, U. A. Th. J . Chromatogr. 1989,474, 388-395. (3) Onuska, F. I.; Terry, K. A. J.High Resolut. Chromatogr. 1989,12, 527-531. (4) Bartle, K. D.; Boddington, T.; Clifford, A. A.; Cotton, N. J.;Dowle, C. J. Anal. Chem. 1991,63, 2371-2377. (5) Daimon, H.; Hirata, Y. Chromatographia 1991, 32, 549-554. (6) Engelhardt, H.; 2app;J.; Kolla, P. Chromatographia 1991,32,527537. 0003-2700/92/0364-2655$03.00/0
to minimize this contamination by keeping the extraction fluid supercritical in the fluid delivery system.
EXPERIMENTAL SECTION All supercritical fluid extractions were performed using SFE ECD/FID certified carbon dioxide that came in a tank with a dip tube (Scott Specialty Gases, Plumsteadville, PA) and an ISCO (Lincoln, NE) Model 260D syringe pump operated at 400 atm. To avoid "shedding" of particles from the seals, the factoryinstalled graphite-filledTeflon piston and wiper seals of the pump were replaced with ultra high molecular weight polyethylene seals (UHMWPE) obtained from ISCO. The pump was also fitted with a water jacket; the water temperature in the jacket was controlled by running tap water through a coil of copper tubing wrapped with heating tape before the water entered the jacket. Taper seal values (High Pressure Equipment Co., Erie, PA) isolated the inlet and outlet of the pump. The flow rate of the supercritical fluid from the pump was controlled by an ll-cmlong fused-silica restrictor (PolymicroTechnologiesPhoenix,AZ). Two sizes were used, 23 or 50-pm i.d., which resulted in flow rates of about 0.6 or 3.0 mL/min, respectively. All supercritical fluid flow rates were measured as a liquid at the pump at 18"C, unless otherwise noted. All SFE-GC analyses were done with a Hewlett-PackardModel 5890 gas chromatograph operating with an electron capture detector. The carrier gas was hydrogen at a linear velocity of 50 cm/s. Data were recorded by a Hewlett-Packard Model 3392 integrator. Effluent from the pump was introduced into a 30-m, DB-5 capillarycolumn (250-pmi.d.; 0.25-pm film thickness;J&W Scientific, Folsom, CA) by directly inserting the SFE restrictor into the on-columninjection port of the gas chromatograph. The GC oven was cooled to 5 "C during the extraction so that the extracted materials would be cryogenically focused on the GC column as they eluted from the SFE restrictor. After 15 mL of carbon dioxide had eluted, the restrictor was removed from the on-column injection port, the COz was flushed from the column for 2 min with carrier gas, and the GC oven temperature was ramped at 30 "C/min to 100 "C, and then to 300 "C at 5 "C/min with a 5-min hold at 300 "C.
RESULTS AND DISCUSSION At the beginning of this study, 15 mL of C02 from the pump was passed through a 23-pm-i.d. restrictor into the GC column and analyzed by the procedures described above. Figure 1A shows the resulting gas chromatogram. A series of large, unresolvedpeaks occupies most of this chromatogram. Clearly, considerable contamination from the SFE system was transferred to the GC column during this extraction. The possible sources of contamination were the COz supply gas, the seals in the valves, and the seals in the pump. To determine if the C02 supply gas was a possible contamination source, 15 mL of liquid C02 was analyzed directly from the supply tank. To do this experiment, a 23pm4.d. restrictor was connected directly to the C02 supply tank. This setup allowed the COzto be introduced to the GC column while bypassing the pump. The resulting chromatogram is shown in Figure 1B. The lack of any major peaks in this chromatogram indicates that the C02 itself is not the source of the contamination. This leaves the seals in the pumps and in the valves as the contamination sources; a strategy was devised to minimize their effect. By heating the CO2 in the pump above its critical temperature (31.1 "C) and by pressurizing the C02 above its 0 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, 1992
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Figure 1. On-line SFE-GC analyses of (A) COz from the pump before “Internal”extractlon, (B) COS!taken directly from the supply tank, and (C) COz from the pump after internal extraction. All chromatograms are at the same attenuation;SFE-GC condltlons are given in the text.
critical pressure (74.8 atm), the CO2 in the pump becomes supercritical. The pump then extracts itself as the contents of the pump are slowly released through the outlet restrictor. This “internal” extraction allows supercritical C02 to pass over the seals in the pump and valves and to leach extractable material from them. If this process is repeated several times, the seals are eventually depleted of their extractable material, resulting in a clean SFE system. This method was implemented on our SFE system by use of the following conditions: (a) The pump temperature was maintained a t 55-65 OC. (b) The pump pressure was held constant a t 400 atm. (c) A 50-pm-i.d.restrictor was used to provide a COZflow rate of about 3-4 mL/min (measured as supercritical C02 from the pump at 60 “C). Approximately 2500 mL of supercritical C o t was passed through the system. The pump used in this study has a 260-mLcapacity,so roughly 10 pump volumes of C02 were used. On the last such extraction, a heat gun was used to bring the outlet valve to about 100 OC in order to also extract this valve with supercritical C02. A cautionary note: Do not heat t h e pump above t h e upper temperature limit of its seals. This could result in seal failure, possibly leading to damage of the pump. The temperature we chose was at least 10 “C below the upper limit for the ultra high molecular weight polyethylene (UHMWPE) seals in the pump. Consult with the pump manufacturer if any questions exist about operating a pump at elevated temperatures.
After the 2500 mL of supercriticalC02 was passed through the system, the temperature of the pump was lowered to 18 “C. The pump was then flushed with three cylinder volumes of CO2, refilled with C02, and pressurized to 400 atm. A 23-pm-i.d. restrictor was attached to the pump’s outlet, and 15 mL of CO2 was then analyzed. The result is shown in Figure 1C. The contamination indicated by Figure 1A has fallen off dramatically, and the resulting chromatogram is almost indistinguishablefrom that of the C02 itself (see Figure 1B). Incidentally, the COZ used to generate Figure 1C was delivered to the column head a t 18 “C. This was preferable to delivering the C02 at 60 “C because the CO2 flow rate is more stable at the lower temperature. However, if the pump is properly insulated it may be possible to use the pump for analyses at elevated temperatures. When our pump was used for an extended period at subcritical temperatures, the contamination level slowly increased. However, when the pump was “internally” extracted at an elevated temperature, the contamination level dropped back to levels indicated by Figure 1C. This observation does not necessarily imply that the seals in the pump are acting as sponges for contaminants in the COz. This hypothesisis not consistent with the data shown in Figure 1B. Rather, we suggest that the diffusion of the contamination to the surfaces of the seals is a slow process. After “internal” extraction of the pump, the surfaces of the seals are clean, but as the pump is used for extended times with subcritical C02, the contaminants diffuse to the surfaces of the seals where they are solvated by the fluid. As a result, the background levels rise.
ACKNOWLEDGMENT We thank the Midwest Regional Center of the National Institute for Global Environmental Change, directed by J. C. Randolph, for the purchase of the SFE pump, ISCO (Lincoln, NB) for the gift of the pump water jacket, and Philip Liescheski (ISCO) for helpful discussions. This work was supported by the U.S.Department of Energy (Grant 87ER60530)and by the NationalInstitute for GlobalEnvironmental Change.
RECEIVED for review May 1992.
4, 1992. Accepted August 7,
