Anal. Chem. 1993, 65, 1038-1042
1038
Solventless Collection of Analytes by Rapid Depressurization after Static Supercritical Fluid Extraction David J. Miller' and Steven B. Hawthorne Energy and Environmental Research Center, University of North Dakota, Box 9018, University Station, Grand Forks, North Dakota 58202-9018 Mary Ellen P. McNally E.I. du Pont de Nemours and Company, Du Pont Agricultural Products Experiment Station, Wilmington, Delaware 19880-0402
A method has been developed to collect analytes from supercrltkal fMd extraction(SFE) that utHlzwnofbw re$trlctor and an empty colkc#onvial. Following datk SFE, the analytcw are coUectd by rapldly(3-30 8 )depressurklngthe CO, etlluent through a 178-lun 1.d. stainless steel tube Into a capped screwtop vlal. Collection efficiencies of analytes as volatile as mheptane are quantitatlve(>95 %), and wet samples showed no evldence of restrictor plugglng In contrast to the plugglng associated with the SFE of w d samples when small 1.d. flow restrictom are used. Solventless collection of PCBs, PAHs, gasoline, and diesel fuel by SFE generally showed high (>90 % ) collection efflclencles, and recoveries normally exceed those obtained using dynamlc SFE with collection In a llquld solvent.
INTRODUCTION Supercritical fluid extraction (SFE) has become an attractive alternative to conventional solvent methods for the removal of analytes from solid samplesbecause of the reduced time needed for extraction and less liquid solvents required. In order to obtain quantitative results, good collection efficiencies are necessary as well as good SFE extraction efficiencies. A wide variety of methods for trapping analytes have been reported including collection in liquid solvent,1-3 collection on sorbent resin traps,@ collection on cryogenically cooled surfaces,' and collection directly into chromatographic columns via on-column or split/splitless injection ports.a11 All of these methods have one thing in common, i.e., they rely on a depressurization step which utilizes a flow restrictor (small i.d. linear restrictor or some type of variable restrictor) at the outlet of the SFE cell. An inherent problem with linear
* Corresponding author.
(1) Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1987,59, 1705-1708. (2) Campbell, R. M.; Lee, M. L. J. Anal. Chem. 1986,58,2247-2251. (3) Lopez-Avila, V.; Dodhiwala, N. S.; Beckert, W. F. J. Chromatogr. Sci. 1990, 28, 468-476. (4) Hedrick, J. L.; Taylor L. T. J.High Resolut. Chromatogr. 1990,13, 312-316. (5) Miller Schantz, M.; Cheder, S. M. J. Chromatogr. 1986,363,397401. (6) Schneiderman, M. A.; Sharma, A. K.; Locke, D. C. J. Chromatogr. 1987,409, 343. (7) Wright, B. W.; Wright, C. W.; Gale, R. W.; Smith, R. D. Anal. Chem. 1987,59, 38-44. (8) Hawthorne, S. B.; Miller, D. J.; Langenfeld, J. J. J. Chromatogr. Sei. 1990, 28, 2-8. (9) Levy, J. M.; Cavalier, R. A.; Bosch, T. N.; Rynaski, A. F.; Huhak, W. E. J. Chromatogr. Sci. 1989,27, 341-346. (10) Hawthorne, S. B.; Krieger, M. S.; Miller D. J. Anal. Chem. 1988, 60, 472-477. (11) Xie, Q.L.; Markides, K. E.; Lee, M. L. J.Chromatogr. Sci. 1989, 27, 365-370. 0003-2700/93/0365-1038$04.00/0
restrictors is the possibility of reduced flow due to plugging which occurs when the sample matrix contains high concentrations of extractable material or water. Several methods have been used to eliminate plugging problems including heating the restridor7J2-15 or maintaining the collection solvent at constant temperature (e.g., 5 "C) during the collection step.16 McNally, Deardorff, and Fahmy have reported the development of a multivesselextractor that first statically extracts analytes and then utilizes a rapid depressurization step to remove extracted analytss from the system. A unique property of their design is the elimination of the use of a flow restrictor, which allows the extraction effluent to be transferred rapidly for collection. With this apparatus, excellent collection efficiencies of nonvolatile analytes such as Diuron (a phenylmethylurea pesticide) spiked on soil (97.3f 6.6% ) were achieved.17 Since solventless collection into an empty vial has only been reported for nonvolatile analytes, the application of this technique to more volatile species was investigated. Determiningcollection efficiencies (rather then extraction efficiencies) was the primary goal of this study; therefore, analytes added to a relatively inert matrix were used to test the collection system. Even though the use of spikes is not always avalid indication of extraction efficiency,18 spike recoveries are an excellent method to determine collection efficiency after SFE. The effects of different trapping parameters on collection efficiencies including vial configuration, exit tube inside diameter (which controls venting rate), extraction temperature, analyte volatility, and collection time were investigated. Collection efficiencies for alkanes (c6430), PAHs, PCBs, gasoline, and diesel fuel are presented.
EXPERIMENTAL SECTION All extractions, except where noted, were performed in triplicate with an ISCO Model SFX 2-10 extractor and a 2.5-mL cell. Approximately 2 g of seaaand (Fisher Scientific,Fair Lawn, NJ) was loaded into the cell and spiked with the test solutions. The cell was then pressurized to 400 atm with SFC-grade COz (12) Burford, M. D.; Hawthorne, S. B.; Miller, D. J.; Braggins, T. J. Chromatogr. 1992,609, 331-342. (13) Wong, J. M.; Kado, N. Y.; Kuzmicky, P. A.; Ning, H. S.; Woodrow, D. P.; Hsieh, D. P. H.; Seiber, J. N. Anal. Chem. 1991, 63, 1644-1650. (14) McNair, H. M.; Frazier, J. 0. Am. Lab. 1991, 23, 24D-I. (15) Campbell, R. C.; Meunier, D. M.; Cortes, H. J. J.Microcolumn Sep. 1989, 1 (6), 302-308. (16) Langenfeld, J. J.; Burford, M. D.; Hawthorne, S. B.; Miller, D. J.
J. Chromatogr. 1992,459,297-307.
(17) McNally, M. E.P.;Deardorff, C. M.; Fahmy,T. M. In Supercritical Fluid Technology: Theoretical and Applied Approaches in Analytical Chemistry;Bright, F. V., McNally, M. E. P., Eds.;ACS Symposium Series 488; American Chemical Society: Washington, DC, 1992; Chapter 12. (18) Burford, M. D.; Hawthorne, S. B.; Miller, D. J. Anal. Chem. In
press.
Q 1993 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 85, NO. 8, APRIL 15, 1993
Cell H e a t e r
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0,007" i,d, E x i t Tube ca. 3 mn 6
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Flgure 1. Schematic diagram of the extractlon/collectlonapparatus. All components except the exit tube and the collection vial are Included with the ISCO SFX 2-10 extractor.
(Scott Specialty Gases, Plumsteadville, PA) at 50 "C, and extractionswere performed statically (noflow out of the extraction cell) for 15min. The normal outlet restrictor was replaced with a 15cm X 1.6 mm (1/16-in. o.d.), 178-pm(0.007-in.) i.d. stainless steel tube (Figure 1).The collectionvessel consisted of an empty 3.7-, lo-, or 22-mL (15 X 45 mm, 19 X 65 mm, 23 X 85 mm, respectively) glass screw-top vial fitted with a Teflon-faced siliconeseptum cap with a single hole that had been prepunctured using a 1/16-in.stainlesssteel rod. (Caution: The collection vial must be securely mounted during depressurization. Although no failures occurred during this study, the possibility exists for the vial to rupture,and appropriate shielding should be provided.) Contrary to conventional SFE collection methods, no solvent or sorbent trap was used during the collection step. The exit tube was inserted through the septum and extended approximately 3 mm into the vial. A t the end of the static extraction time, the inlet valve was closed, and the outlet valve was opened so that the CO2 vented rapidly from the extraction cell (3-30 s) into the empty collectionvial. After the SFE cell was vented, the collected analytes were immediately dissolved by adding 5 mL of CHzC12 or acetone (for PCBs) (Optima Grade, Fisher Scientific, Fair Lawn, NJ) to the collection vial. The test solutions used as spikes in this study were 50 pL of C&O n-alkanes (101.4 mg/mL in CHzClz total alkanes, ca. 4 w t % each, Aldrich Chemical Co., Milwaukee, WI), 10 pL of a PCB solution (4.1 mg/mL Aroclor 1254, Chem Service, West Chester,PA), 10pL of a PAH solution (naphthalene, 6.7 mg/mL; phenanthrene, 6.6 mg/mL; pyrene, 7.1 mg/mL; chrysene, 7.1 mg/ mL; benzo[b]fluoranthene, 2.3 mg/mL; Aldrich Chemical Co., Milwaukee, WI), 50 pL of gasoline or 50 pL of no. 1diesel fuel. Quantitative standards were prepared by adding the appropriate volume of spiking solution to 5 mL of the appropriate solvent. All extractsand standards were analyzed by GC after the addition of 10 WL of the appropriate internal standard, Le., 24.5 mg/mL phenanthrene (for n-alkanes),1.64mg/mL 1-chloronaphthalene (for PCBs),24.2 mg/mL 1,3,5-triisopropylbenzene(for PAHs and gasoline), or 20.4 mg/mL pyrene (for no. 1 diesel fuel). All analyses were performed using a Hewlett-Packard Model 5890 Series I1 gas chromatograph (Avondale, PA) with electron capture detection(ECD)(for PCBs) or flame ionization detection (FID) (for all other analytes). Chromatographicseparationswere accomplished with a 25-m HP-5 (0.32-mm i.d., 0.17-pm film thickness, Hewlett-Packard) column for n-alkanes, PAHs, and fuel components and a 60-m DB-5 (0.25-mm i.d., 0.25-pm film thickness,J & W Scientific,Folsom,CA) column for PCB analysis. Half microliter split (40:l split ratio) injections of n-alkane, PAH, and fuel samples were made at an initial column temperature of 10 O C . The column oven was held at the injection temperature for 2 min, then raised at 12 "C/min to a final temperature of 320 O C , and held there for 5 min. PCB samples were injected at an initial oven temperature of 150 O C (held for 5 min) and then raised at 6 OC/min to a final temperature of 320 O C .
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Figure 2. Percent recovery of Ce-C30 n-alkanes by collection In a 10-mL glass vial for 30 s with (0)capped with a Teflon-llned septum and with a 0.007-In. (178-pm) 1.d. exit tube, (0)capped with a Teflonlined septum and with a 0.010-In. (254-pm) 1.d. exit tube, (V)open vlal with a 0.007-In. (178-pm) 1.d. exit tube. Each sample was extracted in trlpllcate as described In the text. Error bars not shown are smaller than their symbol.
RESULTS AND DISCUSSION Optimization of n-Alkane Recoveries. Initial experimenta were performed with n-alkane (c6-0) spikes to determine the optimal configuration of the collection vial and the i.d. of the exit tube which in turn controls the venting rate. (Note: Care must be taken to ensure that the glass collection vial is securely held during depressurization.) Collection vials with and without a Teflon-lined septum cap were used to collect the n-alkanes that had been spiked onto sand and statically extracted for 15min at 50 OC and 400 atm with COz, and then the extraction cell was vented until all visible and audible signs of depressurization ceased (typically 30 s) through a 0.007-in. (178-pm) or a 0.010-in. (254-pm) i.d. exit tube. To avoid any possible losses of volatile analytes after venting, the solvent was added immediately after collection was finished. Figure 2 is a plot of n-alkane chain length vs percent recovery for collections in a 10-mL vial with and without a cap and for 0.007411. and 0.010-in. exit tubes. All of the collectionmethods showed excellent reproducibility with typical % RSDs of 90% recovery for CS and above. Again, these values are in excellent agreement with reported values for diesel fuel components from dynamic supercriticalfluid extractions collected in liquid solvents. For example, reported values for the dynamic extraction and liquid solvent collection of C,, Cg,and Cg extracted from a sandy loam soil are 67 f 6, 83 f 5, and 90 f 5.4 % , re~pectively,2~ which are statistically identical to recoveries reported in Table V (73 f 2.4, 85 f 1.5, and 90 f 2.9% for C,, CS,and C9, respectively).
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CONCLUSIONS A method has been developed to collect analytes from supercritical fluid extraction (SFE) that utilizes no flow restrictor and an empty collection vial. The results of this study demonstrate that solventless collection of rapidly depressurized SFE extracts can be useful for recovering a wide range of analytes including relatively volatile species (e.g., hexane and heptane) with efficiencies comparable to (or better than) dynamic extraction with collection in a liquid solvent. Solventless collection is especially well suited for the recovery of less volatile materials such as PCB congeners. Some compound classes (PAHs) may require multiple static extractions to achieve quantitative recovery but can still be efficiently collected without a solvent. Also, the fact that no sdvent is present during collection allows one to choose the best dilution solvent based on considerations of the subsequent analytical method rather than on collection parameters. The purpose of this study was only to investigate collection efficiencies resulting from solventless collection, and more rigorous extraction conditions (e.g., modifiers, temperature) may be required for real samples. In addition to being a viable collection method, the absence of a smalli.d. restrictor eliminates plugging problems associated with their use.
ACKNOWLEDGMENT Funding for this work was provided by the U.S.Environmental Protection Agency, (EMSL, Las Vegas, NV). The authors would like to thank ISCO (Lincoln, NE) for instrument loans.
RECEIVEDfor review December January 8, 1993.
7, 1992.
Accepted
