Anal. Chem. 1994,66, 3581-3586
Solvent Trapping of Some Chlorinated Hydrocarbons after Supercritical Fluid Extraction from Soil 8. W. Wenclawlak,'~tG. Maio,t*i Ch. v. Holst,s and R. Darskus* Universitat-GH Siegen, Anal'ische Chemie I, Adolf-Reich wein-Strasse, 57068 Siegen, Germany, Institut Fresenius, Hamburger Strasse 1, 552 18 Ingelheim, Germany, and Institut Fresenius, Labor Hamburg, Andreas-Meyer-Strasse 3 1-35, 22 1 13 Hamburg, Germany
The recovery of chlorobenzenes (di- to hexachloro) and hexachlorocyclohexane (HCH) isomers from supercritical carbon dioxide by trapping in cold solvent was studied. By changes in supercritical pressure, flow rate, nature and temperature of the solvent, and trap geometry, recoveries of chlorobenzenes varied in the range 80-100%,andthose of HCH in the range 50-94%. For the optimum set of variables, the average recovery of all compounds was 95%. With these trapping conditions, an environmental soil sample gave average supercritical fluid extraction (SFE) recoveries (compared to Soxhlet extraction) of 60-85%, depending on the SFE conditions. Supercritical fluid extraction (SFE) is increasingly becoming a powerful tool for the removal of analytes from solid The main advantages over the Soxhlet extraction method are (a) less extraction time, (b) less solvent waste, and (c) the possibility of easily altering extraction conditions during e x t r a ~ t i o n . ~ In order to develop quantitative SFE methods, three major steps have to be considered. The first step is to overcome analyte-matrix interaction and dissolve the analyte in the supercritical fluid (SF). The second step is to carry the analytes through the cell. Finally, quantitative collection of the analytes has to be ensured. Often the collection step is not sufficiently considered and might lead to misinterpretation of low and variable recovery data. Analytical chemists have employed different techniques for off-line collection after S F E . ' & l 3 Basically, the collection t Universitat-GH Siegen.
Institut Fresenius, Ingelheim. 1 Institut Fresenius, Hamburg.
(1) Lopez-Avila, V.; Dcdhiwala, N. S.; Beckert, W. F. EPA/600/S4-90/026, March, 1991. (2) Beckert, W.; Lopez-Avila, V.; Cram, S . Am. Enuiron. Lab. 1991, 10, 21-24. (3) Fahmy, T. M.; Paulaitis, M. E.; Johnson, D. M.; McNally, M. E. P. Anal. Chem. 1993, 65, 1462-1469. (4) Paschke, T.; Hawthorne, S. B.; Miller, D. J.; Wenclawiak, B. J. Chromatogr. 1992, 609, 333-340. ( 5 ) Hawthorne,S. B.; Langenfeld, J. J.; Miller, D. J.; Burford, M. D. Anal. Chem. 1992, 64, 1614-1622. (6) Langenfeld, J. J.; Hawthorne, S. B.; Miller D. J.; Pawliszyn, J. Anal. Chem. 1993, 65, 338-344. (7) David, F.; Verschuere, M.; Sandra, P. Fresenius J. Anal. Chem. 1992, 344, 479-485. (8) Jahn, K.; Wenclawiak, B. Chromatographia 1988, 26, 345-350. (9) Wenclawiak, B. W. Analysis with Supercrifical Fluids-Extraction and Chromatography, 1st ed.; Springer: Heidelberg, 1992. (10) Ashraf-Khorassani, M.; Houck, R. K.; Levy, J. M. J . Chromafogr.Sci. 1992, 30. - ,361-366. - - -- - ~ (1 1) Snyder, J. L.; Grob, R. L.; McNally, M. E.; Oostdyk, T. S. J . Chromatogr. Sci. 1993, 31, 183-191. (12) Furton, K. G.; Jolly, E.; Tein, J. J . Chromatogr. 1993, 629, 3-9. (13) Wenclawiak, B.; Paschke, Th. Nach. Chem. Tech. Lab. 1993, 41, 806-810. 0003-2700/94/0368-3581$04.50/0 0 1994 American Chemlcal Society
procedure is either to adsorb or deposit the analytes on solid surfaces and remove them afterwards or to trap the analytes in organic solvents. Sorbent surfaces include CISand XAD traps.1417 Levy et al. have introduced cryogenically cooled silanized glass beads.18 The main advantage of solid trapping seems to be the possibility of using higher flow rates while still keeping good collection efficiency. Obviously, after adsorption or deposition of the analytes, a second step is needed to remove them from the sorbent trap. This step introduces new parameters to be optimized. Furthermore, modifiers can remove already trapped analytes from the sorbent. In off-line SFE, the most commonly used collection method is depressurizing C 0 2 into organic solvents. This method is relatively simple to perform, and the resulting extract is immediately ready for analysis. Langenfeld et al. have reported goodcollection recovery for 66 compounds at a solvent temperature of 5 O C . 1 9 They have also reported some losses of analytes (e.g., a-pinene) due to purging of the analytes out of the collection solvent. Porter et al. reported lower recovery values (e.g., for isopropyl alcohol) on adding inert bodies to the collection solvent .(compared to those with the collection solvent alone).20 Negligible loss of C 9 4 7 hydrocarbons due to purging from the solvent has been reported in the same study. Thompson et al. report recoveries of 90% and better for several polar analytes using mixed collection solvents.21A further collection method was introduced by McNally et al., who rapidly depressurized the SF without a restrictor into a closed empty via1.22 While spike recovery studies can be used for optimizing analyte collection, extracting real-world samples (e.g., contaminated soil) with those extraction conditions often gives poor r e c ~ v e r i e s .Therefore, ~~ extraction conditions from spike recovery studies can be used only as a starting point for further (14) Miller-Schantz, M.; Chester, S. N. J . Chromatogr. 1986, 363, 397. (15) Hedrick, J. L.; Taylor, L. T. J. High Resolur. Chromatogr. 1990, 13, 312. (16) Schneiderman, M. A.;Sharma, A. K.; Locke, D. C.J. Chromatogr. 1987,409, 343. (17) Mulcahey, L. J.; Hedrick, J. L.; Taylor, L. T.Anal. Chem. 1991,63, 22252232. (18) Levy, J. M.; Ravey, R. M.; Houck, R. K.; Ashraf-Khorassani, M. Fresenius J. Anal. Chem. 1992, 344, 517-520. (19) Langenfeld, J. J.; Burford, M. D.; Hawthome, S. B.; Miller, D. J. J . Chromatop. 1992, 594, 297-307. (20) Porter, N. L.; Rynaski, A. F.; Campbell, E. R.; Saunders, M.; Richter, B. E.; Suranson, J. T.; Nielsen, R. B.; Murphy, B. J. J. Chromatogr. Sci. 199530, 367-373. (21) Thompson, P. G.; Taylor, L. T.; Porter, N. L.; Richter, B. E. Technical Note 204; Dionex Corp.: Sunnyvale, CA, 1993. (22) McNally, M. E. P.; Miller, D. J.; Hawthorne S. B. Anal. Chem. 1993, 65, 1038-1042. (23) Burford, M. D.; Hawthoren, S. B.; Miller, S. J. Anal. Chem. 1993,65, 14971505.
Analytical Chemistry, Vol. 66,No. 21, November 1, 1994 3501
Table 1. Spike Solutions (mg/mL) Used for SFE Collection Efficiency Studies SPSl SPS2 SPS3 SPS4b 1,3-diCB 1,2-diCB 1,3,5-TriCB 1,2,4-triCB 1,2,3-triCB 1,2,3,5-tetCB 1,2,3,4-tetCB pentaCB hexaCB wHCH 7-HCH /3-HCH 6-HCH e-HCH vol used for spiking (pL)
1.77 1.99 -
2.13 2.11 0.94 1.56 1.10 -
-
-
2.31 -
1.00 0.66 -
0.40 0.59 0.19 0.32 0.19 0.36 0.21 0.21 0.19 0.38 0.14 -
-
-
5
3
5
-4
0.01 0.01 0.02 1.ooc 7.74 3.81 0.27 0.66 0.01 0.93 0.01 0.03
250
a A dash (-) means not present in spike solution. SPS4 is a real sample toluene extract. Sum of 1,2,4,5-tetCB and 1,2,3,5-tetCB.
improvement of the extraction efficiencies of real samples. SFE recovery data are usually compared with recovery data from standard soxhlet e x t r a c t i ~ n . ~ ~ ~ ~ ~ The purpose of the present study is to test the feasibility of SFE to simplify analytical procedures used in hazard assessment and remediation of soil in a closed insecticide/ herbicide plant. Main contaminants are chlorobenzenes (CBs) and hexachlorocyclohexane (HCH) isomers, which at present are Soxhlet-extracted with toluene and quantified by gas chromatography with electron capture detectors after cleanup by column chromatography. As a first step, some factors which may influence the efficiency of solvent trapping were investigated.
EXPERIMENTAL SECT1ON Equipment. All SFE extractions were performed on a multivessel Dionex SFE-703 supercritical fluid extractor (Dionex, Idstein, Germany). A H P 5890 Series I1 gas chromatograph equipped with two electron capture detectors (GC-ECD) using a dual column technique was used (HewlettPackard, Hamburg, Germany). The GC-ECD was equipped with a DB 17 (30 m X 0.32 mm i.d., film thickness 0.25 pm) and a DB 1301 (30 m X 0.32 mm i.d., film thickness 1.0 pm) capillary column (both from J & W Scientific, Mainz, Germany), both connected to a 50 cm X 0.32 mm i.d. uncoated deactivated precolumn (Chrompack, Frankfurt, Germany) with a Y-shaped inlet splitter. Standards and Reagents. Four different spike solutions (Table 1) were used for spike experiments. All spiking solutions were prepared in toluene and stored in a refrigerator at -18 "C. SPS4 was not prepared using standards but was a toluene Soxhlet extract of a real-world soil sample. This Soxhlet extract was used to more nearly represent the actual contaminant spectrum found at the site. Nanograde toluene and diethyl ether were obtained from Promochem GmbH (Wesel, Germany); pro analysi grade cyclohexane, ethyl acetate, acetone, and dichloromethane were (24) Snyder, J. L.; Grob,R. L.; McNally, M. E.; Oostdyk, T. S . Anal. Chem. 1992, 64, 194C-1946. ( 2 5 ) Wenclawiak, B.; Rathmann, C.; Teuber, A. Fresenius J . Anal. Chem. 1992, 344, 497-500.
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Analytical Chemistry, Vol. 66, No. 21, November 1, 1994
purchased from Riedel de Haen (Seelze, Germany). SFC grade carbon dioxide with helium head pressure (Air Products, Hattingen, Germany) was used for all experiments. The glass beads were purchased from Sigma Chemical Co. (Deisenhofen, Germany), while the glass fiber filters were obtained from Whatman International Ltd. (Frankfurt, Germany). Soils. Dry sand without contaminants (verified by Soxhlet extraction and GC-ECD) was used for some spike experiments. A real-world sample (contaminated soil) was taken from a contaminated industrial site in Hamburg. After the soil was allowed to dry overnight under a hood and then sieved through a 2 mm sieve, distilled water was added (10 w/w %). The wet matrix was further homogenized using a 3-D Turbula device. The sample was stored in a refrigerator. Soxhlet Extraction. Ten gram portions of contaminated soil, mixed with 10 g portions of sodium sulfate were extracted 4 h using 170 mL of toluene. The extracts were used without further concentration. SFE Extraction Conditions. Ten milliliter Keystone Scientific extraction vessels were used. Theoven and restrictor temperatures were kept at 70 " C . Restrictors with a reported flow rate of 500 or 250 mL of gaseous fluid per minute at 340 atm were used. Two to fiveextraction cells were run in parallel for 50 min. SFE conditions are given in Table 2. The extracts were diluted to a final volume of 20 mL with nanograde toluene and analyzed by GC. Trapping Studies. For most spike experiments, the standard mixtures (SPS1-3) were placed in empty extraction cells in order to eliminate the matrix effect for collection efficiency studies. Experiments were designed to determine the effect of solvent mixtures, solvent temperature, and the presence of glass beads, glass frits, and glass fiber filters on the trapping of CBs and HCHs. SPS4 was spiked on dry, uncontaminated soil (the soil was placed in the extraction cell before spiking, and the cell was closed immediately after spiking). Here the extraction vessel was filled as follows: preextracted cotton wool, 0.4 g of sand, 0.4 g of copper powder,20 11 g of soil, and preextracted cotton wool. To establish an equivalent 100%recovery, an appropiate spike of the same analyte solution was diluted to 20 mL with toluene. Dionex Collection Assembly. A dual-chamber trapping vial arrangement is used. The end of a heated restrictor is placed inside a glass tube, which is sealed by a septum (Figure 1). During extraction, the expanding SF travels down the inner glass tube (glass frit and glass fiber filter shown in Figure 1 are modifications made by the authors) and bubbles through the collection solvent. The sample vials are cooled using Peltier cooling. Depending on the room temperature, the solvent was cooled down to about 0 "C to -6 "C. A vent needle is placed in the septum to allow the expanded gas to exit. Behind the vent needle, a filter cartridge is placed for additional trapping.26 This was analyzed only in a few cases. SFE of Real- World Sample. Nine gram portions of the contaminated soil were each mixed with 3 g of sodium sulfate. After preextracted cotton wool, 0.4 g of sand, and 0.4 g of copper powderz0were placed in a 10 mL extraction vessel, the mixed sample and more preextracted cotton wool were added. The collection device is described in Table 4 C. Addition of (26) Dionex GmbH, Jolstein, Germany
Restrictor
Vent Needle
Figure 1. Modified collection vial assembly.
Table 3. Recovery Data (%) from Spike Experiments Udng Toluene as Collection Solvent@ A (vial) B1 (vial) B2 (vial filter) C (vial)
+
1,2-diCB 1,2,4-triCB 8-HCH
94 (2.7) 94 (2.4) 30 (8.9)
92 (1.6) 88 (4.0) 66 (37.9)
93 (2.6) 89 (4.3) 95 (2.5)
90 (1.8) 86 (2.1) 48 (24.6)
"All experiments were performed in triplicate. CV is given in parentheses. A 50 min extractions at 380 atm were performed with an aerage flow rate of about 1.6 mL of condensed fluid per minute (restrictor type 500). B: a pressure program was introduced, starting with 150 atm holding for 15 min, 250 atm holding for 20 min, and 380 atm holding for 15 min, with the same restrictors as in A. C: the same three-ste pressure pro ram as in B was used, but restrictor ty 250 was used wit[ an average low rate of ca. 0.5 mL of condensed &d per minute. Table 4. Effecl of Temperature, G l a r RH, and G i a r Fiber Filter on Collectlon Effklency recovery (CV)" 0
1,3-diCB 1,2-diCB 1,3,5-triCB 1,2,4-triCB 1,2,3-triCB 1,2,3,5-tetCB 1,2,3,4-tetCB pentaCB hexaCB CY-HCH Y-HCH 8-HCH 6-HCH t-HCH
m
s
a
A
B
C
93 (6.3) 92 (6.4) 98 (5.1) 97 (4.4)
95 (2.1) 95 (2.4) 98 (2.5) 99 (2.8)
98 (3.4) 72 (6.2) 73 (8.4) -
97 (7.5) -
99 (1.5) 99 (1.3) 99 (1.0) 00 (0.8) 99 (1.3) 01 (1.0) 99 (3.4) 95 (5.7) 94 (5.5) 92 (7.0) 91 (6.2) -
-
-
-
82 (5.6) 86 (8.4) -
D -b
97 (2.2) 98 (2.1) 96 (2.1) 95 (1.5)c 94 (1.6) 94 (2.0) 93 (1.6) 94 (1.9) 110 (6.3) 76 (5.6) 99 (2.2) 88 (4.6)
Average of five parallel experiments, except for experiment A, with
n = 4. All data in %A: collection device was filled with 9 mL of toluene/
111
e, E
a2
-
V ) I
E
.t 'a8m LI
6 -- z sc &
0 -
I
0
e e4
5:
cyclohexane, 30 g of glass beads (2 mm d), Peltier cooling, and restrictor type 500. B: collection device was filled with 9 mL of toluene/ cyclohexane, 30 g of glass beads (1 mm d), Peltier cooling, and restrictor type 500. C: collection device was filled with 9 mL of toluene/ cyclohexane, 30 g of glass beads (1 mm d), glass frits ( rosity 1) placed at the end of the inner glass liner, glass fiber filter a g d on top of the glass frit, Peltier cooling, restrictor type 500. For A, B, and C, spiking was performed into empty cells. For D, spiking was performed onto sand placed into extraction cells. Extraction was performed at 70 OC oven and restrictor temperatures, extraction pressure starting at 150 atm (5 min), raising to 400 atm in 7 min, and holding the final pressure for 38 min. Peltier cooling and collection device assembly were as described in C. A, B/C, and D are different standards. A dash (-) means not present in standard. Sum of 1,2,4,5-tetCB and 1,2,3,5-tetCB.
modifier was performed by pipetting 200 p L of nanograde toluene into the filled cell. Separation and Quantification. SFE extracts were appropriately diluted with toluene and analyzed directly. Soxhlet extracts were first cleaned up as described below. Anal'icaiChemistry,
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The toluene extract (1 mL) was placed on a chromatography column (5.3 mm i.d. glass disposable Pasteur pipet) containing (from bottom to top) quartz wool; 100 mg of silica gel 60,70-230 mesh (E. Merck, Darmstadt, Germany); 600 mg of 25% sulfuric acid on silica gel; 300 mg of basic alumina activity I1 (2.3% water; ICN Biochemicals, Eschwege, Germany); and 100 mg of anhydrous sodium sulfate. The column was eluted with toluene to give a total of 5 mL of eluate. The CBs and HCHs were separated and quantified by gas chromatography with electron capture detection using a dual column technique. Three microliters of extract was injected onto the precolumn. The injection port temperature was set 3 OC higher than the oven temperature. The temperature program started at 85 OC, rising 5 deg/min to 115 "C and then 10 deg/min to 250 OC and 30 deg/min to 280 OC. The final temperature was held for 14.5 min. Helium 5.0 (99.999% purity) was the carrier gas at a flow rate of 3 mL/min per column, and nitrogen 5.0 was added used as the makeup gas to each detector at 60 mL/min. Quantification was performed using a seven-point external calibration over the range 2.5160 pg/&
RESULTS Trapping Studies. Initially, the collection of three analytes added to the empty cell (in order to avoid matrix effects) was studied. SFE conditions were varied as described in Table 2 1-111. As shown in Table 3, 1,Zdicb and 1,2,4-tricb are trapped to about 90%, with a low coefficient ofvariation (CV) regardless of conditions used. P-HCH behaves differently: recoveries in toluene are below 70% and highly variable. When the content of the filter cartridge is taken into account (experiment B2), however, the three analytes show similar high and reproducible recoveries, indicating that the HCH is lost at the collection stage, not the extraction stage. As a test of the backup medium (filter cartridge behind the extraction vessel), an experiment was performed collecting an extended range of analytes in empty (cooled) sample vials under the conditions of Table 2 IV. After SFE, the sample vials and filter cartridges were rinsed and eluted, respectively, with 10 mL of toluene. Negligible (
