Selective Supercritical Fluid Extraction of Organochlorine Pesticides and Herbicides from Aqueous Samples

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1 Selective Supercritical Fluid Extraction of Organochlorine Pesticides and Herbicides from Aqueous Samples Ian J. Barnabas John R. Dean*, and Steven Μ. M. Hitchen Department of Chemical and Life Sciences, University of Northumbria at Newcastle, Ellison Building, Newcastle-upon-Tyne NE1 ΝΕ1 8ST, UK Susan P. Owen Analytical & Environmental Services, Northumberland Dock Road, Wallsend, Tyne & Wear NE28 OQD, UK Abstract Supercritical fluid extraction (SFE) of organochlorine pesticides and two classes of herbicides is achieved from a water matrix using solid-phase extraction prior to SFE. This technique allows for the removal of water prior to supercritical elution with carbon dioxide. Selectivity of extraction between these two groups of compounds is demonstrated by adding 10% methanol modifier to the CO 2 after an initial extraction only. The first extraction (CO 2 only) is used to preferentially remove organochlorine pesticides (approximately 90%) with minimal extraction of the herbicides (less than 5%). Modified CO 2 is then used to extract the herbicides, and approximately 90% is recovered. The extracts are analyzed by either gas chromatography with mass selective detection for organochlorine pesticides or reversed-phase high-performance liquid chromatography for the herbicides. This SFE selective extraction may prove useful in segregating pesticides from herbicides prior to analysis. In addition, chromatographic selectivity is also achieved because organochlorine pesticides cannot be determined using reversed-phase high-performance liquid chromatography, and the herbicides are not directly amenable to gas chromatographic separation. Introduction solvent extraction step (2). These traditional extraction methods frequently do not meet the requirements of modern pesticide monitoring, which include rapid sample throughput and low organic solvent usage. Supercritical fluid extraction (SFE) is a relatively new technique that may solve some of the problems associated with solvent extraction. The use of supercritical fluids as extraction solvents is attractive because of their unique solvation properties. Low viscosities combined with high diffusion rates offer high mass transfer and hence rapid extraction. Carbon dioxide is the conventional extracting solvent because it is relatively inexpensive, nontoxic, and excellent for extraction of nonpolar and moderately polar analytes. With the addition of a polar modifier, it can also be used to extract more polar samples (3). One aspect of SFE that is seldom demonstrated is the ability to change the solvating power of the fluid and hence selectively extract different analytes by altering the density of the fluid or by changing the fluid itself. Selectivity between low and high molecular weight hydrocarbons has been achieved by either increasing the carbon dioxide density (4) or by changing the extraction fluid to one with a greater solvent strength (5). The addition of a polar modifier has also led to selectivity between organochlorine and organophosphorus pesticides (6). The continual increase in the use of organic pesticides as crop protection chemicals (1) necessitates that their monitoring, often in many varied matrices, is rapid and simple to perform. Some form of sample preparation is almost always required to remove and concentrate the target analytes from their matrices. The majority of analysis protocols involve a solvent extraction stage prior to detection to achieve concentration. This is illustrated by the Environmental Protection Agency's (EPA) chromatographic protocols for pesticide analysis, which almost always contain a * Author to whom correspondence should be addressed. Figure 1. Schematic of the Jasco SFE. Reproduction (photocopying) of editorial content of this journal is prohibited without publisher's permission. 547

2 Table 1. Recoveries of Herbicides Using the Carlo Erba SFE* Compound Extraction 1 (%) Extraction 2 (%) Total (%) Simazine ND ND 0 Propazine Trietazine 2 ND 2 Chlortoluron Isoproturon Diuron * Conditions: pressure, 13.5 MPa; temperature, 50 C; mobile phase, pure CO 2. Not detected. Table II. Recoveries of Herbicides Using the Carlo Erba SFE* Compound Extraction 1 (%) Extraction 2 (%) Total (%) Simazine Propazine Trietazine Chlortoluron Isoproturon Diuron * Conditions: pressure, 40 MPa; temperature, 50 C; mobile phase, CO 2 and 400 μl MeOH. Table III. Percent Recoveries of OCPs and Herbicides Using C 0 2 Only (Jasco SFE) SFE has been used to extract a range of pesticides mainly from solid samples because of their direct compatibility with conventional extraction cell geometry. Organochlorine pesticides (OCPs) were extracted from soils using SFE. This method was compared with solvent extraction methods (7). Extraction of 5-triazine herbicides from sediment has also been achieved using supercritical carbon dioxide (8). Extraction from aqueous samples, however, has proved difficult with SFE, primarily because of the solubility of water (approximately 0.3%) in supercritical carbon dioxide (9). This carryover of water causes restrictor blocking due to ice formation and problems with subsequent chromatographic detection. These problems have been solved by trapping the analytes of interest onto a solid sorbent before elution using SFE (10,11). This paper describes the selective extraction of three OCPs and two classes of herbicides (uron and 5-triazine) from a water matrix. Solid-phase extraction utilizing C 1 8 Empore disks (Jones Chromatography; Glamorgan, Wales) was used to initially trap all of the analytes prior to elution with both pure and methanol-modified supercritical CO 2. Both OCPs and herbicides have been shown to be effectively trapped onto the C 1 8 packing used (12,13). This selectivity between OCPs and herbicides may be useful because herbicides cannot be analyzed with conventional gas chromatographic split/splitless injection systems because of their thermal instability. They are usually detected by high-performance liquid chromatography (HPLC) (14). Many OCPs, however, are not sensitive to the UV-vis detectors used in HPLC and are commonly analyzed by split/splitless gas chromatography (GC). Compound Mean recovery (%) OCPs Heptachlor ,92.9,91.7, 92.3,88.2 Isodrin ,114.5, 98.1,96.3,94.1 Dieldrin , 88.2, 85.1, 85.4, 81.7 Herbicides Simazine ,1.5,0.9, 1.0,0.9 Propazine ,2.0, 0.4, ND, 0.4 Trietazine , 2.9, ND, 2.2,3.1 Chlortoluron ,5.4,1.5, 1.7,4.5 Isoproturon , 3.9, 2.3, 2.0,4.0 Diuron ,4.3, 2.7, 2.8, 2.7 Individual recovery SD % RSD Experimental Instrumentation Initial SFE was carried out using a Carlo Erba SFE 30 extraction system (Milan, Italy), which is described in detail elsewhere (15). The modified collection unit (also detailed) was used to prevent any analyte loss due to aerosol formation. This fixed restrictor system was used to extract the herbicides studied, with both pure and methanol-modified CO 2. It was found, however, to inefficiently remove all of the herbicides from the solid C 1 8 packing material within one extraction, and a Jasco SFE (Mettler-Toledo Ltd.; UK), which possessed a second pump for modifier addition, was used for all subsequent extractions (Figure 1). The OCPs and herbicides were initially trapped onto a C 1 8 Empore extraction disk prior to extraction from a 10-mL extraction cell (Jasco; Easton, MD). The cell was kept at a constant 50 C throughout all extractions. Subsequent detection of OCPs was carried out using a Hewlett-Packard GC 548

3 (Model 5890; Wilmington, DE) with a Model 5971A mass selective detector (MSD). Extract (1μL)was injected into a split/splitless injection port maintained at 250 C and operated in splitless mode at the time of injection (split at 0.75 min). Helium was used as a carrier gas with a column head pressure of 9 psi. An HP-1 fused-silica capillary column (25 m χ 0.2-mm i.d.; 0.32-μm film thickness) was used to achieve separation with the following temperature program: initial oven temperature of 85 C, hold 2 min; then 16 C/min to 285 C and hold for 5 min. The MSD was operated in single ion monitoring mode (SIM) with a 7-min solvent delay at a constant temperature of 280 C. All herbicides (uron and 5-triazine) were analyzed using reversed-phase HPLC. A Gilson pump (Model 305; Anachem, UK) was used to isocratically pump a 55% methanol-water mixture (1 ml/min) through a C 1 8 column (25 cm χ 4.6-mm i.d.) (Phase Separations; UK), which was maintained at 35 C in a column oven (Gilson). Detection of the herbicides was by a UV-vis spectrophotometer (Jasco, Model UV 975) at 240 nm. An LDC/Milton Roy integrator (Model CI 10, Thermo Separations; San Jose, CA) was used for peak analysis. Reagents Supercritical fluid grade C0 2 was supplied by Air Products (UK). OCPs (heptachlor, dieldrin, isodrin, and β-endosulphan as the GC-MSD internal standard) and herbicides (simazine, propazine, trietazine, chlortoluron, isoproturon, and diuron) were obtained from Promochem (St. Albans). The solvents, including hexane (collection solvent for OCPs), methanol, and water, were all HPLC-grade or better and obtained from various sources. Empore extraction disks were obtained from Jones Chromatography, and the C 1 8 Sep-Pak cartridges (Waters; Milford, MA) (used in the modified collection unit) were supplied by Fisons (UK). Extraction procedure Initial extractions were concerned with extraction of herbicides only because previous work had indicated that some Table IV. Percent Recoveries of Herbicides Using Modified CO 2 (10% MeOH, Jasco SFE) Compound Mean recovery (%) Individual recovery SD % RSD OCPs may be extracted from solid-phase extraction (SPE) disks only at moderate pressure conditions (15). A 200-mL distilled water sample was spiked with all six herbicides at a 10- pg level. The sample was then pretreated by adding approximately 5 ml methanol and adjusting the ph to less than 2 with hydrochloric acid prior to SPE extraction. The disk pretreatment has been discussed previously (6). The sample was filtered through the disk in around 5 min, and the disk was partially dried under vacuum at the pump. A further drying stage (45 C for 20 min) was found to be necessary to prevent restrictor plugging due to ice formation. The disk was then rolled and placed into the extraction cell. The disk containing the herbicides was then extracted with C0 2 only at a pressure of 13.5 MPa. This pressure, which has been experimentally determined, is known to extract OCPs including lindane, aldrin, and dieldrin (6) using 30mL of CO 2 passed dynamically preceeded by a 30-min static extraction period. The long static period was found to be necessary to allow sufficient modifier-analyte interaction in methanol-modified extractions. Further extractions were carried out with methanol-modified CO 2 at an increased pressure of 40 MPa, with 400 μl methanol being added directly into the extraction cell. The amount of methanol added was kept at this low volume because it was found that any increase caused severe restrictor blockage and therefore greatly reduced dynamic flow rates. All herbicide extracts were collected in the HPLC mobile phase, as any deviation from this injection solvent was found to affect the resultant chromatography. Subsequent extractions were carried out using the variable restrictor Jasco SFE system, which does not suffer from restrictor blocking due to modifier addition. Also, the Jasco SFE has a second pump that allows modifier to be continually added to the cell during extraction. This system was used to selectively extract OCPs (100 μg to allow GC-MSD detection) from the herbicides. The SPE procedure discussed previously was used to trap all nine OCPs and herbicides onto the extraction disk. The disk was then extracted at 250 kg/cm 2 only at a flow rate of 2 ml/min. The extract containing the OCP fraction was collected in hexane, and an internal standard (β-endosulphan) was added. The disk was then re-extracted under identical conditions with the addition of 10% methanol modifier. This herbicide fraction was collected in HPLC mobile phase. Simazine , 108.3,112.3, 90.7,108.1 Propazine , 80.3, 93.4, 83.0, 95.3 Trietazine ,81.8,92.5, 87.1,95.2 Chlortoluron , 82.4, 90.7, 86.2, 96.0 Isoproturon , 83.4, 92.6, 87.2, 94.3 Diuron ,80.5,90.7, 88.4, Results and Discussion The recoveries obtained for the extracted herbicides using the Carlo Erba SFE with CO 2 only (13.5 MPa) are shown in Table I. Even after two long extractions (30 min static followed by 30 ml C0 2 in dynamic extraction), a maximum of 5% is extracted. It is obvious from these results that C0 2 is ineffective in removing the herbicides from the extraction disk. This is because of the lower solubility of the herbicides in non- 549

4 polar CO 2. The differences in polarity between the OCPs and herbicides can be illustrated by the octanol/water partition coefficients (log P) of heptachlor and simazine, which are approximately 5.5 and 2.0, respectively (16). The smaller log Ρ value of simazine indicates that it is relatively polar, and there- Figure 2. GC-MSD chromatogram of OCP extraction (CO 2 only). Peaks: 1, heptachlor; 2, isodrin; 3, dieldrin; 4, internal standard. Figure 3. HPLC chromatogram of herbicide extraction: A, CO 2 only; B, CO 2 plus methanol as the modifier. Peaks: 1, simazine; 2, chlortoluron; 3, isoproturon; 4, diuron; 5, propazine; 6, trietazine. fore its extraction will require a modifier to increase the solvating power of the fluid. However, the OCPs are nonpolar (higher values oflogp) and should be removed effectively only. A methanol modifier (400 μl) was then added directly to the disk, and the pressure was increased to 40 MPa (the extraction was carried out for an equal length of time). The recoveries obtained using methanol-modified CO 2 are shown in Table II. After a second extraction, where another 400-pL aliquot of methanol was added to the disk, the recoveries were approximately 90%. However, it was not possible to achieve this near quantitative recovery in the first extraction because as soon as the dynamic extraction period was started, the methanol modifier was flushed from the cell, and the herbicides were no longer extracted. The addition of a greater volume of methanol was not possible because increasing the methanol concentration caused restrictor blockage due to freezing. Also, an extended static extraction period in which methanol-analyte interactions take place was impractical because a lengthy extraction period was already used (30 min). The Jasco SFE was therefore used for all subsequent extractions because it incorporated not only a variable restrictor that did not suffer from blockage but also a second pump used to continually deliver modifier throughout the extraction. The percentage recoveries obtained for extraction of both OCPs and herbicides using the Jasco SFE are shown in Table III. These initial extractions were carried out at a pressure of 250 kg/cm 2, a temperature of 50 C, and a flow rate of 2 ml/min with pure CO 2 only. It is obvious from the results that herbicides are not recovered at this pressure only, whereas two out of the three OCPs (heptachlor and isodrin) are extracted with over 90% recovered. However, the results for dieldrin were less quantitative; only 85% was recovered. This may be due to the presence of an epoxide ring in the structure of dieldrin that may bond to sites on the Empore disk more effectively than the other OCPs. All five disks were re-extracted under the same pressure and temperature conditions with the addition of 10% methanol as the modifier using Jasco's second pump (Table IV). The herbicide recoveries from this extraction were now all approximately 90% with the addition of a modifier. This recovery does not include the small amount of herbicide extracted in the first (CO 2 only) extraction. Recoveries of OCPs are not shown because they were already selectively removed from the disk in the CO 2 extraction. The results shown in Tables III and IV indicate that selectivity is possible between OCPs and herbicides using SFE. This selectivity may be best illustrated by consideration of example chromatograms obtained during these extractions. Figure 2 shows the GC-MSD trace for the initial OCP extraction only. (Note the absence from the chromatogram of any of the herbicides that are not extracted.) 550

5 Figure 3A shows the HPLC chromatogram for the herbicide extraction only, and Figure 3B shows the modified-co 2 extraction trace. The difference in the HPLC chromatograms clearly indicates both the extraction selectivity of SFE and the separation/detection system. Conclusion Selective extraction of OCPs from two different classes of herbicides in a water sample has been demonstrated. Using an SPE-SFE methodology in which the analytes are initially adsorbed onto a C 1 8 Empore extraction disk allows the analytes to be extracted from the water matrix without any of the problems associated with SFE from aqueous samples. Almost complete selectivity is achieved by the use of methanol-modified CO 2 which quantitatively removes the herbicides that remain after an initial extraction for OCPs only. An SFE system that incorporated a second pump for modifier addition and a variable restrictor was used because preliminary extractions of herbicides proved to be incomplete with a fixed restrictor (one pump) system. This selectivity between OCPs and herbicides may prove useful in their subsequent detection because the two classes of compound are conventionally analyzed by GC and HPLC, respectively. Acknowledgment We gratefully acknowledge the financial support of Analytical and Environmental Services Ltd., Northumbria Water plc. References 1. F.L. McEwan and G.R. Stephenson. The Use and Significance of Pesticides in the Environment John Wiley & Sons, New York, NY, 1979, pp D. Barcelo. Environmental protection agency and other methods for the determination of priority pesticides and their transformation products in water. J. Chromatogr. 643: (1993). 3. J.R. Dean. In Applications of Supercritical Fluids in Industrial Analysis. J.R. Dean, Ed. Blackie Academic & Professional, Glasgow, UK, 1993, ρ S.B. Hawthorne and D.J. Miller. Extraction and recovery of organic pollutants from environmental solids and tenax-gc using supercritical C0 2. J. Chromatogr. Sci. 24: (1986). 5. J.M. Levy, R.A. Cavalier, T.N. Bosch, A.F. Rynaski, and W.E. Huhak. Multidimensional supercritical fluid chromatography and supercritical fluid extraction. J. Chromatogr. Sci. 27: (1989). 6. I.J. Barnabas, J.R. Dean, S.M. Hitchen, and S.P. Owen. Selective extraction of organochlorine and organophosphorus pesticides using a combined solid phase extraction-supercritical fluid extraction approach. Anal. Chim. Acta. 291: (1991). 7. E.G. van der Velde, W. de Haan, and A.K.D. Liem. Supercritical fluid extraction of polychlorinated biphenyls and pesticides from soil Comparison with other extraction methods. J Chromatogr. 626: (1992). 8. V. Janda, G. Steenbeke, and P. Sandra. Supercritical fluid extraction of Striazine herbicides from sediment. J. Chromatogr. 479: (1989). 9. M.S. Kuk and J.C. Montagna. Chemical Engineering at Supercritical Fluid Conditions. M.E. Pailitis, J.M. Penninger, R.D. Gray, and K.P. Davidson, Eds. Ann Arbor Science, New York, NY, J.B. Pawliszyn and N. Alexandrou. Indirect supercritical fluid extraction of organics from water matrix samples. J. Water Pollut. Res. 24(2): (1989). 11. P.H. Tang, J.S. Ho, and J.W. Eichelberger. Determination of organic pollutants in reagent water by liquid-solid extraction followed by supercritical fluid elution. J. Assoc. Off. Anal. Chem. Int. 76: (1993). 12. J.J. Saady and A. Poklis. Determination of chlorinated hydrocarbon pesticides by solid-phase extraction and capillary GC with electron capture detection. J. Anal. Toxicol. 14: (1990). 13. A. Balinova. Solid-phase extraction followed by high-performance liquid chromatographic analysis for monitoring herbicides in drinking water. J. Chromatogr. 643: (1993). 14. S. Scott. Determination of derivatized urea herbicides in water by solid-phase extraction, methylation and gas chromatography with a nitrogen-phosphorus detector. Analyst 118: (1993). 15. I.J. Barnabas, J.R. Dean, S.M. Hitchen, and S.P. Owen. Supercritical fluid extraction of organochlorine pesticides from an aqueous matrix. J. Chromatogr. 665: (1994). 16. A. Noble. Partition coefficients (n-octanol-water) for pesticides. J. Chromatogr. 642: 3-14 (1993). Manuscript received September 22,