Environ. Sci. Techno/. 1995, 29, 1259-1266
Determination of Pesticides and Polychlorinated Biphenyls in Water: A Low-Solvent Method V O O N S . O N G A N D R O N A L D A . HITES* School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
A low-solvent method was developed to replace EPA Method 608 for the determination of chlorinated pesticides and polychlorinated biphenyls in water and wastewater. The sample is first passed through large, octad e cylsili c a, Iiquid-solid extraction disks; the disks and the particles filtered from the water are then extracted with supercritical carbon dioxide to elute the analytes into a small vial of hexane. Using this method, solvent consumption was reduced by over 90%, and recoveries of at least 80% were obtained for most of the analytes. The extracts were analyzed by gas chromatographic electron-capture mass spectrometry (GC/ECMS), which is particularly sensitive and selective for the quantitation of chlorinated analytes. GC/ECMS also provides unambiguous analyte identification in a single gas chromatographic analysis.
Introduction The Environmental Protection Agency (EPA) relies on a series of laboratory-based analytical methods to determine the concentrations of organic pollutants in wastewater and drinking water (1, 2). The “600 series” methods were promulgated during the 1970s for the analysis of organic compounds in wastewater; the complete set of methods were given in the FederuZRegister (1). Later, the “500series” methods were developed for the analysis of organic compounds in drinking water (2). Today, many of these methods are widely used in analytical laboratories across the country. There is a problem however. Some of these methods generate large amounts of hazardous waste solvents. Since one of the EPA’s goals for the 1990s is “pollution prevention” (3), it is clear that official EPA methods that generate large volumes of hazardous solvents need to be eliminated. Method 608 is a particular problem. This is a gas chromatographic (GC) method for the determination of organochlorine pesticides and polychlorinated biphenyls (PCBs) in municipal and industrial wastewater (I); see Figure 1 for the structures of the compounds covered by this method. In Method 608, approximately 1 L of water is extracted with at least 200 mL of dichloromethane in a separatory funnel. The dichloromethane extract is then dried over sodium sulfate, solvent-exchanged to hexane, and concentrated to a volume suitable for analysis by GC with an electron-capture detector. Most of the dichloromethane is evaporatedand released into the environment. This is unfortunate because dichloromethane is a possible carcinogen (41, and its environmental release is regulated under the Clean Air Act hazardous air pollutants listing (5). Method 608 is also one of the most widely used of the 600 series methods. It is used for at least 27 000 analyses per month by at least 370 laboratories (6‘). Based on this usage and on a conservative estimate of the volume of dichloromethane used to perform each analysis, we calculate that more than lo5kg of dichloromethane is released into the environment annually from just this one method alone. Clearly, minimizing the amount of solvent used for Method 608 is important, and we sought to develop a lowsolvent method. We considered alternative methods based on liquid-solid extraction (LSE) and supercritical fluid extraction (SFE), both of which can significantly reduce solvent consumption. Although current analytical SFE systems are not suitable for the extraction of water samples, such samples can be indirectly extracted by first passing them through an adsorbent to collect the analytes of interest and then extractingthe adsorbent by SFE (7,8). In addition, particle-laden water samples can be extracted by LSE-SFE in one step; thus, particles in the water need not be filtered and extracted separately. Method 608 specifies the use of a gas chromatograph with an electron-capture detector to analyze the extracts. The method also calls for compound identification to be supported by at least one additional qualitative technique. The official method recommends analytical conditions for a secondary GC column to confirm results from the primary column. In our method, gas chromatographic electron* Corresponding author e-mail address: HITESR@INDIANA. EDU.
0013-936W95/0929-1259$09.00/0
0 1995 American Chemical Society
VOL. 29, NO. 5,1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1259
c1
CQJ’ (3)
Cl
(4)
44 %% 4 4 4 &1 4 14 c
c1 c1
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(5)
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(13)
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FIGURE 1. Compounds studied: (1) a-hexachlorocyclohexane(e-BHC); (2)/3-hexachlorocyclohexana (b-BHC); (3) &hexachlorocyclohexane (d-BHC); (4) y-hexachlorocyclohexane or lindane (g-BHC); (5) aldrin; (6) dieldrin; (7) endrin; (8) endrin aldehyde (endrin eld); (9) heptachlor, (10) heptachlor epoxide (hepta epox); (11) oxychlordane; (12)a-chlordane (a-chlordane); (13) y-chlordane (g-chlordane); (14) trans-nonachlor (t-nonachlor); (15) endosulfan I (endo I);(16) endosulfan II (endo 11); (17) endosulfan sulfate (endo sulfate); (18) p,p’-DDE; (19) p,p’-DDT; (20) general structure of polychlorinated biphenyls (PCBs).
capture mass spectrometry (GCIECMS) is used for sample analysis. Past studies conducted by various researchers (9-12) and in our laboratory (13-17) have shown that electron-capturemass spectrometry is particularly sensitive and selective for the determination of trace amounts of chlorinated organic environmental contaminants. By using GC/ECMS, only a single gas chromatographic analysis is needed, and errors associatedwith compound identification by GC-only methods can be avoided.
Experimental Section Solvents and Chemicals. All solvents were glass-distilled grade obtained from EM Science (Gibbstown, NJ). A 1280 rn ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 5,1995
standard mixture of organochlorine pesticides in 1:l to1uene:hexane at a concentration of 2000pglmL each was purchased from Ultra Scientific (North Kingston, N). Additional compounds in solid form (a-chlordane, y-chlordane, oxychlordane, and DDT) were obtained from the U.S. EPA Pesticides and Industrial Chemical Repository (Research Triangle Park, NC), and standard stock solutions (100 pglmL) containing these compounds were made up in hexane or toluene. Polychlorinated biphenyls were purchased as a mixture (2.00 pglmL) in isooctane from Supelco, Inc. (Bellefonte, PA). Spiking solutions for the compounds were made up from the above stock solutions with methanol for subsequent dilutions. Isotopically-
labeled standards, &-lindane and [37CWheptachlor epoxide, were obtained from Cambridge Isotope Laboratories, Inc. (Andover,MA) and from the EPA Pesticides and Industrial Chemical Repository, respectively. PCB congener 166 (100 pg/mL in hexane) was purchased from Ultra Scientific.The calibration mixture used for quantitation was prepared in toluene from the same standard stock solutions as those used to make up the spiking solutions. Water Samples. Laboratory tap water was used in all recovery experiments, except as indicated. Unless otherwise noted, 1-L water samples were spiked with a representative set of organochlorine pesticides at the 2 ppb level and PCBs at the 0.5 ppb level for recovery experiments. Influent water from the municipal wastewater treatment plant in Bloomington, IN, was used as a representative wastewater sample. Liquid-Solid Extraction System and Procedure. Octadecylsilica Empore LSE disks were obtained from the 3M Corporation (St. Paul, MN); two disk sizes (47 or 90 mm diameter) were used. A standard Millipore (47 or 90 mm) glass vacuum filtration apparatus was used for drawing the water samples through the LSE disks. The extraction disk (both sizes) was first rinsed with 15 mL of hexane in a beaker. This rinsing step is important to remove any accumulated contaminants from the manufacturing process and from exposure of the disk to the environment. The disk was then placed in the filtration apparatus and attached to a vacuum source. Another 15 mL of hexane was then added to the filtration reservoir and allowed to soak through the disk (without vacuum) for 3 min. After air was drawn through the disk for a few minutes, 20 mL of methanol was added and allowed to soak through the disk, but the surface of the disk was not allowed to dry. A 1-L water sample was then added to the filtration reservoir, and the flow was adjusted to about 60-70 mLlmin so that the total extractiontime was 15-20 min. After all the sample had been extracted, air was drawn through the disk for 10 min to remove most of the residual water. The disk was removed and extracted as follows: Supercritical Fluid Extraction System and Procedure. The extraction disk was tightly packed (using a steel ramming rod) into a cylindrical stainless steel SFE vessel (3.5 or 10.4 mL) purchased from Keystone Scientific, Inc. (Bellefonte, PA). These extraction cells were fitted with a 20 cm x 45 pm i.d. fused silica restrictor as the outlet. The restrictor was used to regulate the flow rate at approximately 2 mllmin. The 3.5-mL SFE vessel measured 50 mm x 9.4 mm id.; the 10.4-mLvessel measured 150 mm x 9.4 mm i.d. When the 10.4-mLvessel was used for SFE, the void volume of the vessel was filled with anhydrous sodium sulfate purchased from Fisher Scientific (Fair Lawn, NJ). For both vessels, the entire vessel was inserted into a heated copper tube, and the extraction was effected with 120 mL of SFE grade carbon dioxide (Scott Specialty Gases, Inc., Plumstedville, PA) at 60 "C and 400 atm. An Isco (Lincoln, NE) Model 260D syringe pump supplied the supercritical COz at constant pressure. The outlet end of the flow restrictor was immersed in a test tube containing 15 mL of hexane to trap the extracted analytes. This test tube was maintained at 45 "C using a warm temperature bath. After SFE, the hexane extract was concentrated to about 1 mL under a gentle stream of nitrogen. Instrumental Conditions. Analyses of sample extracts were carried out on a VG 30-250 triple quadrupole mass spectrometer equipped with a Hewlett-Packard 5890 gas
chromatograph. Injections (1pL) of standards and sample extracts were made on-column into a 30 m x 250 pm i.d. DB-5 (0.25 pm film thickness) fused silica column U&W Scientific, Rancho Cordova, CAI. The GC temperature program was 45 "C to 160 "C at a rate of 30 "Clmin, held at 160 "C for 8 min, 160 "C to 280 "C at a rate of 5 "Clmin, and held at 280°C for 5 min. Helium was used as the carrier gas, and the linear velocity was set to 45 cmls at 150 "C. The GClMS transfer line was maintained at 280 "C. Mass spectrometer performance opti ;iization and mass axis calibration were carried out using the ions at mlz 633 and 452 of peduorotributylamine (PFTBA PCR Research Chemicals, Inc., Gainesville, FL) and the ion at mlz 193 of pentafhorobenzonitrile (PFBN; PCR Research Chemicals, Inc.). Methane (99.99% purity, Air Products, Allentown, PA) was used as the enhancement gas; optimum sensitivity was obtained at 0.20 Torr ion source pressure. The source temperature was maintained at 120 "C. Total ion chromatograms were reconstructed by the mass spectrometer scanning from 200 to 500 Da.
Results and Discussion Liquid-Solid Extraction. Octadecylsilica has demonstrated excellent extraction capacities for trace quantities of organochlorine pesticides and PCBs (18-21). It is available as prepacked cartridges or disks from several manufacturers. The most commonly-encountered form is the prepacked cartridge,which resembles a plastic syringe barrel. Typically, it has frits at both ends, with 100-1000 mg of 40-pm stationary-phase particles in the middle. Although widely used, this configuration has certain disadvantages. It has a narrow internal diameter that limits the flow rate to a range that requires lengthy processing times. For example, a cartridge LSE procedure suggested in EPA Method 525 recommends flow rates of 5-10 mLl min, leading to a total extraction time of 2-3 h (22). On the other hand, artificially high flow rates through the LSE cartridge can cause reduced mass transfer efficiency, preventing the recovery of certain analytes (23). Also, water samples with a relatively high particle content can plug the small cross-sectional area of a cartridge, causing restricted or even stopped flow. Bed channeling is another potential problem with dry-packed cartridges; if this happens, the recovery of various analytes may be substantially reduced (24). LSE disks will also efficientlytrap trace organicpollutants in aqueous samples (24-281, and their high cross-sectional area can alleviate many of the problems faced by LSE cartridges. The basic disk configuration consists of a polytetrafluoroethylene fibril network that holds octadecyl bonded silica particles. The particles are 8 pm in diameter and comprise about 90% of the disks weight. The extraction process is performed in a standard all-glass filtration apparatus to reduce contamination from plasticizers that may be found in cartridges. We preferred this disk configuration because the high cross-sectional area would be less susceptible to plugging problems from particleladen wastewater samples, and this would facilitate faster extraction rates. Supercritical Fluid Extraction. Supercritical carbon dioxidewas used to extract the LSE disks because it dissolves analytes that are soluble in solvents such as hexane, benzene, and dichloromethane (29). Practical advantages of C O 2 include its relative inertness and its lack of toxicity. Since it is a gas at ambient temperatures, it can be vented VOL. 29, NO. 5, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
1261
into the environment, and no liquid waste is generated. Carbon dioxide has been used successfully in SFE for the extraction of environmental contaminants from solid matrixes, such as soils (30-321 and adsorbents (7, 8,331. Thus, there is no need to separate the particles from the water sample; the LSE disks with the adsorbed analytes and with the particles filtered from the water can simply be folded up, inserted into an appropriate SFE vessel, and extracted with Con. SFE of Empore LSE Disks. One of our initial concerns as we were developing this method was the integrity of the LSE disk under supercritical fluid extraction conditions. The following experiment was carried out to ensure that the disk did not degrade and did not contain any potential interferents when extracted with supercritical COZ. A 90mm LSE disk was tightly packed into a 3.5-mL (9.4 mm x 50 mm) SFE vessel and extracted for 30 min. The collected extract was analyzed by GCIECMS. The resulting chromatogram was free of any contamination that would hinder identification and quantitation of the analytes of interest. Residual Water. Initial supercritical fluid extraction experiments, using the 90-mm LSE disks, showed problems with residualwater. Although a 10-mindryingstep (drawing air through the disk) was included, the LSE disk still retained a significant amount of water. This caused plugging in the restrictor because of ice formation and created erratic (or stopped) flows. Also, having water in the collected extract after the SFE process required an additional drying step (passing the wet extract through a sodium sulfate column). To eliminate this problem, the following procedure was used: The LSE disk was tightly packed into the COZinlet end of a high volume SFE vessel (10.4 mL), and the remainder of the vessel was filled with anhydrous sodium sulfate (approximately 10 g). The sodium sulfate, packed into the outlet end of the SFE vessel, served to absorb any residual water as the LSE disk was being extracted. Extracts collected in this way were free of water, and fluctuations of COZflow rates were minimized. SFE at 400 atm and 60 "C. Initially, the 90-mm disks were extracted at 400 atm and 60 "C using 40-mL of COz because this volume had been recommended for the 47 mm LSE disks (7, 8). To test this procedure, selected pesticides were spiked into 1-L drinking water samples (at 2 ppb concentration levels) and extracted using the LSESFE combined method. The resulting extract was then analyzed by GCIECMS, and the percent recovery of each pesticide was calculated by comparing the peak area of the most abundant ion (typically, the molecular anion in its mass spectrum) to that of a known external standard. Under these conditions, good recoveries (greater than 80%)were obtained for compounds eluting from the GC column earlier than DDE (except for aldrin). However, poorer recoveries were obtained for later eluting compounds such as endrin aldehyde and endosulfan sulfate. These results suggested that a higher volume of COZ was required for efficient extraction of the 90-mm disks. An experiment was then carried out to determine the volume of COz required for better extraction recoveries of the later eluting organochlorine pesticides. This was done by collecting a new hexane fraction at 15-min intervals during the SFE process. The flow rate of CO2 through the SFE vessel was 2.0 mL/min, so each fraction corresponded to increments of 30 mL of COz. The results are shown in Figure 2; each line corresponds to the cumulative SFE efficiency for a particular pesticide at various extraction 1262
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 5, 1995
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volumes of CO2. For early eluting and lower molecular weight pesticides, such as &BHC, the extraction process was virtually complete after only 30 mL of COZ. However, later eluting and more polarpesticides, such as endosulfan aldehyde, required larger volumes of COz for efficient recovery. Recoveries for most pesticides reached a plateau above 100mL of Con, suggestingthat quantitative recoveries could be obtained with 120 mL of COZ. All subsequent SFE experiments were performed with this volume. Flow Rate vs Volume. In an effort to find the optimum COZ flow rate for maximum extraction efficiency, we compared the percent recoveries for SFE at 400 a m and 60 OC at two different COz flow rates (0.50and 2.0 mL/min). No differenceswere observed, but the higher flow rate gave a substantial savings in total extraction time and was used in subsequent extractions. With our current experimental setup, it was not practical to use flow rates higher than 2.0 mLlmin due to increased losses of the collection solvent (hexane) caused by rapid evaporation. RecoveryJkperimentswithDrinkingWater. The LSESFE method was applied to the full suite of compounds including the organochlorine pesticides and the polychlorinated biphenyls (seeFigure 1). Laboratory drinkingwater, which had been previouslyverified to be free of the selected pesticides, was used. One liter samples were spiked with 2.0 ppb of the chlorinated pesticides (exceptfor DDT, which was at 7 ppb) and 0.50 ppb of each PCB congener. The spiked drinking water sample was then subjected to the LSE-SFE procedure. To determine reliable recoveries, a known amount of aquantitation standard ( [37CbJheptachlor epoxide) was added to the extract prior to analysis by GC/ ECMS. Recoveries were then calculated based on response factors for different analytes measured with a standard solution containing all the analytes and the quantitation standard. The recoveries of the various analytes are given in Figure 3 (gray bars). In general, the results suggested that quantitative recoveries (more than about 80%)can be obtained for a majority of the analytes. The PCBs showed remarkably uniform recoveries across different levels of chlorination with an average of about 75-80%. For the pesticides, however, recoveries showed more compound-
%Recoveries 0
20
40
60
80
100
a-BHC b-BHC g-BHC d-BHC HEPTACHLOR ALDRIN OXYCHLORDANE HEPTA EPOX g-CHLORDANE ENDO I a-CHLO RDANE t-NONACHLOR DIELDRIN ENDRIN ENDRIN ALD DDE DDT ENDO II ENDO SULFATE PCB 77 PCB 118 PCB 105 PCB 126 PCB 153 PCB 138 PCB 128 PCB 188 PCB 187 PCB 180 PCB 170 PCB 200 PCB 195 PCB 206 PCB 209
120
140
160
m LSE ONLY
LSE-SFE
0
20
40
60 80 100 %Recoveries
120
140
160
FIGURE 3. Recoveries of the pesticides and PCB studied under LSE-SFE and LSE-only conditions.
to-compound variability, with aldrin having the lowest recovery at about 55%. The hexachlorocyclohexanes, on the other hand, exhibited elevated (greater than 100%) recoveries for reasons which are not yet clear. Comparison to LSE-Only Recoveries. Another spiked drinking water sample was analyzed by an LSE-only procedure (34). In this procedure, analytes adsorbed on the extraction disk were eluted with two 15-mL aliquots of dichloromethane (instead of by SFE); these aliquots were combined, preconcentrated, and solvent-exchanged into hexane for analysis by GC/ECMS. To allow direct comparison, quantitation was performed the same way as the LSE-SFE method. The recoveries of the analytes from the LSE-onlyprocedure were compared to those obtained from
the LSE-SFE method, and the results are summarized in Figure 3 (black bars). In general, the recoveries from the LSE-only procedure were about 10% lower than from the LSE-SFE method, especially for the PCBs. The pesticides showed more variability; a fewwere better recovered using the LSE-only procedure. The recoveries for the hexachlorocyclohexanes were still among the highest with the @-isomerhaving the largest value (120%). Clearly, supercritical fluid extraction gives comparableor better recoveries for most of the compounds. Wastewater Experiments. This LSE-SFE method was extendedto include the determinationof selected pesticides and PCBs in wastewater. The intluent water to a local wastewater treatment facility was used as a representative VOL. 29. NO. 5. 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1263
wastewater sample. We suspected that the high suspended solids concentration in the sample (about 200 mg/L) would provide a challenge for the LSE disk in terms of sample throughput. Indeed, although drinkingwater flow rates in excess of 100 mL/min were commonly encountered, wastewater flow rates were much slower. At first, 47-mm LSE disks were tested to see if they could be used to extract 1-Lwastewatersamples. They couldnot; flowthroughthese disks was essentially blocked after approximately 300 mL of wastewater. The 90-mm LSE disks performed better; most of the extractions were complete in under 1 h, which translated to flow rates on the order of about 20 mL/min. To further alleviate the flow problem, wastewater samples were stored overnightin their sampling containers to allow the solids to settle. Although, all of the suspended solids in the water samples did not settle during this time, there was a marked improvement in the flow when only the first 900 mL of each sample was drawn through the LSE disk before the final 100 mL (which contained a higher suspended solid concentration). Aside from restricting the flow through the LSE disk, the suspended solids also limited the extent to which the LSE disk could be dried before SFE. We noticed that the LSE disks that had been used for wastewater samples were significantly wetter than those used for drinking water samples. However, the 10 g of sodium sulfate that was inserted into the SFE vessel prior to extraction was sufficient to absorb the extra residual water left on the LSE disk. No erratic flow problems were observed. We first applied this LSE-SFE method to the wastewater sample without any spiked compounds, and we found no detectable analytes of interest (see below for detection limits). An experiment to determine recoveries of the analytes of interest from wastewater samples was then performed. The wastewater was spiked with the same amounts of the various pesticides and PCBs as used in the previously described drinking water experiment. Early experiments indicated that the reproducibility of these recoveries was a problem. This was overcome by using additional internal standards for quantitation. These internal standards were spiked into the water sample along with the analytes, and the calculation of analyte recoveries was based on the relative response from the internal standard. The internal standards were ds-hdane (for hexachlorocyclohexanes), [37clSlheptachlor epoxide (for other pesticides and chlordanes), and PCB congener 166 (for polychlorinated biphenyls). The latter congener was chosen because it is not present in any of the Aroclor mixtures. Results of a wastewater experiment, carried out in triplicate, are given in Table 1, which gives the recoveries of the various analytes in three extractions, their mean, standard deviation, and percent relative standard deviation. For most of the analytes, the recoveries were reproducible, with an average relative standard deviation of 6%(omitting the high value for aldrin). Although the standard deviation for the recovery of aldrin is low, the relative standard deviation value is high because of the low mean recovery of the analyte (only 23%). The recoveries for the hexachlorocyclohexanes were all close to loo%, probably because &-lindane was used for quantitation. The other pesticides showed more scatter in mean recoveries because only one internal standard ([37Cl~l heptachlor epoxide) was used for their quantitation; as expected, this standard was recovered with somewhat different efficiencies compared to the other 1264 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 5, 1995
TABLE 1
Percent Recoveries of Different Analytes in Triplicate (recl, rec2, rec3) and Their Mean, Standard Deviation, and Relative Standard Deviation analyte
recl
rec2
rec3
mean
SD
rsd (YO)
a-BHC b-BHC g-BHC d-BHC heptachlor aldrin oxychlordane hepta epox g-c hlorda ne endo I a-chlordane t-nonachlor dieldrin endrin endrin ald DDE DDT endo II endo sulfate PCB 77 PCB 118 PCB 105 PCB 126 PCB 153 PCB 138 PCB 128 PCB 188 PCB 187 PCB 180 PCB 170 PCB 200 PCB 195 PCB 206 PCB 209
98 110 101 103 58 21 59 79 61 106 72 55 76 80 61 42 45 113 67 101 96 96 104 93 92 94 94 99 101 101 98 102 104 102
94 110 98 104 71 20 62 79 69 113 78 62 75 79 58 46 45 119 65 106 102 99 106 96 100 92 100 102 99 98 94 95 91 86
98 102 100 102 69 27 64 77 73 112 90 65 77 83 50 51 51 115 69 99 98 96 100 95 97 96 100 99 99 99 98 100 104 102
97 107 100 103 66 23 62 78 68 110 80 61 76 81 56 46 47 116 67 102 99 97 103 95 96 94 98 100 100 99 97 99 ,100 96
2 5 1 1 7 4 3 1 6 4 9 5 1 2 6 5 3 3 2 4 3 1 3 1 4 2 4 2 1 2 2 3 7 9
2
4 1 1 10
ia
4 2 9 3 12 9 2 2 10 10 7 3 3 4 3 2 3 1 4 2 4 2 1 2 2 3 7 10
pesticides. The PCBs, on the other hand, showed excellent reproducibilityin recovery. With the exception of congeners 206 and 209, all the relative standard deviations were less than 5%. GCIECMS. EPAMethod 608 recommendsusing GC with an electron-capture detector for analysis of the sample extracts (I). However, for analyte confirmation, Method 608 also recommends using a secondary gas chromatographic column to confirm identifications made with the primary column. In our method, sample extracts were analyzed by gas chromatographic electron-capture mass spectrometry (GUECMS). In the past, we have found that ECMS is particularly sensitive and selective for the measurement of electrophilic compounds such as chlorinated pesticides (35-37) and chlorinated dibenzo-p-dioxinsand dibenzofurans (14,38,39). Briefly, in ECMS, an enhancement gas such as methane is introduced into the ion source at a pressure of about 1 Torr and bombarded with 100 eV primary electrons to produce a population of secondary, thermal electrons: 2CH4 -t2e-(primary)
-
+
CH,' CH,' 2e-(thermal)
+H + + 2e-(primary)
The thermal electrons react with electrophilic molecules to rapidly and selectively produce negative ions that are specific for each compound. The resulting electron-capture mass spectrum can then be used to confirm the identity of a particular compound.
TABLE 2
Ion Used for Quantitatior of Each Analyte of Interest and Its Corresponding Minimum Detectable Amount (mda) analyte
quant ion
mlZ
mda (pg)
a-BHC b-BHC g-BHC d-BHC heptachlor aldrin oxychlordane hepta epox g-chlordane endo I a-chlordane t-nonachlor dieldrin endrin endrin ald DDE DDT endo II endo sulfate PCB 77 PCB 118 PCB 105 PCB 126 PCB 153 PCB 138 PCB 128 PCB 188 PCB 187 PCB 180 PCB 170 PCB 200 PCB 195 PCB 206 PCB 209
(M - CI)(M - CI)(M - CI)(M - CI)(M - 2HCI)(M - CI H)MMMM(M - 4CI - 2H)MMMMM(M - HCI)M(M - HC1)MMMMMMMMMMMMMMM-
255 255 255 255 300 330 424 388 410 404 266 444 380 380 380 318 318 406 386 292 326 326 326 360 360 360 394 394 394 394 430 430 464 498
9 55 6 27 36 330 43 35 21 8 33 22 36 33 23 45 280 4 4 5 4 4 4 5 5 4 6 4 3 3 4 3 3 4
+
Because fullmass spectra (mlz200-500) were obtained during each scan of the mass spectrometer, the most abundant ion for an analyte (typically,its molecular anion) could be used for quantitation, and the full mass spectrum could be used for analyte confirmation. In all the analyses involving wastewater, we found no potential interferences when using mass chromatograms for quantitation. This is a testament to the selectivity of electron-capture mass spectrometry. When using mass chromatograms for quantitation, each mass chromatogram was found to be noise-free (even for wastewater). Therefore, the minimum detectable amount for each analyte was calculated based on the amount of analyte required to generate enough ion current t o produce a registered peak (with a minimum peak area that was operationally-defined). This minimum detectable peak area was estimated experimentally, and the minimum detectable amount for each analyte was computed by extrapolatingits corresponding calibration curve (peakarea versus amount injected) down to the minimum peak area. Calibration curves were typically constructed using injected amounts that spanned 3 orders of magnitude. Minimum detectable amounts and the major ions used for quantitation of each analyte are given in Table 2. All of the quantitation ions represent the most abundant ion produced for each analyte, except for a-chlordane for which the (M-4C1-2H)-fragment ion was used. This is because a-chlordane co-eluted with endosulfan I and both compounds had similar molecular weights (the molecular
weights of a-chlordane and endosulfan I are 406 and 404, respectively). Therefore, for quantitation, the molecular anion of endosulfan I was used while a fragment ion specific only to a-chlordane (mlz 266) was used. The minimum detectable amounts for pesticides were in the 5-50 pg range, except for aldrin and DDT, which exhibited minimum detectable amounts of about 300 pg. Although aldrin is a hexachlorocyclopentadienederivative like most of the pesticides (see Figure 11, its minimum detectable amount is about an order of magnitude worse. This is because it undergoes dissociative electron-capture to produce mainlyC1- ions. Because the mass spectrometer was not scanned below 200 Da, the signal for C1- at mlz 35 and 37 was not detected) thus giving a low response for aldrin. When we compared responses for the pesticides based on total ion production measured by scanning the mass spectrometer from mlz 30 to mlz 500 (to incorporate the signal from Cl-1, aldrin gave the same response as the other structurally-related pesticides. Although DDT has one more chlorine than DDE, its higher minimum detectable amount probably reflects its aliphatic character (no molecular anion was observed) (16).The PCBs showed excellent minimum detectable amounts for individual congeners with an average of 4 pg. With GClECMS, not onlywas quantitation feasiblebut also unambiguous analyte identificationwas achieved in a single gas chromatographic analysis.
Conclusions In addition to requiring large solvent volumes, the liquidliquid extraction procedure now used in Method 608 has other drawbacks. Some samples can form intractable emulsions when shaken, and most extraction solvents are not very selective. Another concern is the purity of the extractingsolvent itself;solvent impurities at the nanogram per liter level can accumulate and cause significant background interferences. In addition to solvent minimization, the procedure described in this paper reduces these other problems. Solvents were only necessary in this combined LSE-SFE procedure for the LSE disk prewashing step and for analyte collection from the SFE process. A total of 45 mL of hexane and 20 mL of methanol were used, compared to at least 200 mL af dichloromethane in the current EPA method. Furthermore, most of the hexane and methanol can be recovered and incinerated. Only about 15 mL of hexane is evaporated into the atmosphere during concentration of the extract after SFE. This represents a reduction in solvent pollution of over 90%. More importantly, the use of dichloromethane has been completely eliminated.
Acknowledgments The authors thank 3M Corporation (J. K. Mitchell) for providing the Empore extraction disks and Craig Markell at 3M for helpful conversations. The authors also acknowledge M. A. Gudeman at the Dillman Road Wastewater Treatment Facility in Bloomington, IN, for providing the wastewater samples. Support for this project was provided by the U.S. EnvironmentalProtectionAgencythrough Grant CR-816842.
Literature Cited (1) Fed. Regist. 1984, 49 (2091, 43234. (2) National Technical Information Service (5285 Port Royal Rd., Springfield, VA 22161; order Nos. PB-89-220461 and PB-91108266).
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Received f o r review July 28, 1994. Revised manuscript received January 19, 1995. Accepted January 27, 1995.@ ES940472B @Abstractpublished in Advance ACS Abstracts, March 1, 1995.
