1510
Anal. Chem. 1991, 63, 1510-1513
Solid-Phase Extraction of Pesticides from Water: Possible Interferences from Dissolved Organic Material Warren E. Johnson,* Nicholas J. Fendinger,’ and J a c k R. Plimmer Natural Resources Institute, Environmental Chemistry Laboratory, USDA-ARS, Beltsville, Maryland 20705
A multlreddue analysis for trlfluralln, shnazlne, atrazlne, pr+ pazlne, dlazlnon, parathlonmethyl,aiachior, malathion, parathlon, chlorpyrlfos, pendhethalin, methldathlon, and DEF In water that utlllzes Ilquld-solid extractlon (LSE) wlth octadecyi-bonded slllca cartridges (C,,BSCs) followed by gas chromatography/mass spectrometrk analyels was developed. Recoverles of most pesticides were greater than 80% wlth C,,BSCs from fortlfled water at concentratlon levels from about 1 to 500 ppb. Recoverles wlth C,,BSCs, from an optlcaily adjusted humk acld solutlon (10 ppm dlssohred organk carbon) made to slmulate a natural water wlth a hlgh dlssolved organlc content, ranged from 29 to 153% and in general were lower than recoveries obtained from pure water. “CLabeled dlazlnon and parathion were recovered from the humlc acld solution at levels of 57 and 68%, respectlveiy, wlth C,,BSCs; the remainder of the labeled pesticides was found in the cartrldge eluents. Partltlon coefficients wlth humic acld were calculated based on recovery of “Clabeled pesticides from the C,,BSCs.
INTRODUCTION Pesticides are released into the environment during manufacture, transport, handling, and application to crops. Because of their widespread use, pesticide residues have been found in groundwater (1, 2), surface waters (3),rain ( 4 ) , and fog water (5). There is an increasing need for rapid reliable methods to measure pesticide concentrations in natural waters to minimize risks associated with pesticide use. The use of bonded-phase silica sorbents has gained widespread acceptance as a technique to concentrate contaminants in natural waters prior to liquid chromatographic (LC) or gas chromatographic (GC) analysis. The advantages of liquid-solid extraction, commonly referred to as solid-phase extraction (SPE), over conventional liquid-liquid extraction techniques include decreased use of and exposure to hazardous solvents, methodology adaptable to automation, less time consuming, and extractions that are not hindered by the formation of emulsions. In addition, bonded-phase silica sorbents also have advantages over polymeric resins such as XAD because they do not require extensive cleanup and there are usually fewer chromatographic interferences during analysis (3). Although there are a number of published pesticide analysis techniques that use LSE (3, 6-13) only a few studies have made direct comparisons to liquid-liquid extraction methodology or investigated how naturally occurring organic material may affect LSE extraction of pesticides. Because pesticide residues often occur a t concentrations near 1 ppb, most extractions require 1-2 L of water to obtain sufficient pesticide for analysis. However, C18-bonded silica cartridge (C18BSC) extraction efficiency of pesticides from natural waters with
* Corresponding author. Current address: The Procter and Gamble Co., Ivorydale Technical Center, Cincinnati, OH 45217.
high dissolved organic carbon (DOC)concentrations may be much lower than expected because dissolved organic matter (DOM) may saturate sorptive sites or the C18BSCsmay be inefficient in extracting pesticide associated with DOM. This paper describes a multipesticide residue technique that utilizes CleBSCs and illustrates how naturally occurring organic material may interfere with the extraction efficiencies of bonded silica phases. EXPERIMENTAL SECTION Reagents. Pesticide-grade methanol, hexane, methylene chloride, and acetone were obtained from Burdick & Jackson (Muskegon,MI). Water used in this investigation was produced by the Hydro Services and Supples, Inc. (Rockville, MD), Model 4C2-18 ultrapure water system with a critical applications column. Cl&SCs (SepPak, Waters Associates, Milford, MA) containing a nominal weight of 0.4 g of octadecyl-bonded silica with a 0.5-mL cartridge volume were prepared for we by first eluting with 5-10 mL of methanol followed by 5-10 mL of water. Pesticides (alachlor, atrazine, chlorpyrifos, DEF, diazinon, malathion, methidathion, parathion-methyl, parathion, pendimethalin, propazine, and simazine) were obtained from the U.S. Environmental Protection Agency’s Pesticide and Chemical Repository (Research Triangle Park, NC) and had purities of greater than 99.4%. “C-Labeled diazinon was obtained from Ciba-Geigy (Greensboro, NC) and had a reported radiochemical purity of 97.2% and chemical purity of >99% determined by gas chromatography. ‘%-Labeled parathion was obtained from Sigma Chemical Co. (St. Louis, MO) and had a reported radiochemical purity of >98%. All pesticides were used without additional purification. Humic acid sodium salt was obtained from Aldrich Chemical Co. (Milwaukee, WI). Optically standardized humic acid solutions were prepared by dissolving 0.10 g of humic acid sodium salt in 200 mL of 0.1% sodium hydroxide (14). The solution was stirred for about 12 h and centrifugedat 27 100 average relative centrifugal force (25 “C) for 1 h. The supernatant was diluted 20-fold and then adjusted to pH 6 with 0.1 N sulfuric acid. The resulting solution was diluted with water to obtain an absorbance of 0.2 (at 366 nm with a 1-cm cell), equivalent to an organic carbon content of about 10 ppm. This solution was diluted 1 part humic acid solution with 1 part water and 1 part humic acid solution with 3 parts water to produce 1/2 and 1/4 humic acid solutions, respectively. Water Extraction and Pesticide Analysis. Water samples (3-4 L) amended with pesticide were prepared by injecting an appropriate amount of stock pesticide solution made up in methanol. Spike levels ranged from 1 to 500 ppb. The spiked water samples were then mixed well and extracted within 1-3 h. Liquid-liquid extraction of water samples (approximately 1 L) was done in a 2-L separatory funnel with methylene chloride (ratio methylene chloride to water 1:lO). Sodium sulfate was added to the samples to produce a concentration of 20 g L-’ prior to extraction. Solvent evaporation was done with a Kuderna-Danish apparatus on a steam bath. Final solvent evaporation was done after adding 500 pL of methanol as a “keeper solvent” and evaporating to approximately 800 pL under a stream of nitrogen at room temperature. Water samples were pumped through preconditioned ClJ3SCs at a rate of 8-10 mL min-’ with valveless metering pumps (Fluid Metering Inc., Oyster Bay, NY; Model RP, SY). In some cases, a second backup C18BSCwas used to determine if saturation of
This article not subject to U.S. Copyrlght. Published 1991 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL.
adsorptive sites in the first cartridge had occurred that would result in pesticide breakthrough. Sample volumea (approximately 1 L) were measured by collecting effluent from the metering pumps into graduated cylinders. Cl&SCs were then dried by passing air through them at a rate of >lo0 mL min-' for 15-30 min. The dried cartridges were eluted with 4-6 mL of 1:l hexane:acetone. The solvent was exchanged with 500 pL of methanol and concentrated to approximately 500 p L under a stream of nitrogen at room temperature. Deuterated phenanthrene, used as an internal standard, was added to all extracts just prior to injection into a gas chromatograph/mass spectrometer. River water was collected from the Patuxent River at Benedict, MD, approximately 60 km southeast of Washington, D.C. Approximately 8 L of water was collected in amber glass bottles from a pier 10-moff-shore. The peeticideamended and unaltered river water samples were extracted as described above except that a disposable glass fiber filter (Waters, Millex AP20 fiiter, 0.8-8-pm pore size) was connected in-line to a Cl&SC. Pesticide analyses were done on a Hewlett-Packard (Palo Alto, CA) Model 5890A gas chromatograph interfaced with a Hewlett-Packard Model 5970 mass-selective detector (aD).The gas chromatograph was equipped with a 30-m X 0.25-mm4.d. X 0.25pm f h thickness fused silica DB-1 (J&W Scientific, Folsom, CA) capillary column. The chromatographic conditions were as follows: injection, splitlew; carrier gas, UPC He at a linear velocity of 30 cm s-l; injector temperature, 175 OC; transfer line temperature, 220 "C; and oven temperature programmed from an initial temperature of 50 "C held for 1 min to 200 "C at a rate of 5 "C mi&. The MSD was operated in the selective ion monitoring (SIM)mode. Three characteristic ions were monitored for each pesticide analyzed. The most abundant ion for each pesticide was used for quantitation, while the other two ions were used to confirm the presence of the pesticide. The minimum detectable quantity (MDQ) for the analysis with a 1-pL injection volume was approximately1 ng for most of the peaticidea analyzed. W activity was measured by liquid scintillation spectrometry with a Beckman Model LS6OOOIC liquid scintillation spectrometer. Samples were counted in Handifluor scintillation cocktail (Mallinckrodt, Inc.) and corrected for background and quench. Dissolved organic carbon measurements were made with Oceanographic International Corporation (College Station, TX) Total Organic Carbon Analyzer Model 700.
RESULTS AND DISCUSSION Pesticide Extraction and Recovery. Recovery data of 13 pesticides extracted from water with Cl&SCs are listed in Table I. Pesticide recovery from water extracted with ClsBSCs was nearly quantitative throughout the range of concentrations with no evidence of pesticide breakthrough up to sample volumes of 1 L with about 10 ppm DOC from Aldrich humic acid. These results are consistent with recoveries of pesticides from water extracted with C1&SCs and C&SCs reported previously (3,6,7). Recovery data of pesticides from humic acid solutions liquid-liquid extracted with methylene chloride and liquid-olid extracted with C18BSCs are listed in Table 11. Pesticide recovery data from humic acid solutions by liquid-liquid extraction into methylene chloride were similar to recoveries obtained from water using Cl@Cs. Liquid-liquid extractions of the humic acid solutions were hindered by the formation of emulsions that were separated by centrifugation. Recoveriea of trifluralin, malathion, chlorpyrifos, methidathion, and DEF were found to be significantly lower by the null hypothesis a t the 95% confidence interval. Although differences in recovery of other pesticides were determined to be statistically similar, recoveries of pesticides from Cl@C-extracted samples were in most cases lower than recoveries achieved with methylene chloride. GC/MS analysis of methylene chloride extracts of Cl&SC eluents of humic acid solution indicated that measureable amounts of pesticide passed through the two extraction cartridges. The significantly lower pesticide recoveries from humic acid solutions with the C18BSCs may be caused by (1)saturation
63,NO. 15, AUGUST
1, 1991
1511
Table I. Recovery of Pesticides from Water Extracted with CIaBSCs
spiked concn, anal*
n
PdL
recovery, %
RSD
trifluralin
3 4 4 4 4 4 4 4 4 4 4 4 4 4 4
460 41.1 8.2 2.1 24.6 4.9 1.2 25.1 5.1 1.2 28.7 5.7 1.4 52.0 10.4 2.5 2.6 28.1 5.7 1.4 237 24.9 5.0 1.2 71.8 14.3 3.5 180.2 36.1 9.0 25.5 5.1 1.2 12.6 2.5 35.4 1.6 47.6 9.5 2.4
84
5 17 10 8 9 3 14 2 2 9 2 1 13 6 4
simazine atrazine propazine
diazinon
1
parathion-methyl alachlor
malathion parathion chlorpyrifos
pendimethalin methidathion
DEF
4 4 4 4 3 4 4 4 3 4 4 4 4 4 4 4 4 4 4 3 2 4 4 4
78 74 82 99 73 74 106 87 72 103 89 80 86 79 113 81 114 81 97 112 106 93 99 123 81 119 87 78 82 99 92 88 74 133 72 73 66 94 118
8 11 5 40 4 4 7 30 2 6 23 26 4 26 12 1 28 20 18 20 15 24 53
of sorptive sites on C18BSCs by the humic material, (2) association of pesticide to humic acid retained by Cl&3Cs and not desorbed during solvent extraction, or (3) lower pesticide affinity to C18BSCS when associated with humic acid in solution. Evidence that a portion of the humic material was sorbed onto the C18BSCs during extraction was the discoloration of the cartridge packing by the humic material. The eluent from the Cl&SCs also retained the characteristic amber color of the humic acid, indicating that only a portion of the humic acid was removed from the solution. To determine if sorptive sites in the Cl$SCs were saturated, a second Cl&SC was placed after the first to determine if pesticide breakthrough occurred. The second cartridge also turned brown from sorption of humic material but at a slower rate than the f i t cartridge. Extraction and analysis of the second cartridge by GC/MS indicated that less than 1%of the pesticide spike was found in that cartridge. Extraction and analysis of 14Clabeled diazinon and parathion from the optically standardized humic acid solution gave recoveries (Table 111)from the first C&SC that were within the standard deviation of recoveries determined by GC/MS (Table 11). Recovery of "C-labeled diazinon and parathion from the unextracted second backup cartridge was about 1%. If the saturation of sorptive sites in the first C18Bsc caused low recoveries of pesticides, then a higher percentage of pesticide should have been present in the backup cartridges. Therefore, it is unlikely that saturation of sorptive sites in the primary C18BSC caused the low re-
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1991
Table 11. Recovery of Pesticides from Optically Standardized Humic Acid Solutions Extracted with CIIBSCsor Methylene Chloride
analyte trifluralin
simazine
atrazine
propazine
diazin on
parathionmethyl
alachlor
malathion
parathion
chlorpyrifos
pendimethalin methidathion
DEF
Table 111. Recovery of '%-Labeled Parathion and Diazinon from an Optically Standardized Solution of Humic Acid (10 ppm) with CIBBSCsa
extrachumic spiked tion acid dil concn, recovery, technique factor n rg/L % RSD
LL" LSEc LSE LSE LSE LL LSE LSE LSE LSE LL LSE LSE LSE LSE LL LSE LSE LSE LSE LL LSE LSE LSE LSE LL LSE LSE LSE LSE LL LSE LSE LSE LSE LL LSE LSE LSE LSE LL LSE LSE LSE LSE LL LSE LSE LSE LSE LL LSE LSE LL LSE LSE LSE LSE LL LSE LSE LSE LSE
I* 1 1
2d 4e 1 1 1
2 4 1 1 1 2 4 1 1 1 2 4 1 1 1 2 4 1 1 1
2 4 1 1 1 2 4 1 1 1
2 4 1 1 1
2 4 1 1 1
2 4 1 1
2 1 1 1
2 4 1 1 1
2 4
3 3 3 2 3 3 3 3 2 3 3 3 3 2 3 3 3 3 2 3 3 3 3 2 3 3 3 3 2 3 3 3 3 2 3 3 3 3 2 3 3 3 3 2 3 3 3 3 2 3 3 3 1 3 3 3 3 3 3 3 3 2 3
24.7 24.7 8.4 24.7 24.7 14.7 14.7 4.9 14.7 14.7 15.0 15.0 4.9 15.0 15.0 17.2 17.2 5.6 17.2 17.2 31.0 31.0 10.3 31.0 31.0 16.8 16.8 5.5 16.8 16.8 15.0 15.0 4.9 15.0 15.0 42.9 42.9 14.2 42.9 42.9 108.0 108.0 35.8 108.0 108.0 15.3 15.3 5.0 15.3 15.3 7.5 7.5 7.5 19.5 19.5 14.5 19.5 19.5 28.5 28.5 9.5 28.5 28.5
94 40 49 48 70 81 60 65 70 72 83 67 69 92 82 80 63 62 93 85 57 50 29 73 72 94 80 65 74 72 78 69 63 83 77 128 85 91 92 87 71 71 59 80 78 77 55 54 74 81 91 153 105 94 76 31 52 56 157 50 57 48 51
7 14 15
23 17 14 12
parathion recovery, 70 RSD 1:l hexane:acetone eluent primary ClsBSC backup CI8BSC eluted water a Average
f
66.8 0.9 1.4 35.3
diazinon recovery, % RSD
1.6 0.2 1.0 2.6
56.4 1.1 1.4 37.8
5.9 0.1 0.6 1.8
RSD for three replicate analyses.
4 11 12 7
Table IV. log Organic Carbon Partition Coefficients for Parathion and Diazinon pesticide
Aldrich humic acid
5 20 10 10
parathion' diazinon
4.72°3c 4.835.C
4 8 20 67 9 17 11
2 9 13 11 12 3 18 11 6 7 18 8 8 5 15 5 9 25 23 3 29 12 14 19 26 12 20 15
a LL indicates methylene chloride liquid-liquid extraction. *Indicates optically standardized humic acid solution (absorbance = 0.2 at 366 nm with 1-cm cell). (LSE indicates liquid-solid extraction with C18BSCs. Indicates a 2-fold dilution of the optically standardized humic acid solution. 'Indicates a 4-fold dilution of the optically standardized humic acid solution.
covery of pesticide from the humic acid solution. l4C-Labeled diazinon and parathion were also used to determine if formation of a pesticide-humic acid complex irreversibly bonded to the CIBBSCSwas responsible for lower
measd
calcd
3.68b 5.824.04C*d
2.8ab 2.76b
Indicates value determined in this study. The soil sorption coefficient normalized to organic carbon content ( K w ) (17). 'The pesticide-DOC partition coefficient ( K m ) . Schomburg et al. (18).
recoveries of pesticides from humic acid solutions. During elution of the cartridges with 1:l hexane:acetone, most of the humic material remained on the cartridge packing as indicated by the brown color of the packing after extraction. Therefore, the possibility exists that a pesticidehumic acid complex may also be retained by the cartridge packing after extraction. However, analysis of cartridge packings after use indicated that only about 1%of the 14C-labeledpesticide remained on the cartridge packing after extraction with 1:l hexane:acetone (Table 111). Recovery experiments done with 14C-labeleddiazinon and parathion from the optically standardized humic acid solution indicated that approximately a third of the pesticide present passed through the primary and backup CleBSCs (Table 111). These results indicate pesticides associated with humic material either as a complex or by simple adsorption are not retained by the Cl&SCs. Separation of humic bound organics from "freely dissolved" organic by ClsBSCs was previously reported (15,16).If it is assumed that all '%-labeled pesticide measured in the eluent from the primary and backup extraction cartridges was bonded to humic acid and all 14C-labeled pesticide extracted with C18BSCS and eluted with 1:l hexane:acetone was freely dissolved, then a partition coefficient (KDoc)describing the distribution of free and humic associated pesticide can be calculated from the following equation: (g of pesticide eluent)/ (g of
DOC)
KDOc= (g of pesticide extracted)/(g of water) The partition coefficients that describe pesticide affinity for dissolved organic carbon and particulate organic carbon are differentiated in this paper with the terms KDoc and Koc, respectively. Recent studies have suggested the need for this distinction (16). Knowledge of K m values can be important in predicting pesticide movement in the environment. Our measured parathion and diazinon partition coefficients ( K m ) for an optically standardized humic acid solution are compared to measured (KW,Kwc) and calculated (KOc) values found in the literature (Table IV). Humic acid KDW values measured in this study were 2 orders of magnitude greater than calculated KW values (17). Measured K m values in fog water by Schomburg et al. (18)ranged from 1 to 2 orders of magnitude greater than humic acid KDocvalues in the present
ANALYTICAL CHEMISTRY, VOL. 63,NO. 15, A W S T 1, 1091 Table V. Recovery of Perticider from Patuxent River Water with C,&SCI
malm trifluralin simazine
atrazine propazine
diazinon parathion-methyl alachlor parathion chorpyrifos
pendimethalin methidathion DEF malathion a
spiked concn, pg/L
W recoveries for
24.7 14.7 15.0 17.2 31.0 16.8 15.0 108.0 15.3 7.5 19.5 28.5
57,56 104,93 112,101 117,107 60,62 105,94 105’ 101,89 70,61 71,70 139,107 53,32 NDb
duplicate anal.
Indicates only one analysis. * ND = not determined.
study. Landrum et al. (15)found PAH, PCB, and biphenyl
K m values for natural DOC to differ from those measured with Aldrich humic acid and also showed seasonal variation. &die et al. (16), however, did not observe seasonal variations in measured K m values in Great Lakes waters. They reported that Km’s were consistantly larger than Km’s and varied to a greater extent with contaminant solubility. Different K m values for pesticides in various natural waters or seasonal variaion effects could alter pesticide recovery by C&SC extraction. Therefore, when LSE is used for extraction of pesticides in natural waters, matrix spikes with target analytes or field spikes with surrogate compounds that are chemically similar to the analytes should be employed to account for variablility in extraction efficiencies that may be encountered. Although we experienced no breakthrough of pesticide into the second backup cartridge, we strongly recommend the use of backup extraction cartridges under field conditions in the event that saturation of the primary cartridge occurs. Recovery of most pesticides from Patuxent River water with C18BSC was near 100% (Table V). It was not determined whether the relatively low recoveries of some pesticides from the river water (trifluralin, diazinon, chlorpyrifos, and DEF) was caused by the same mechanisms responsible for low recoveries of all pesticides from humic acid solutions.
1119
obtained for all of the pesticides from humic acid solutions and four of the pesticides from a natural water sample. Experiments done with “C-labeled diazinon and parathion suggest the formation of a pesticide-humic acid complex that is not efficiently extracted by the C18BSCs. With more experimentation, this may prove to be a useful technique for determining the amount of pesticide complexed to DOM in natural waters. In addition, when LSE is the method of choice for pesticide residue studies in natural waters with relatively high DOC concentrations, we recommend field spiking surrogate compounds similar to pesticide analytes to account for low recoveries that may occur.
ACKNOWLEDGMENT We acknowledge Ciba-Geigy Corp. for kindly supplying I4C-labeled diazinon. &&try No. Water, 7732-185;trifluralin,158209-8; nimadne, 122-34-9;atrazine, 1912-24-9;propazine, 139-40-2;diazinon, 333-41-5;parathion-methyl, 298-00-0;alachlor, 15972-60-8;malathion, 121-75-5;parathion, 56-38-2;chlorpyrifos, 2921-88-2; pendimethalin, 40487-42-1;methidathion, 950-37-8.
LITERATURE CITED M e , H. B.; Welt)‘, D. E. Water Res. 1989, 23, 1031-1037.
Pknke, H. B.: Welly, D. E.; Lucas, A. D.; Urban, J. 8. J . En-. -1. 1988, 17, 76-84. Hnckley, D. A,; Bidleman, 1. F. Mvkon. Sd. Tedmd. 1989, 23. 995-1000. .- - . - - -.
Richards. R. P.; Kramer, J. W.; Baker, D. B.; Krieger, K. A. N e t m 1987. 327. 129-131. W&, d. E.; Sdber, J. N.; LllJedahl,L. A. Netwe 1987, 325, 602-605. Junk, 0. A.; Rlchard, J. J. Anal. Chem. 1988, 60, 451-454. Anal. Chem. 1988, Bardaleye, P. C.; Wheeler, W. 8. Znt. J . En-. 25, 105-113. Swinefcfd, D. M.; BeHde, A. A. En-. T o x W . Chem. 1989, 8 , 465-468. wokon, A. w.; c. J. -top. i981,4, 1459-1472. Sew, W. A.; Glhrert, J . @. c h r o m e m .1980, 3 , 1753-1765. Wells, M. J. M.; Michael, J. L. Anal. Chem. 1987, 50, 1739-1742. Bushway, R. J. J . chrometog.1981, 211, 135-143. West, S. D.; Donrlla, 0. K.; Pode, G. M. J . Assoc. Off. Anal. Chem. 1983, 66, 111-114. Zepp, R. G.; Baughman, (3. L.; Schbtzhauer, P. F. C%e”@m 1981, 10, 109-117. Landrum, P. F.; Nihart, S. R.; Eadle, B. J.; Gardner, W. S. En-. Sci. Technd. 1984, 18, 187-192. Eadle, B. J.; Moreheed, N. R.; Landrum, P. F. chsmos.1990, 20, 161-178. Kenaga, E. E. Ecotox. Envkm. &few 1980, 4, 26-38. Schomburg, C. J.; Gktfelty, D. E.;Selber, J. N. E n W . Scl. Technd. 1991, 25, 155-160.
cw,
w.
CONCLUSIONS LSE utilizing Cl@SCs provided rapid near quantitative recoveries of 13 pesticides from water. Lower recoveries were
RECEIVED for review October 9, 1990. Revised manuscript received February 11,1991. Accepted April 9,1991.
