Environ. Sci. Technol. 1992, 26, 66-74
program. Averaged snow monitoring will of course indicate current deposition levels. Acknowledgments
We are grateful that the Snoqualmie Pass Sewer district for providing study space, winter 1987. The management of the Mission Ridge Ski Area kindly allowed instillation of our collectors during the winter 1989. Terry Lee, Dan Stauffer, and Steve Wagner provided analytical services for the study. Registry No. H, 12408-02-5; Ca, 7440-70-2; Mg, 7439-95-4; K, 7440-09-7; NH4, 14798-03-9; Na, 7440-23-5.
Literature Cited Rogers D. C.; Baumgardner D.; Vali, G. J. Appl. Meteorol. 1983, 22, 153. Jacob, D. J.; Waldman, J. M.; Munger, J. W.; Hoffman, M. R. Enuiron. Sei. Technol. 1985, 19, 730. Barrie, L. A,; Schemenauer, R. S. Water,Air, Soil Pollut. 1986, 30, 91. Weathers, K. C.; Likens, G. E.; Bormann F. H.; Bicknell, S. H.; Bormann, B. T.; Daube, B. C., Jr.; Eaton, J. S.; Galloway, J. N.; Keene, W. C.; Kimball, K. D.; McDowell, W. H.; Siccama, T. G.; Smiley, D.; Tarrant, R. A. Enuiron. Sci. Technol. 1988, 22, 1018. Logan, R. M.; Derby, J. C.; Duncan L. C. Enuiron. Sei. Technol. 1982, 16, 771. Laird, L. B.; Taylor, H. E.; Kennedy, V. C. Enuiron. Sei. Technol. 1986, 20, 275.
(7) Duncan, L. C.; Welch, E. B.; Ausserer, W. Water,Air, Soil Pollut. 1986, 30, 217. (8) Berndt, H. W.; Fowler, W. B. J. For. 1969, 67, 92. (9) Hindmann, E. E.; Borys, R. D.; DeMott, P. J. Water Resour. Bull. 1983, 19, 619. (10) Berg, N. H. Arc. Alp. Res. 1988, 20, 429. (11) Berg, N. H.; Dunn P. H. Proceedings,International Symposium on High Elevation Watersheds; University of California Press: Berkeley, CA, in press. (12) Borys, R. D.; Hindmann, E. E.; Demott, P. J. J. Atmos. Chem. 1988, 7, 213. (13) Mitchell, D. L.; Lamb D. J. Geophys.Res. 1989,94,14831. (14) Chan, W. H.; Tang, A. J. S.; Chung, D. H. S.; Reid, N. W. Enuiron. Sei. Technol. 1988, 21, 1219. (15) Bormann, B. T.; Tarrant, R. F.; McClellan, M. H.; Savage, T. J. Environ. Qual. 1989, 18, 149. (16) Chan, W. H.; Chung, D. H. S. Atmos. Enuiron. 1986,20, 1397. (17) Topol, L. E. Atmos. Environ. 1986, 20, 347. (18) Hegg D. A. J. Geophys. Res. 1983,88, 1369. (19) Scott, B. C. J. Appl. Meteorol. 1981, 20, 619. (20) Basabe, F. A.; Chang, W., Larson, T. V.; Edmonds R. L.
Proceedings, The Effects of Air Pollution on Western Forests, Air Pollution Control Association Meeting June 1989, Anaheim, CA.
Received for review July I , 1991. Accepted July 10,1991. This work was partially supported by Central Washington University, the WashingtonState Department of Ecology, and the National Park Service.
Method Development for Monitoring Pesticides in Environmental Waters: Liquid-Solid Extraction Followed by Liquid Chromatography Antonio Di Corcia” and Marcello Marchettl Dipartimento di Chimica, Universitg “La Sapienza”, Piazza Aldo Mor0
rn Our recent method based upon liquid-solid extraction of pesticides from drinking water followed by high-performance liquid chromatography (HPLC) was modified to make it suitable for monitoring 89 pesticides in groundwater and river water. The modified method involved passing 2, 1.5, and 0.5 L, respectively, of drinking water, groundwater, and river water through a 300-mg Carbopack cartridge at 120-130 mL/min. The complete separation of base-neutral pesticides from acidic ones was accomplished by a stepwise elution. After concentration of the two extracts, pesticides were separated and quantified by reversed-phase HPLC with UV detection. The goal of analyzing base-neutral pesticides with a low probability of false positives was reached by using a C-18 HPLC column as the primary column and a cyano column as the confirmational column. Except for few pesticides, the limits of quantification (5 times the limit of detection) of this method for the pesticides considered in municipal water and groundwater samples were lower than 0.1 pg/L. Introduction
At our laboratory, we have been able to elaborate very sensitive high-performance liquid chromatographic (HPLC) methods for determining various classes of organic contaminants (1-7) in environmental aqueous samples. The liquid-solid extraction (LSE) of these analytes from water has been performed with small cartridges filled with fine particles of Carbopack B, which is a well-known ex66
Environ. Sci. Technol., Vol. 26,No. 1, 1992
5, 00185 Rome,
Italy
ample of graphitized carbon black. Very recently, the mechanisms of adsorption from water onto the Carbopack B surface of 35 pesticides of various chemical nature and desorption from it by suitable organic solvent mixtures were described (8). The presence of few quinone groups contaminating the Carbopack surface, which can irreversibly adsorb particular compounds able to react with them, was demonstrated and the way of eliminating this unwelcome effect was also described. When the performance of the Carbopack cartridge was compared with that of a very commonly used, chemically bonded (C-18) silica cartridge, it appeared that the former sorbent was much more effective for trapping polar pesticides than the latter one. The higher permeability of the Carbopack B cartridge also enabled the extraction of pesticides from water to be done in a much shorter time than that with the C-18 cartridge. Due to the presence on the Carbopack surface of a positively charged, oxygen-containing chemical complex (9),we have been able to separate base-neutral pesticides from acidic ones by stepwise elution. This reduces the probability of false positives and gives additional evidence for the presence of a certain pesticide. In this paper, we report the verification of this method by extension to 89 pesticides of environmental interest having a broad range of polarity and volatility and to aqueous samples from various environmentalsources. The effectiveness of using two HPLC columns, one as the primary column (C-18) and the other as the confirmational
0013-936X/92/0926-0066$03.00/0
0 1991 American Chemical Society
column (cyano), for decreasing the probabilities of false positives in the analysis of base-neutral pesticides is also shown.
Experimental Section Materials. Authentic phenoxy acids were purchased from Eurobase (Milan, Italy). All other pesticides considered in this work were from Riedel-de-Haen, Selze, Germany. Individual standard solutions were prepared by dissolving 100 mg of each pesticide in 100 mL of methanol. By the chromatographic systems adopted, in no way it was possible to separate all the base-neutral pesticides considered by a single chromatographicrun. For recovery studies, we prepared two, distinct, composite working standard solutions by mixing 20-300 pL of each pesticide standard solution and diluting to 10 mL with methanol. A working standard solution containing only hexazinone was also prepared, as this compound produced a peak overlapping those for either aldicarb or metoxuron, when eluted on the C-18 column. Recovery studies of hexazinone were performed by adding it to water samples and chromatographingthe final extract isocratically on the C-18 column. Another composite working standard solution was prepared by mixing 100 pL of each acidic pesticide considered, except for bromoxynil, ioxynil, and warfarin, whose volumes were 50, 50, and 200 pL, respectively, and diluting to 10 mL with methanol. For HPLC, distilled water was further purified by passing it through a Norganic cartridge (Millipore, Bedford, MA). Methanol of HPLC grade was from Baker (Deventer, Holland), while acetonitrile of gradient grade was from Riedel-de-Haen. Trifluoroacetic acid (TFA) was supplied by Fluka AG (Buchs, Switzerland),and ascorbic acid was from Carlo Erba (Milan, Italy). All other solvents were of reagent grade and were used as supplied. Carbopack B (120-400-mesh size) as well as the other materials used for preparing LSE cartridges was from Supelco Inc. (Bellefonte, PA). The preparation and the pretreatment of the 300-mg Carbopack cartridges were carried out as previously reported (8). The trap was fitted into a side-arm filtering flask, and liquids were forced to pass through the cartridge by vacuum from a water pump. Water was collected in empty solvent bottles and stored at 4 "C until used. Groundwater samples having a dissolved organic carbon (DOC) concentration between 0.9 and 1.6 mg/L were collected from various sources near Rome. River water samples (3.6-8.2 mg/L DOC) were sampled from various rivers flowing between Florence and Rome. Unless they contained large amounts of suspended sediments, river water samples were extracted unfiltered. When necessary, Whatman GF/C glass-fiber pads (pore size 10 pm) were used. Procedure. Occasionally, when extracting groundwater and river water samples spiked with very low amounts of the pesticides considered, we obtained unexpected low recoveries of those pesticides that are more prone to oxidative degradation, in particular of 4,6-dinitro-o-cresol. Likely, this was due to the presence in certain water samples of some unknown oxidative species. This misleading effect was eliminated by adding 0.2 g/L sodium sulfite to the water samples. Sodium sulfite was also added to spiked samples of municipal water containing hypochlorite. Thereafter, 2, 1.5, and 0.5 L, respectively, of drinking water, groundwater, and river water samples were spiked with known volumes of the composite working standard solution of acidic pesticides and, alternatively, with one of the two base-neutral composite working standard so-
lutions of base-neutral pesticides. The extraction of the analytes from aqueous samples was accomplished as previously described (8). Water remaining in the cartridge was partially removed by drawing room air through the cartridge by vacuum for 1min. The water content was further decreased by slowly passing 0.7 mL of water/methanol(50:50, v/v) through the cartridge. Again, the trap was air-dried for 1min. Then the water pump was disconnected, a round-bottom glass vial with an inside diameter of 1.4 cm was located below the cartridge, and the base-neutral pesticides were eluted by passing through the trap, at flow rates of about 5-6 mL/min, 1mL of methanol followed by 2 X 3 mL aliquots of methylene chloride/methanol(9010, v/v). When glass vials narrower than those described above were used, a persistent double layer, the lower one being constituted mainly of methylene chloride, was occasionally formed during the solvent blowdown step. In this case, it may happen that the final extract still contains some methylene chloride, which interferes with the subsequent separation and quantification by HPLC. Acidic pesticides were collected into a second glass vial by drawing through the trap 2 X 4 mL aliquots of methylene chloride/methanol (80:20, v/v) acidified with TFA (0.2%, v/v). When not being used, this solution was stored at 4 "C to decrease the rate of esterification of TFA by methanol. Additionally, this solution was freshly prepared every 2 days. The last drops of this eluant system were forced out by an additional decrease of the pressure into the flask. Before solvent removal, the acidic fraction was partially neutralized by addition of 50 p L of a water/ methanol solution of ammonia, obtained by taking 2 mL of concentrated ammonia and diluting to 10 mL with methanol. When the acidic extract was dried without the addition of ammonia, severe losses of those pesticides that are more readily esterified by methanol in an acidic ambient, such as coumafuryl and warfarin, were noted. The base-neutral and the acidic fractions were then placed in a water bath at 30 OC under a nitrogen stream for solvent removal. The base-neutral extract was blown down to 240-260 pL, while the acidic extract was dried and the residue reconstituted with 300 pL of water/methanol (60:40, v/v) acidified with TFA (0.05%, v/v). After the exact final extract volume of the base-neutral fraction was measured, 40 pL of it was injected into the HPLC apparatus, while 50 pL of the acid extract was chromatographed. A 20-pL aliquot of the base-neutral extract was reanalyzed on the confirmational cyano column. Higher volumes of the extract injected on the cyano column provoked broadening of the early-eluting peaks. HPLC Analysis. Liquid chromatography was carried out with a Varian (Walnut Creek, CA) Model 5000 chromatograph equipped with a Rheodyne Model 7125 injector having a 50-pL loop and with a Model 2550 UV detector (Varian); 25 cm X 4.5 mm i.d. columns filled with 5 pm of siliceous materials were used. For the base-neutral fraction, the primary column contained a LC-18 DB packing and the confirmational column contained a LC-CN (cyano) packing. The acidic fraction was chromatographed on a LC-18 column. All these columns were from Supelco. For separating the base-neutral pesticides on the LC-18 DB, the initial mobile-phase composition was 80% water, containing 1 mmol/L phosphate buffer (pH 6.71, and 20% acetonitrile. This was programmed linearly to 85% acetonitrile after 45 min. For the LC-CN column the initial mobile-phase composition was 88% buffered water and 12% acetonitrile, held for 7 min, and then programmed linearly to 57 % acetonitrile after 50 min. The N
Environ. Sci. Technol., Vol. 26,
No. 1, 1992 67
Table I. Liquid Chromatography Retention Times and Recovery of Pesticides Added to 2 L of a T a p Water Sample (Spike Level 0.1-1.5 pg/L)
pesticide 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
oxamyl methomyl mevinphos IC chloridazon dimethoate mevinphos I1 hexazinone aldicarb metoxuron simazine bromacil monuron cyanazine metribuzin dichlorvos propoxur carbofuran pirimicarb atrazine chlortoluron fluometuron carbaryl diazinon isoproturon ethiofencarb monolinuron diuron difenoxuron metobromuron paraoxon propachlor propham propazine
class" carbamate (I) carbamate (I) organophosphate (I) pyridazinone (H) phosphorodithioate (A) organophosphate (I) triazinedione (H) carbamate (I) phenylurea (H) triazine (H) uracil (H) phenylurea (H) triazine (H) triazine (H) organophosphate (I) carbamate (I) carbamate (I) carbamate (I) triazine (H) phenylurea (H) phenylurea (H) carbamate (I) phosphrothioate (I) phenylurea (H) carbamate (I) phenylurea (H) phenylurea (H) phenylurea (H) phenylurea (H) organophosphate (I) acetanilide (H) carbamate (I) triazine (H)
thiadiazinone (H) 2 dicamba methoxybenzoic (H) 3 bromoxynil phenol (H) 4 dinitro-o-cresol phenol (H) coumarin (R) 5 coumafuryl 6 2,4-D phenoxy acid (H) 7 ioxynil phenol (H) phenoxy acid (H) 8 MCPA phenoxy acid (H) 9 dichlorprop 1 bentazone
retn time, min C-18 % DB CN recb 3.2 3.8 6.1 6.6 7.6 8.6 11.0 11.1 11.1 11.9 11.9 12.7 13.5 13.6 13.7 15.2 15.9 15.9 16.6 16.6 17.1 17.4 17.8 18.0 18.0 18.3 18.4 19.2 19.4 19.4 19.8 20.4 20.9
8.1 8.5 9.5 9.9 11.8 12.3 12.7 12.9 16.8
-
pesticide
Base-Neuti * a l S 34 propanil 3.2 93 35 terbutylazine 3.6 98 4.4 97 36 dichlobenil 6.2 92 37 linuron 6.8 96 38 azinphos-methyl 5.6 97 39 chloroxuron 9.4 101 40 fenamiphos 7.2 83 41 chlorbromuron 11.2 97 42 molinate 7.8 95 43 propyzamide 9.4 94 44 parathion-methyl 11.6 100 45 chlorpropham 12.5 98 46 metolachlor 8.1 97 47 procymidone 9.1 86 48 malathion 9.4 98 49 fenitrothion 10.9 99 50 rotenone 9.4 95 51 azinphos-ethyl 11.6 95 52 neburon 17.4 101 53 eptam 17.4 98 54 fenthion 17.8 97 55 parathion-ethyl 7.8 94 56 sulfallate 17.8 98 57 coumaphos 14.9 93 58 cycloate 17.4 92 59 phorate 22.2 99 60 phoxim 27.0 97 61 disulfoton 19.8 98 62 pirimiphos-methyl 21.5 96 63 butylate 16.1 94 64 methoxychlor 16.7 93 65 chlorpyriphos 16.7 94 66 pendimethalin 67 pirimiphos-ethyl 68 trifluralin 69 bromophos-ethyl 70 DDT 71 fenvalerate 99 95 100 96 98 93 99 92 99
Acids 10 11 12 13 14 15 16 17 18
mecoprop warfarin 2,4,5-T 2,4-DB MCPB 2,4,5-TP dinoseb dinot erb pentachlorophenol
class"
retn time, min C-18 % DB CN recb
propionalide (H) triazine (H) benzonitrile (H) phenylurea (H) phosphorodithioate (I) phenylurea (H) phosphoroamidate (N) phenylurea (H) thiocarbamate (H) benzamide (H) phosphorotioate (I) carbamate (H) acetanilide (H) dicarboximide (F) phosphorodithioate (I) phosphorothioate (I) benzopyranone (A) phosphorothioate (I) phenylurea (H) thiocarbamate (H) phosphorothioate (I) phosphorothioate (I) dithiocarbamate (H) phosphorothioate (I) thiocarbamate (H) phosphorodithioate (I) phosphorothioate (I) phosphorodithioate (I) carbamate (I) thiocarbamate (H) organochlorine (I) phosphorothioate (I) dinitroaniline (H) carbamate (I) dinitroaniline (H) phosphorothioate (I) organochlorine (I) pyrethroid (I)
21.8 22.2 22.6 23.5 24.0 24.0 24.3 24.5 24.6 25.8 26.5 26.5 27.8 28.4 28.4 29.0 29.0 29.4 29.4 29.6 32.7 32.7 32.9 33.9 34.1 34.5 34.8 35.2 35.2 36.6 36.9 39.8 39.8 40.2 40.2 44.3 44.8 45.6
phenoxy acid (H) coumarin (R) phenoxy acid (H) phenoxy acid (H) phenoxy acid (H) phenoxy acid (H) phenol (H) phenol (H) phenol (H)
17.2 17.6 18.1 20.1 20.4 22.7 25.0 26.1 33.9
25.8 22.0 21.5 28.3 31.8 34.5 31.7 30.4 17.8 27.1 36.1 30.0 27.8 34.6 35.4 39.8 38.2 38.7 37.3 23.2 42.5 43.2 38.2 44.8 31.3 41.2 46.0 42.2 39.8 34.6 49.1 48.0 47.6 44.8 51.6 51.2 54.6 57.2
99 95 84 96 99 96 98 97 91 99 96 97 100 99 95 98 99 101 99 82 96 97 99 92 89 92 96 92 100 82 99 93 99 98 93 100
99 102 100 100 94 99 101 101 102 102 88
I, insecticide; H, herbicide; A, acaricide; N, nematicide; F, fungicide; R, rodenticide. Mean values obtained from three determinations. 'As supplied, this pesticide standard was a mixture of two isomers.
acidic fraction was chromatographed with a premixed methanol/acetonitrile (82:18, v/v) solution as organic modifier and water acidified with TFA (0.17%, v/v). The initial mobile-phase composition was 50% organic modifier and 50% acidified water, which was linearly increased to 78% organic modifier after 34 min. In all cases, the flow rates of the mobile phases were 1.5 mL/min. The base-neutral fraction was monitored with the detector set at 220 nm, while the acidic fraction was monitored at 230 nm. The concentrations of the pesticides in water were calculated by measuring the peak height of each pesticide and comparing them with those obtained from standard solutions. These were prepared by taking known and appropriate volumes of the working standard solutions, evaporating the methanol, and reconstituting the residue 68
Environ. Sci. Technol., Vol. 26,No. 1, 1992
with 250 p L of water/methanol (50:50, v/v) for baseneutral and 300 pL of water/methanol (60:40, v/v) acidified with 0.05% (v/v) TFA for acids.
Results and Discussion The pesticides considered in this study were selected according to the criteria of persistence and mobility in the aquatic environment,of extensive usage in agriculture, and, obviously, of being UV-absorbing species. Table I lists the pesticides selected together with their chromatographic retention times and recovery data obtained by adding appropriate volumes of the working standard solutions to 2-L aliquots of a municipal water sample and analyzing. Before the pesticides were added, hypochlorite contained in the water sample was reduced by adding 0.2 g/L Na2SOB.
19.20
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,
54,55
69 71
65.66
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32
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70
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8
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20
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36
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48
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Flgure 1. HPLC chromatograms obtained by injecting simultaneously the two working composite standard solutions of base-neutral pesticides and that containing hexazinone. Chromatographic conditions are reported in the Experimental Section. Amount of each pesticide injected 10-150 ng. (A) C-18 DB column; attenuation 0.04 AUFS. (B) Cyano column; attenuation 0.02 AUFS. Peak numbering corresponds to that reported in Table I .
Figures 1 and 2 show chromatograms obtained by injecting aliquots of the composite working standard solutions of pesticides.
With respect to our previously described method (8), the analytical procedure was modified to adapt it to the determination of those pesticides not considered in the Environ. Sci. Technol., Vol. 26, No. 1, 1992 69
Table 111. Recovery of Selected Water-Soluble Pesticides with Increasing River Water Volumes (Spike Level 0.5 d L )
18
I
% recovery*
compound"
I
0
I
I
4
8
I
12
1
16
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20
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Figure 2. HPLC chromatogram obtained by injecting the composite working standard solution of the acidic pesticides. Chromatographic condtions are reported in the Experimental Section. Amount of each pesticide injected 43-172 ng. Attenuation 0.02 AUFS. Peak numbering corresponds to that reported in Table I .
Table 11. Recovery of Some Selected Pesticides after Partial Solvent Removal % recovery
temp, "C
unwashed trap
washed trap 30
30
30
34
30
30
final vol, p L
350
250
200
250
350
250
dichlobenil (5.5) molinate (55) malathion (1.2) eptam (250) cycloate (60) phorate (8.3) disulfoton (1.8) butylate (130) trifluralin (1.3) bromophos-ethyl (0.45) DDT (0.0013) fenvalerate (0.0019)
99 98 100 91 95 96 97 92 99 100 100 100
83 90 97 82 89 91 92 83 91 97 99 101
77 84 85 78 84 86 86 77 82 85 85 88
74 81 84 75 87 89 89 71 81 94 93 92
85 91 93 80 91 87 87 67 82 83 76 76
66 86 93 63 52 80 81 36 75 71 71 75
Mean values calculated from three determinations. Values in parentheses are reported vapor pressures (X mmHg) taken from various sources.
previous work and, chiefly, to make it suitable for the monitoring of pesticides in more complex matrices, such as well and river waters. The most significant modifications made are reported below and discussed. In order to attain a high enrichment factor and contain evaporative losses of the most volatile pesticides, the most suitable experimental conditions for concentrating the extract were studied. In Table I1 are reported the results relative to only those pesticides prone to evaporative losses when the experimental conditions for concentrating the extract were varied. When, according to the previously the aqueous methanol wash step (see reported method (8), the Experimental Section) was eliminated, losses of even those pesticides having a low volatility but a high hydrophobicity occurred. Likely, this effect is due to an anom70
Environ. Sci. Technol., Vol. 26, No. 1, 1992
Aniene
Arno
DOC,mg/L
3.6
3.6
6.6
6.6
8.2
8.2
volume, L
0.5
1.0
0.5
1.0
0.5
1.0
oxamyl (280) methomyl (58) mevinphos ( V S ) ~ dimethoate (25) hexazinone (33) aldicarb (6) metribuzin (1.2) propoxur (2) dichlorvos (10) pirimicarb (2.7) dicamba (7)
92 96 102 94 96 87 98 96 89 94 84
25 49 99 52 97 39 86 97 56 94 54
93 95 98 93 97 89 94 98 92 93 75
35 51 98 58 97 48 92 97 60 96 36
91 97 96 99 98 93 97 95 90 95 71
32 46 97 48 96 49 93 94 53 94 20
36
TIME( m i " . )
compoundb
~~
"Values in parentheses are reported data of solubilities in water, expressed as grams per liter, taken from various sources. bMean values obtained from three determinations. vs, very soluble.
I
28
Tevere
alously large escaping tendency of highly hydrophobic compounds when dissolved in a watery moiety. A water bath temperature of 30 "C and a final volume of the extract not lower than 250 pL was found the best compromise in terms of sensitivity and accuracy of the analysis. No loss of the acidic pesticides was noted on drying the extract, provided the extract was partially neutralized by ammonia (see the Experimental Section) and the vial was taken away from the water bath immediately after the last drop of the solvent mixture was evaporated. Preliminary extraction experiments performed on eight different 2-L well water samples spiked with the pesticides considered for the previously proposed 250-mg Carbopack cartridge (8) showed that those pesticides having the highest mobilities on the Carbopack column were partially found in the water effluent. In particular, aldicarb and oxamyl gave unacceptable mean recoveries of 50% (range 44-72%) and 63% (56-75%), respectively. If compared to recoveries obtained when they were extracted from a municipal water sample, losses of these two pesticides could be traced to the presence in the groundwater samples of unknown organic contaminants, which partially saturated the Carbopack cartridge. For this reason, the weight of Carbopack in the cartridge was increased to 300 mg and the groundwater sample volumes submitted to extraction were not larger than 1.5 L. The profound influence that the matrix effect can have on the accuracy of an analytical method involving LSE extraction by small cartridges was also assessed by spiking three river water samples, having different DOC contents, with pesticides that, among those considered, have the highest solubilities in water and analyzing 0.5- and 1-L aliquots of each water sample. Table I11 shows that large losses of some of the added pesticides occurred when the volume of water extracted was doubled. The extent of the loss for base-neutral pesticides was independent of the DOC level of the river water samples. This can be explained by considering that fulvic acids, which represent up to 80% of the DOC content in surface waters (IO),are adsorbed on the anion-exchange sites of the Carbopack surface so that they cannot compete with nonacidic pesticides for adsorption on the unspecific sites of the sorbent. According to the former procedure (8),acidic pesticides were displaced from the anion-exchange sites of the Carbopack surface by a KOH-basified organic solvent mixture. However, when analyzing river water samples spiked with
I 11
A
Flgure 3. Chromatograms obtained from (A) 1.5 L of a groundwater sample spiked with the acidic Pesticides at 80 ng/L (except for warfarin (160 ng/L), bromoxynil (40 ng/L), and ioxynil (40 ng/L); attenuation 0.01 AUFS) and (B) 0.5 L of a river water sample (Tevere, October 1990) spiked with the acidic pesticides at 480 ng/L (except for warfarin (960 ng/L), bromoxynil(240 ng/L), and ioxynil(240 ng/L); attenuation 0.02 AUFS). Peak numbering corresponds to that of the pesticides in Table I.
the acidic pesticides, we observed that peaks for dinoterb and pentachlorophenol were often obscured by other ones produced by compounds originally present in the aqueous samples and coextracted with acidic pesticides. These compounds were identified as linear alkylbenzenesulfonates (LAS), which are widely used surfactants commonly present in the aquatic environment at the microgram per liter level. Recently, during development of a selective method for monitoring LAS in aqueous samples ( l l ) ,an acidified methylene chloride/methanol mixture was found to be completely ineffective for eluting LAS from a Carbopack cartridge. Taking advantage of this, LAS interference with the analysis of acidic pesticides was eliminated by selectively eluting the pesticides with a methylene chloride/methanol mixture (80:20, v/v) acidified with TFA (0.20%, v/v). Parts A and B of Figure 3 show typical chromatograms obtained by analyzing, respectively, 1.5 L of a groundwater sample and 0.5 L of a river (Tevere) water sample spiked with acidic pesticides. The initial large front was presumably caused by fulvic acids coextracted with the analytes of interest. When a large number of pesticides, having a broad spectrum of chemical characteristics, are being monitored, a lot of unknown compounds present in a complex aqueous matrix are carried through the analytical procedure and some of these may cause positive biases. For neutral pesticides, no effective cleanup procedure can be adopted without avoiding severe losses of some of the analytes of interest. Compared to the gas chromatographic technique, a serious shortcoming in the HPLC technique is the absence of a reliable and sufficiently sensitive highly specific detector, such as mass spectrometry.
Recently, it was reported (12) that the US. EPA laboratories have developed an HPLC method for determining several nonvolatile pesticides that involves reanalyses of the sample extracts on a second HPLC column to confirm the presence of an analyte identified by using the primary column. Analyses performed by this method of more than 1000 water samples from both rural domestic wells and community systems showed that none of the pesticides detected with the primary column were confirmed by means of the second column. This means that HPLC methods involving the use of only one HPLC column for identification of a large number of pesticides have high probabilities of false positives. In our laboratory, in a joint project between the University of Rome and a public agency (ENEA) promoted for conducting a survey of pesticide contamination of water wells situated in a region near Rome, we have used for 8 months the method here described that involves the use of two HPLC columns. We have found that in only 11% of the cases was the presence of a base-neutral pesticide successfully confirmed upon second analysis. As an example and to show the potential of this method in terms of selectivity and sensitivity, a selected well water sample was divided in two aliquots: one was analyzed as such and the other was analyzed after being spiked with 20 selected pesticides at individual levels ranging between 25 and 100 ng/L. The pesticides were chosen by the criterion of adding those that were suspected of being present in the original water sample after interpretation of the chromatogram (Figure 4A) obtained by injecting the unspiked well water extract on the primary column. Figure 4 shows chromatograms obtained for the two extracts by chromatographing them on both the primary and confirmational columns. It has to be pointed out that, when the HPLC Environ. Sci. Technol., Vol. 26, No. 1, 1992
71
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Flgure 4. Chromatograms obtained from 1.5 L of a well water sample. Extract of the unsplked sample chromatographed on the (A) C-18 DB column and (C) the cyano column. Extract of the spiked sample chromatographed on the (B) C-18 DB column and (D) cyano column. The spike level was 100 ng/L of each pesticide (except for chloridazon (peak 4; 50 ng/L) and carbaryl (peak 22; 25 ng/L)). The extract volume injected onto the cyano column was 20 pL; attenuation 0.005 AUFS. The extract volume injected on the '2-18 DB column was 40 pcLL; attenuation 0.01 AUFS. Peak numbering corresponds to that reported in Table I.
column is operated in the solvent gradient mode with a large variation of the initial mobile-phase composition, relative differences in retention times for the same compound of -1.5% obtained by two successive chromatographic runs is satisfactory. Based upon differences in retention times, a comparison between chromatograms obtained after injecting on the primary column the extracts of both the unspiked and spiked water samples excluded the presence in the unspiked sample of monuron, linuron, propyzamide,fenthion, parathion-ethyl, and phoxim. The presence of 10 other pesticides tentatively identified by the primary column was not confirmed by the secondary column. Of the remaining four pesticides, the presence 72
Environ. Sci. Technol., Vol. 26,
No. 1, 1992
of 1.9 ng/L chloridazon, 14 ng/L chlortoluron, and 8.8 ng/L pendimethalin in the well water sample tentatively identified and quantified by the primary column was not substantiated, even from a quantitative point of view, by the secondary column, as peaks supposed to be produced by them were obscured by others for unknown compounds. Only the presence of 5.9 ng/L carbaryl was both qualitatively and quantitatively confirmed with the cyano column. The spike recoveries of the overall isolation and chromatographic procedures are listed in Table IV. For the sake of conciseness, we reported accuracy data relative only to those pesticides (see above) whose recoveries are dependent upon the particular aqueous matrix in which they
I
1
Table IV. Accuracy of This Method for t h e Determination of Selected Pesticides in Aqueous Environmental Matrices (Spike Level 1 pg/L for River Waters and 0.33 pg/L for Groundwaters)
compound
no. of samples
oxamyl methomyl dimethoate aldicarb dichlorvos bentazone dicamba
16 16 16 16 16 16 16
groundwater av RSD,” recovery, % % 89 98 96 81 92 98 93
6.8 5.3 2.1 7.5 5.9 2.6 3.8
%
no. of samples
79-95 90-101 93-99 77-90 84-96 94-102 89-95
8 8 8 8 8 8 8
range,
river water av RSD,” recovery, % % 92 97 96 90 92 92 78
4.2 3.0 3.0 2.6 2.7 3.4 8.3
range, %
90-96 91-101 92-101 87-94 88-95 90-95 71-85
RSD, relative standard deviation. Table V. Limits of Detection (LOD), Recovery, and Within-Run Precision of This Method at Low and High Concentrations of Pesticides Added to a Groundwater Sample
LOD, concn, recovery f ng/L ng/L RSD,” % 2.1 100 92 f 5.7 oxamyl 5.4 100 98 f 4.4 methomyl 4.0 101 f 5.0 mevinphos I 20 chloridazon 93 f 6.3 1.8 50 dimethoate 20 95 f 3.9 300 mevinphos I1 4.0 95 f 3.8 80 4.7 hexazinone 100 103 f 5.6 6.2 96 f 4.0 100 metoxuron 16 300 83 f 5.9 aldicarb 30 97 f 2.8 1.1 simazine 8.5 102 f 3.5 100 bromacil 101 f 3.6 10 200 monuron 1.8 98 f 2.9 40 cyanazine 97 f 4.1 5.5 100 metribuzin 47 dichlorvos 93 f 5.1 300 97 f 4.4 9.3 propoxur 150 12 carbofuran 102 f 3.6 200 15 95 f 5.3 200 pirimicarb 1.0 30 atrazine 96 f 2.9 chlortoluron 3.3 103 f 2.5 100 99 f 3.3 9.0 200 fluometuron 0.8 20 carbaryl 98 f 2.9 17 diazinon 94 f 3.6 150 102 f 2.4 6 isoproturon 100 ethiofencarb 200 96 f 3.9 8.3 9.3 monolinuron 150 98 f 3.3 101 f 2.9 diuron 100 3.8 100 104 f 2.7 5.6 difenoxuron 15 100 metobromuron 97 k 3.5 8.1 paraoxon 95 f 4.6 100 5.4 propachlor 93 f 5.3 100 propham 4.3 95 f 4.6 100 propazine 1.0 96 f 3.3 30 4.2 102 f 2.9 100 propanil 1.9 terbutylazine 98 2.9 50 4.1 dichlobenil 83 f 6.3 100 linuron 101 f 3.7 3.6 100 azinphos-methyl 4.0 50 98 f 3.9 chloroxuron 100 6.8 103 f 4.0 10 150 fenamiphos 96 f 5.0 chlorbromuron 7.0 100 99 f 3.5 molinate 9.5 100 90 f 4.6 propyzamide 6.0 100 96 f 3.7 parathion-methyl 16 100 94 f 3.9 chlorpropham 8.8 100 100 f 3.3
*
concn, recovery f ng/L RSD,” % 91 f 1.9 1000 1000 98 f 2.0 99 f 2.1 200 92 f 3.3 500 97 f 2.1 3000 97 f 1.1 800 99 f 2.6 1000 95 f 4.6 1000 79 f 2.3 3000 96 f 1.3 300 1000 98 f 1.6 2000 100 f 1.3 400 97 f 1.0 97 f 1.8 1000 92 f 2.0 3000 1500 98 f 2.1 2000 100 f 1.0 94 f 2.8 2000 97 f 1.2 300 99 f 1.3 1000 91 f 1.0 2000 99 f 1.1 200 93 f 2.3 1500 1000 100 f 1.5 2000 95 f 2.8 1500 98 f 2.0 1000 98 f 1.0 1000 101 f 1.0 1000 96 f 1.3 1000 94 f 2.9 1000 92 f 3.3 1000 96 f 3.0 300 98 f 1.3 1000 99 f 1.5 91 f 1.2 500 1000 85 f 5.5 96 f 1.0 1000 97 f 1.4 500 1000 98 f 1.3 1500 95 f 2.3 1000 98 f 1.3 1000 89 f 3.8 1000 94 f 2.8 1000 95 f 3.0 1000 98 f 1.4
LOD, concn, recovery f ng/L ng/L RSD,” % metolachlor 9.1 100 99 f 2.9 procymidone 101 f 3.6 8.0 100 malathion 98 f 4.0 45 300 fenitrothion 1.7 96 f 4.4 100 rotenone 99 f 2.9 8.5 100 azinphos-ethyl 50 3.8 97 f 3.5 3.0 neburon 99 f 3.6 100 9.3 eptam 100 81 f 5.4 fenthion 100 94 f 4.0 7.2 parathion-ethyl 97 f 3.8 16 100 sulfallate 9.8 100 97 f 2.9 12 coumaphos 92 f 4.7 100 8.0 cyc1oate 50 88 f 5.0 phorate 200 91 f 4.7 21 phoxim 12 100 95 f 3.7 disulfoton 27 91 f 4.6 200 pirimiphos-methyl 21 200 96 f 3.7 butylate 14 84 f 5.6 100 12 methoxychlor 98 f 2.9 100 chlorpyriphos 16 100 94 f 3.3 pendimethalin 10 100 104 f 2.8 21 pirimiphos-ethyl 200 95 f 3.4 trifluralin 11 100 90 f 4.4 bromophos-ethyl 5 100 96 f 3.0 DDT 10 100 100 f 2.5 fenvalerate I 150 102 f 2.6 bentazone 5.5 100 97 f 3.3 dicamba 6.6 100 92 f 4.9 bromoxynil 3.8 50 102 f 3.1 dinitro-o-cresol 9.0 93 f 5.0 100 coumafuryl 11 100 97 f 3.8 2,4-D 6.8 100 98 f 4.5 ioxynil 3.7 50 102 f 2.8 MCPA 6.6 100 103 f 2.6 dichlorprop 100 8.2 98 f 3.7 mecoprop 100 1.5 98 f 3.7 warfarin 19 200 96 f 4.2 2,4,5-T 9.1 100 95 f 5.1 2,4-DB 8.3 100 96 f 2.8 MCPB 6.9 100 97 f 2.7 2,4,5-TP 100 8.9 100 f 3.4 dinoseb 11 100 104 f 3.2 dinoterb 12 100 105 f 3.1 pentachlorophenol 9.0 100 90 f 7.6
concn, recovery f ng/L RSD,” % 1000 1000 3000 1000 1000 500 1000 1000 1000 1000 1000 1000 500 2000 1000 2000 2000 1000 1000 1000 1000 2000 1000 1000 1000 1500 1000 1000 500 1000 1000 1000 500 1000 1000 1000 2000 1000 1000 1000 1000 1000 1000 1000
99 f 1.1 98 f 1.0 95 f 2.2 94 f 2.5 100 f 1.5 98 f 1.4 98 f 1.6 83 f 5.3 93 f 1.4 95 f 1.2 97 f 1.4 91 f 3.1 89 f 4.4 92 f 3.9 96 f 2.2 91 f 3.8 97 f 2.0 84 f 4.5 97 f 1.9 96 f 1.8 99 f 1.1 96 f 2.0 92 f 3.0 98 f 1.3 96 f 1.0 99 f 1.0 98 f 1.3 89 f 5.0 100 f 1.0 92 f 3.6 98 f 2.4 98 f 1.9 101 f 1.2 99 f 1.3 97 f 1.5 98 f 2.0 94 f 2.3 94 f 3.3 97 f 1.6 96 f 1.5 98 f 1.7 103 f 2.2 103 f 2.5 94 f 3.6
Mean values and relative standard deviations calculated from six determinations.
are dissolved. Data refer to analyses performed on 0.5- and 1.5-L aliquots of 8 different river water and 16 different groundwater samples, respectively. The precision of the method was assessed by spiking aliquots of a pesticide-free groundwater sample with the pesticides considered at two different concentrations by using appropriate volumes of the three composite working standard solutions and the standard solution of hexazi-
none. Each aliquot was then analyzed six times. Extracts containing low concentrations of the base-neutral pesticides were fractionated on both the primary and confirmational columns. This was done in order to eliminate some overestimations caused by unknown organics contained in the groundwater sample and impurities released by the plastic materials of the cartridge. These impurities affected correct quantification of malathion and methEnviron. Scl. Technol., Vol. 26, No. 1, 1992
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Environ. Sci. Technol. 1992, 26, 74-79
oxychlor on elution from the primary column. Results are presented on Table V together with the limit of detection (LOD) for each pesticide present in a 1.5-L groundwater sample. When gradient elution reversed-phase HPLC is operated in the low-wavelength region (220-230 nm), more than the amplitude of the electrical noise is the slope of the base-line drift, which depends on the content of UVabsorbing impurities present in the mobile phase, which defines the LOD of the analysis. Under the chromatographic conditions selected, the LODs of this method were estimated by assuming arbitrarily that 0.5 cm was the minimum peak height that could be used with reasonable confidence. The LODs for base-neutral pesticides were estimated from peaks produced by them on the cyano column. LODs reported in the table refer to extraction of 1.5 L of a groundwater sample. Thus, the LODs for river waters have to be increased by a factor 3 and for drinking water decreased by a factor 0.75, as this method involves extraction of 0.5 and 2 L of river and drinking waters, respectively. Registry No. DDT, 50-29-3; 2,4-D, 94-75-7; MCPA, 94-74-6; 2,4,5-T, 93-76-5; 2,4-DB, 94-82-6; MCPB, 94-81-5; 2,4,5-TP, 9372-1; H20, 7732-18-5;oxamyl, 23135-22-0;methomyl, 16752-77-5; mevinphos I, 7786-34-7; chloridazon, 1698-60-8;dimethoate, 6051-5; hexazinone, 51235-04-2; aldicarb, 116-06-3; metoxuron, 19937-59-8; simazine, 122-34-9; bromacil, 314-40-9; monuron, 150-68-5; cyanazine, 21725-46-2; metribuzin, 21087-64-9; dichlorvos, 62-73-7; propoxur, 114-26-1; carbofuran, 1563-66-2; pirimicarb, 23103-98-2; atrazine, 1912-24-9;chlortoluron, 15545-48-9;fluometuron, 2164-17-2; carbaryl, 63-25-2; diazinon, 333-41-5; isoproturon, 34123-59-6; ethiofencarb, 29973-13-5; monolinuron, 174681-2; diuron, 330-54-1; difenoxuron, 14214-32-5;metobromuron, 3060-89-7; paraoxon, 311-45-5; propachlor, 1918-16-7;propham, 122-42-9;propazine, 139-40-2; propanil, 709-98-8; terbutylazine, 5915-41-3; dichlobenil, 1194-65-6; linuron, 330-55-2; azinphosmethyl, 86-50-0; chloroxuron, 1982-47-4;fenamiphos, 22224-92-6; chlorbromuron, 13360-45-7; molinate, 2212-67-1; propyzamide, 23950-58-5;parathion-methyl, 298-00-0; chlorpropham, 101-21-3; metolachlor, 51218-45-2; procymidone, 32809-16-8; malathion,
121-75-5;fenitrothion, 122-14-5;rotenone, 83-79-4;azinphos-ethyl, 2642-71-9; neburon, 555-37-3; eptam, 759-94-4; fenthion, 55-38-9; parathion-ethyl, 56-38-2;sulfallate, 95-06-7; coumaphos, 56-72-4; cycloate, 1134-23-2; phorate, 298-02-2; phoxim, 14816-18-3; disulfoton, 298-04-4; pirimiphos-methyl, 29232-93-7; butylate, 2008-41-5; methoxychlor, 72-43-5; chlorpyriphos, 2921-88-2; pendimethdin, 40487-42-1;pirimiphos-ethyl, 23505-41-1; trifluralin, 1582-09-8; bromophos-ethyl, 4824-78-6; fenvalerate, 51630-58-1; bentazone, 25057-89-0; dicamba, 1918-00-9; bromoxymil, 1689-84-5;dinitro-o-cresol,534-52-1; coumafwyl, 117-52-2; ioxynil, 1689-83-4;dichloroprop, 120-36-5;mecoprop, 7085-19-0; warfarin, 81-81-2; dinoseb, 88-85-7; dinoterb, 1420-07-1; pentachlorophenol, 87-86-5.
Literature Cited (1) Di Corcia, A.; Carfagnini, G.; Marchetti, M. Ann. Chim. 1987, 77,825-835. (2) Di Corcia, A.; Marchetti, M.; Samperi, R. J. Chromatogr. 1987, 405, 357-363. (3) Battista, M.; Di Corcia, A.; Marchetti, M. Anal. Chem. 1989, 61, 935-939. (4) Di Corcia, A,; Marchetti, M.; Samperi, R. Anal. Chem. 1989, 61. 1363-1367. ( 5 ) D i Corcia, A.; Marchetti, M. J. Chromatogr. 1991, 541, 365-373. (6) Di Corcia, A.; Samperi, R. Anal. Chem. 1990,62,1490-1494. (7) Borra, C.; Di Corcia, A.; Marchetti, M.; Samperi, R. Anal. Chem. 1986,58, 2048-2052. (8) Di Corcia, A.; Marchetti, M. Anal. Chem. 1991,63,580-585. (9) Andreolini, F.; Borra, C.; Caccamo, F.; Di Corcia, A,; Samperi, R. Anal. Chem. 1987,59, 1720-1725. (10) Saleh, F. H.; Ong, W. A.; Chang, D. Y. Anal. Chem. 1989, 61, 2792-2800. (11) Di Corcia, A.; Marchetti, M.; Marcomini, A.; Samperi, R. Anal. Chem. 1991,63, 1179-1182. (12) Munch, D. J.; Graves, R. L.; Maxey, R. A.; Engel, T. M. Environ. Sci. Technol. 1990, 24, 1446-1451.
Received for review April 29,1991. Revised manuscript received June 26, 1991. Accepted July 1, 1991.
Synthesis of Peroxyacetyl Nitrate in Air by Acetone Photolysis Peter Warneck" and Thomas Zerbach Max-Planck-Institut fur Chemie (Otto-Hahn-Institut), D 6500 Mainz, Germany
The photodecomposition of acetone in air with NOz admixed has been used to generate peroxyacetyl nitrate (PAN) either by batch synthesis in an isolated flask or in a continuous-flow reactor. With NOz mixing ratios near 10 ppm, the method converts in the first case 88% of NOz to PAN and -6% to methyl nitrate within a time period of a few minutes. In the second case, the conversion is -90% and lo%, respectively. The reproducibility is better than 5%. The PAN mixing ratio produced by batch synthesis declines exponentially with time due to wallcatalyzed thermal decomposition. Introduction
Peroxyacetyl nitrate (PAN) is an important nitrogen compound in the atmosphere. It originates from the addition of oxygen and nitrogen dioxide to acetyl radicals, which are generated in precursor reactions involving carbonyl compounds such as acetaldehyde and acetone ( I ) . As a product of photochemical air pollution, PAN has been suggested to serve as an indicator for photochemical smog ( 2 , 3 ) . Under these conditions it is one of the compounds 74
Environ. Sci. Technol., Vol. 26, No. 1, 1992
causing eye irritation and plant damage ( 4 , 5 ) . As a natural constituent of the background troposphere, PAN provides a reservoir of loosely bound NOz, because NO, is released when PAN undergoes thermal decomposition at favorably high temperatures (6). In addition, thermal decomposition is a source of acetylperoxy radicals, which react further to produce methylperoxy, hydroperoxy, and ultimately hydroxyl radicals by reactions with NO and oxygen. Singh ( l ) in , a recent review of reactive nitrogen, has furnished a summary of the occurrence and the effects of PAN in the troposphere. The currently preferred method for the determination of PAN in air is gas chromatography in combination with the electron capture detector (ECD). Its sensitivity is excellent but subject to some variability, so that frequent calibrations are needed. In view of the thermal instability of PAN, some effort is required to make available, for the purpose of calibration, a PAN/air mixture with known mixing ratio. A widely used method is to synthesize PAN in an inert solvent such as octane, stabilize it by refrigeration, and then prepare from it PAN/air mixtures by on-site dilution with air (7-10). Alternatively, PAN may
0013-936X/92/0926-0074$03.00/0
0 1991 American Chemical Society
