Pesticide Stability Studies upon Storage in a Graphitized Carbon Black-Extraction Cartridge CARLO CRESCENZI, A N T O N I O D I CORCIA," MAGDY DIAB MADBOULY,+ AND ROBERTO SAMPERI Dipartimento d i Chimica, Uniuersita "La Sapienza" di Roma, Piazzale Aldo Mor0 5, 00185 Roma, Italy
The stability of 34 selected pesticides extracted from water onto the graphitized carbon black (GCB) surface was evaluated under various storage conditions. The best results were obtained by first minimizing the water content into the GCB extraction cartridge by a suitable methanol washing and then freezing the cartridge. Under these conditions and over a storage period of 3 weeks, the stability of pesticides extracted from four river water samples onto the GCB surface was assessed and compared with that in water a t 4 "C with and without an inhibitor of biological degradation, such as HgC12. Results indicated that storage on the GCB material was a far better preservation procedure than storage in water a t 4 "C. Several of the pesticides considered were completely degraded when stored in water in the presence of HgC12. Extensive environmental surveys require the analysis of a large number of samples. Aqueous environmental samples are usually collected in glass bottles and shipped to a laboratory where the rest of the analytical procedure is carried out. In order to avoid possible chemical and biochemical analyte alterations, field samples should be analyzed immediatelyafter collection. Since it is impossible to do this for many environmental laboratories, serious problems of sample stability arise. Therefore, when developing a method for determining target compounds in environmental waters, a sample preservation study should be regarded as an essential component of the whole analytical procedure. In this vein, stability studies of pesticides spiked into well water samples were conducted by the US.Environmental Protection Agency (EPA) (I).Of the 147pesticides considered,26 were completelylost upon storing at 4 "C for 14 days in amber glass bottles well water samples that had been biologically inhibited with HgC12. Vice versa, all but two pesticides remained unaltered in *To whom correspondence should be addressed. Present address: The National Center for Social and Criminal Research, Awkaf City, Cairo, Egypt. +
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D 1995 American Chemical Society
stored sample extracts. The method of isolating target compounds from water by liquid-liquid extraction (LLE) and storing the sample extracts until chromatographic analysis can solve problems of analyte instability and save storage space. However, the problem of extracting a large number of samples in a relatively short time by a laborious and time-consuming technique, such as LLE, still remains. Compared to the classical LLE technique, that based upon extraction by small sorbent cartridges (SPE) offers several advantages. One of these is that SPE is adaptable to field extraction by using newly submersible instrumentation. By doing this, organic compounds can be isolated immediately from the aqueous matrix so that unwelcome effects of chemical and biological degradation should be eliminated. The small-volume trap could then be easily stored in a cold bag and transported to the laboratorywhere it is kept frozen until analyte desorption. Green and Le Pape (2)reported that hydrocarbons from crude oil stored on sorbent cartridges medwith bothXAD-2 macroreticular resin and octadecyl bonded silica (C-18) for periods of up to 100 days in the presence of an oleophilic bacterial population proved to be stable, while equivalent stored water samples were completely degraded after 10- 15 days. Senseman et al. (3) compared the stability of 12 pesticides in water and on SPE membrane filters containing C-18 material. Data indicated a higher stability of pesticides when adsorbed on C-18 material. Partial loss of some pesticides stored on the extraction membrane was traced to hydrolysis reactions by residual water remaining on the adsorbent even after a prolonged air drawing through the membrane. Phenylurea herbicides extracted from a river water sample by graphitized carbon black (GCB) showed them to be stable upon storage on this adsorbent over 15 days of storage at ambient temperature ( 4 ) . Recently at our laboratory, we developed a multiresidue high-performanceliquid chromatographic (HPLC)method for monitoring 105 pesticides in environmental waters at part per trillion levels (5- 7). To extract pesticides, we use GCB-containingSPE cartridges. Compared to C-18 cartridges and solvent extraction, the GCB material proved the highest efficiency in extracting very polar pesticides from water ( 7 ) . Although GCB behaves as a natural reverse phase, its surface is contaminated by a small number of oxygen chemical complexes having structures similar to hydroquinones, quinones, chromene, and benzpyrilium salts (8).At verylow surface coverages, these active centers may have a profound influence on the adsorption of polar compounds (9). We found that traces of chloroanilines extracted from water by a GCB cartridge were largely lost after 1-day storage on the adsorbent unless hydrazine was added to the water sample (10). Recently, stability studies of the 11 US.EPA priority pollutant phenols on the GCB surface were conducted (1I ) . Surprisingly, after 7 days of storage at 4 "C, the three most acidic phenols could be quantitativelyreextracted only passing through the cartridge an eluant phase volume 50% larger than that needed for eluting nonincubated phenols.
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TABLE 1
List of 34 Pesticides Selected for Analyte Preservation Studies
a
pesticide
class
pesticide
class
2,4-D aldicarb atrazine methyl azinphos bentazone bromacil carbaryl carbofuran chloridazon chlorpyriphos diazinon dimethoate dinoseb disulfoton ethiofencarb fenamiphos fenitrothion
phenoxyacid (H) carbamate (I)a triazine (HIb phosphorodithioate (1) benzothiadiazinone (H) uracil (H) carbamate (1) carbamate (I) pyridazinone(H) phosporothioate (1) phosporothioate (I) phosphorodithioate (I) dinitrophenol (H) phosphorodithioate ( I ) carbamate (I) phosphate (NP phosporothioate (I)
fenthion malathion metamitron methomyl metribuzin molinate monuron ethyl parathion phoxim pirimicarb ethyl pirimiphos propachlor propanil propham propoxur propyzamide vam idot h ion
phosporothioate (1) phosphorodithioate (I) triazinone (H) carbamate (I) triazine (H) thiocarbamate (H) phenylurea phosporothioate (1) phosporothioate ( I ) carbamate (I) phosporothioate (I) anilide (H) anilide (H) carbanilate (H) carbamate (I) amide (H) phosporothioate (I)
I, insecticide.
H, herbicide. N, nematicide.
TABLE 2
Various Storage Treatments Used for Selected Pesticides Dissolved in Two Different Matrices experiment desciption
no. of pesticides stored
matrix
experiment 1A experiment 1B experiment I C
34 34 34
distilled water distilled water distilled water
experiment 2A experiment 28 experiment 2C
12 12 12
distilled water distilled water distilled water
experiment 3A
34
experiment 38
34
experiment 3C experiment 3 0
34 34
total storage period (day)
Storage Stability Study 7 bottle stored, 19 "C 7 C-18 cartridge stored, 19 "C 7 GCB cartridge stored, 19 "C Deactivation Study 7 GCB cartridge stored, 4 "C 7 GCB cartridge stored, -18 "C 7 GCB cartridge stored, CH30H washing, 4 "C
Application and Comparison of Methodology river water 21 GCB cartridge stored, 1 day at 4 "C, CH30H washing, 20 days at - 18 "C river water 21 GCB cartridge stored, CH30H washing, 1 day at 4 "C, 20 days at -18 "C river water 21 bottle stored, 4 "C river water 21 bottle stored, 4 "C HgC12
The objectives of this work were (a) to compare the stability of 34 selected pesticides upon storage on the GCB surface with the pesticide stabilities upon storage on C-18 material and in water; (b) to devise a suitable sample preservation method for minimizing degradation over time of the analytes adsorbed on the GCB surface; (c) to evaluate the stability of the selected pesticides adsorbed from river water samples onto the GCB surface under two storage options and compare stability with those obtained by following two accepted storage methods (amber glass bottles at 4 "C with and without a biological inhibitor).
Experimental Section Materials. The list of the 34 pesticides chosen for this analyte preservation study is presented in Table 1. Individual standard solutions were prepared by dissolving 100 mg of each pesticide in 100 mL of acetonitrile. Under the chromatographic conditions used, the 34 pesticides could be separated by injecting two suitably prepared standard solutions. However, three standard solutionswere prepared and alternatively added to water samples. This precaution was taken for minimizing the probability of interferences with the analysis of intact pesticides by degradation 2186
1
storage treatment
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products eventually formed during storage. Standard solutions were prepared by mixing 1-3 mL of each individual standard solution and diluting to 100 mL with acetonitrile. GCB (120-200 mesh size), commercially referred to as Carbograph 1,and 1-gprepackedc-18cartridgeswerefrom Alltech Associates,Inc. (Deerfield,IL). The other materials for preparing the GCB cartridges were supplied by Supelco Inc. (Bellefonte, PA). Cartridges were fitted into side-arm filtering flasks, and water was forced to pass through them by vacuum from a water pump. With aqueous environmental samples, a limited pesticide preservation study was conducted. Surface water samples (4.5-13.8 mg/L dissolved organic content, pH = 7.9-8.6) were collected from various rivers flowing around Rome. Before pesticide fortification, field samples were analyzed by the method previously reported (6) to determine if any of the analyte of interest or analyte interferences were present. On this basis, four river water samples were selected. Storage Treatments. To reach the three objectives mentioned above, three sets of experiments were conducted. The list of the storage treatments is presented in
Table 2. The first set of experiments was designed for distinguishing between generic and specific long-term mechanisms of sample alteration taking place on the GCB surface. In the second set of experiments, only those pesticides shown to be unstable on storage on the GCB surface at 19 OC were considered. The rationale behind the combined storage treatments of the GCB cartridge in the third set of experiments was that of depicting a practical procedure of field sampling by GCB extraction cartridges where precautions against sample alteration are in part taken in situ and in part taken upon arrival of samples in the analytical laboratory. All experiments were conducted by placing 250 mL of water samples in amber glass bottles, which were spiked alternatively with known volumes of one of the three standard solutions to obtain individual pesticide concentrations of 5- 15pglL. After bottle storage, pesticides were extracted by GCB cartridges, and their percent recovery was determined. DataTreatment. Under any storage condition, percent recoveries of the pesticide concentrations originally added to water were measured and their means calculated. For each experiment set, to estimate whether means of percent recovery differed significantly from each other and from those at zero-day storage, the one-way analysis ofvariance (ANOVA) was performed, and differences were compared with the least significant difference (LSD) at a 0.05 level of significance (12). Extraction and Elution Procedures. GCB Cartridge. The preparation, the pretreatment of the 1-gGCB reversible cartridge, and the extraction and the reextraction procedures were carried out as previouslyreported (5- 7). Briefly, the cartridge was pretreated with an ascorbic acid solution to convert quinones present on the GCB surface in the less reactive hydroquinones. After sample extraction,waterwas in part removed from the cartridge by air-drying for 2 min. The water content into the cartridge was further decreased by passing through it 0.9 mL of methanol. Again, the cartridge was air-dried for 2 min. The cartridge was then reversed, and pesticides eluted with 1 mL of methanol followed by 6 mL of methylene chloridelmethanol (80:20, vlv). When water samples spikedwithpesticidesof solution 3 were analyzed, the elution of the analytes from the GCB cartridge was accomplished by acidifying the solvent mixture mentioned above with trifluoroacetic acid (6).The acidification of the eluent phase was necessary for reextracting the three acidic pesticides, that is bentazone, 2,4D, and dinoseb, contained in solution 3. As reported elsewhere (3, neutral solvent mixtures are unable to displace anionic organic compounds from some positively charged adsorption sites present on the GCB surface. Partial removal of the solvent mixture was carried out in a water bath at 27 "C under a gentle stream of nitrogen, until a final volume of about 0.25 mL was reached. After measuring the exact volume, 25 pL of the final extract was injected into the HPLC apparatus. C-18 Cartridge. Before pesticide extraction, the prepacked 1-g C-18 cartridge was washed with 5 mL of methanol followed by 10 mL of distilled water. The breakthroughvolume ofmethomylonthe 1-g C-18 cartridge was estimated to be 120-130 mL. In order to avoid loss of this analyte in the water effluent, only 100 mL of 250-mL water samples spiked with the pesticide solution 3 containing methomyl were taken and extracted by C-18 cartridges. C-18 material is unable to retain compounds bearing an electrical charge. Therefore, water samples
TABLE 3
Percent Recovery at Zero-Day Storage of 34 Selected Pesticides Extracted from Distilled Water by C.18 and GCB Cartridges %
pesticides 2,4-0
aldicarb
atrazine methyl azinph10s bentazone bromacil carbaryl carbofuran
chloridazon c h I or py ri p h os diazinon dimethoate dinoseb disulfoton et h iofencarb fenamiphos fenitrothion fenthion malathion metamitron methornyl metri buzin molinate monuron parathion
phoxim pirimicarb
pirimiphos propachlor propanil propham
propoxur propyzam ide vamidothion
recovery'
c-18
GCB
91 94 101 95 90 95 95 102 93 77 94 97 98 95 94 91 97 94 96 95 86 97 92 98 99 91 95 85 101 103 92 99 101 90
99 96 99 103 96 102 97 100 97 88 95 93 100 94 95 90 98 102 99 92 94 96 96 97 99 89 95 91 99 100 95 95 99 93
Mean values obtained from six determinations.
spiked with the pesticide solution 3 containing three acidic pesticides were acidified to pH = 3 prior to extraction. After the passage of the sample, water remaining in the cartridge was partially removed by air-drying for 2 min. Pesticides were recovered by eluting them with 6 mL of methanol. The extract was concentrated to about 0.6 mL by following the same procedure as described above. After dilutingwith an approximately equal volume of water and measuring the exact volume, 25 pL of this solution was injected into the HPLC apparatus. The pesticides in the final extracts from both GCB and C-18cartridges were then separated and quantified by HPLC with U V detection at 210 nm under chromatographic conditions reported elsewhere (6). In particular, extracts containing the three acidic pesticides were chromatographed in the ion suppression mode (6).
Results and Discussion Of the 105 pesticides considered in our previous works, 34 were included in this study. The selection was made with the criteria of studying the behavior upon storage on GCB surface of those pesticides that (i) represent important classes of pesticides; (ii) are unstable by their nature in the presence of water, such as most of the insecticides (13); and (iii) showed in the past an anomalous behavior upon adsorption on the GCB surface, that is metamitron, metribuzin, and chloridazon (5). VOL. 29. NO. 9 , 1 9 9 5 / ENVIRONMENTAL SCIENCE &TECHNOLOGY rn 2187
TABLE 4
TABLE 5
Effect of Various Storage Treatments on Percent Recovery of 34 Selected Pesticide after 7 days of Storage Temperature at 19 "I:
Effect of Temperature Treatments on Percent Recovery of 12 Selected Pesticides Adsorbed on the GCB Surface
storaae treatment
recovery'
recovery' pesticides
bottled water
c-18
GCB
LSDb
2,4-D aldicarb atrazine methyl azinphos bentazone bromacil carbaryl carbofuran chloridazon chlorpyriphos diazinon dimethoate dinoseb disulfoton ethiofencarb fenamiphos fenitrothion fenthion malathion metamitron methomyl metribuzin molinate monuron parathion phoxim pi rim icarb pirimiphos propachlor propanil propham propoxur propyzamide vamidothion
97 95 102 101 97 98 80 100 95 85 99 99 96 80 85 95 83 78 80 99 94 95 92 94 95 77 99 85 98 95 91 100 65 93
91 95 100 97 91 97 97 98 91 45 97 92 98 84 94 82 97 85 83 96 88 94 96 99 88 80 100 50 96 99 91 96 79 88
95 96 100 100 97 101 56 101 92 89 93 66 91 72 15 86 79 83 34 32 92 75 97 100 93 78 97 73 97 102 95 97 71 90
4.5 3.1 3.4 3.6 3.1 2.6 5.4 2.8 5.1 6.2 3.9 3.7 4.0 6.3 3.7 4.1 3.9 4.5 4.1 3.8 3.6 3.9 3.6 3.1 6.0 4.3 3.3 4.8 4.5 3.3 3.7 3.1 5.9 4.2
a Mean values obtained from six determinations. Least significant difference. lfthe difference between the twocornparedvalues isgreater than the LSD, the values are considered to be statistically different. If the difference between the two compared values is smaller than the LSD, the values are considered statistically similar.
To assess the effects of the various storage treatments, recovery data at zero-day storage were determined by extracting pesticide-spiked distilledwater samples with both C- 18 and GCB cartridges, immediately eluting pesticides, and measuring their percent recoveries (Table 3). Storage Stability Study. Distilled water samples spiked with the 34 pesticides were in part stored in bottles (experiment lA), in part extracted and stored on C-18 material (experiment 1B) and GCB material (experiment 1C). A stability study of pesticides stored on C-18 material was included in this set of experiments to answer the question of whether the analyte stability upon storage on a sorbent surface could be affected by the particular nature of the sorbent itself. To make evident sample alterations in a relatively short time, both bottles and cartridges were left for 7 days at ambient temperature. Results are reported in Table 4. Twelve (carbaryl, dimethoate, disulfoton, ethiofencarb,fenitrothion, fenthion, malathion, metamitron, metribuzin, phoxim, pirimiphos, and propyzamide) of the 34 pesticides were to a larger or lesser extent degraded upon storage on the GCB cartridge. By comparing recovery data obtained after the three storage treatments, it appears 2188
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pesticides
4 "C
-18 "C
4" C CHIOH washing
LSDb
carbaryl dimethoate disulfoton ethiofencarb fenitrothion fenthion malathion metamitron metribuzine phoxim pirirniphos propyzarnide
94 84 83 45 92 94 70 65 86 89 89 86
98 90 97 56 97 96 82 98 98 98 98 98
90 81 66 28 93 94 76 91 95 96 92 92
2.3 2.8 6.9 6.3 2.4 2.3 7.5 4.2 2.8 3.8 4.8 3.7
a Mean values obtained from six determinations. Least significant difference. If the difference between the two compared values is greater than the LSD, the values are considered to be statistically different. If the difference between the two compared values is smaller than the LSD, the values are considered statistically similar.
that pesticides exhibited the lowest stability upon storage on the GCB material. The percentage losses of propyzamide, disulfoton, fenitrothion, fenthion, and phoxim observed after GCB cartridge storage were comparable to those obtained after storing the five analytes in water and, with the exception of fenitrothion, on C-18 surface. These data seemed to indicate that hydrolytic decomposition by that fraction of water remaining into the cartridges after air-drying them was responsible for the loss observed. The reason for the fact that fenitrothion did not undergo attack by water on storing it on C-18 material was unclear to us. Carbaryl, ethiofmcarb, malathion, and pirimiphos also proved to be partially degraded by storing them in water. The rate of degradation increased upon GCB cartridge storage. This increase might be explained by taking into consideration that some hydrolysis reactions can be catalyzed upon adsorption on the GCB surface. For the four pesticides mentioned above, storage stability data on C-18 material were difficult to interpret. Adsorption on C- 18surface showed the preservation of the two carbamate derivatives, that is, carbaryl and ethiofencarb, from hydrolytic degradation. On the contrary, no preservation effect was present for the two esters, i.e., malathion and pirimiphos. This latter compound was even more rapidly degraded upon storage on C-18 material than in water. With respect to C-18 material, the more pronounced catalytic action exhibited by the GCB sorbent might be due to the fact that this adsorbent has a higher surface chemical heterogeneity (8). Only after sample storage on the GCB surface, the loss of metamitron, metribuzin, and dimethoate was observed. No remarkable loss of the three pesticides mentioned above was observed on reextracting them after storage for few hours on the GCB surface. This singular effect might be accounted for by assuming that over a large period of time particular adsorbates migrate slowly on the sorbent surface toward oxygen chemical complexes contaminating the GCB surface, where chemisorption takes place. A n analogous mechanism of irreversible adsorption by residual weakly acidic silanol groups present on the chemically modified
TABLE 6
Effect of Various Storage Treatments on Percent Recovery of Selected Pesticides Dissolved in River Water after 21 Days storage treatment (% recovery')
pesticides
GCB, 4 "C for 1 d, methanol washing, -18 "C for 20 d
GCB, methanol washing, 4 "C for 1 d, -18 "C for 20 d
bottled water, 4 "C for 21 d
bottled water HgC12, 4 "C for 21 d
LSDb
2,4-D aldicarb atrazine methyl azinpho' S bentazone bromacil carbaryl car bof u ra n chloridazon chlorpyriphos diazinon dimethoate dinoseb disulfoton ethiofencarb fenamiphos fenitrothion fenthion malathion metamitron methomyl metribuzin molinate monuron parathion phoxim pi ri m icarb pirimiphos propachlor propanil propham propoxur propyzamide vam idot h ion
96 92 96 95 96 98 96 97 94 98 99 99 92 87 56 91 94 96 90 96 99 94 95 96 95 95 97 93 95 98 95 101 96 91
96 98 94 98 95 100 97 97 92 105 102 102 91 89 76 92 98 91 95 97 98 95 94 103 97 95 97 92 104 99 96 94 99 100
94 96 97 98 93 95 70 94 95 70 92 90 91 60 74 83 98 103 68 88 82 93 91 99 63 55 90 48 90 98 93 95 50 91
95 92 96 ndc 94 93 89 96 92 nd 5 nd 92 3 99 87 nd nd nd 42 98 94 93 95 nd nd 95 nd 94 97 95 98 nd 20
3.8 4.2 3.6 4.2 3.8 6.8 5.1 4.1 7.2 5.4 4.4 3.7 7.3 7.8 8.1 4.7 3.5 6.1 6.2 6.0 5.1 6.2 6.0 5.1 5.6 6.2 4.0 9.1 5.2 4.0 4.3 4.2 8.6 4.8
+
a Mean values obtained from duplicate measurements for each of the four river waters sampled. Least significant difference. If the difference between the t w o compared values is greater than the LSD, the values are considered t o be statistically different. If the difference between the t w o compared values is smaller than the LSD, the values are considered statistically similar. nd, not detected.
silica surface could also explain the severe loss of pirimiphos and chlorpyriphos, which are weakly basic compounds, upon storage on C-18 material. The results of this first set of experiments indicated that three mechanisms of slow analyte chemical degradation may concurrently be operative upon GCB cartridge storage: (a1 noncatalyzed hydrolysis by residual water, (b) hydrolysis reactions catalyzed by the GCB surface as a whole or by its surface chemical heterogeneities, and (c) chemisorption caused by the same surface active sites. Deactivation Study. To eliminate adverse effects discussed above, various devices were investigated. One device was that of storing cartridges by maintaining them refrigerated (experiment 2A) or frozen (experiment 2B). A second device was that of washing the cartridge with 0.9 mL of methanol before storage to minimize the amount of residual water remaining into the cartridge (experiment 2C). As previously reported (7), 0.9 mL is the maximum volume of methanol that can be passed through the I-g GCB cartridge without loss of those pesticides having the largest mobilities on the GCB bed. Results are reported in Table 5. The methanol washing after sample extraction and before cartridge storage had the general effect of increasing the stability of the 12 pesticides considered.
Interestingly,the methanol washing resulted in quantitative recovery of metamitron, metribuzin, and dimethoate, which showed the tendency to be irreversibly adsorbed on the GCB surface. This result makes evident that chemisorption effects of adsorbates on the GCB surface can only occur in the presence of water. From a practical point of view however, recovery of disulfoton, malathion, and chiefly ethiofencarb after storage on the GCB material was still unsatisfactory. GCB cartridge storage at low temperatures had the expected effect of remarkably increasing the analyte stability. Although significantly reduced, signs of degradation of ethiofencarb and, to a lesser extent, of malathion upon cartridge storage at - 18 "C were however still evident. Application and Comparison of Methodology. In a third set of experiments, the coupled effects of the two storage procedures, that is, methanol washing and storage at low temperature, on the stability of 34 pesticides stored on the GCB surface were investigated. Moreover, in order to assess if matrix effects and bacterial attacks could affect the stability of the pesticides upon a long storage period in the GCB cartridge, this third set of experiments was performed by adding the 34 pesticides considered to riverwater samples. With these samples, storage experiments were performed by simulating a field sampling VOL. 29, NO. 9,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
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procedure. In this situation, one can plan that sorbent cartridges, after using them to isolate pesticides from the matrix at the sampling site, are kept refrigerated in a suitable bag until arrival at the laboratory. Here, if analytes cannot be immediately desorbed, additional precautions are taken to preserve them from degradation. Keeping in mind this scheme, two alternative analyte preservation procedures were considered. After sample extraction, the first procedure involved a cartridge washingwith 10 mL of sterilized water. Thereafter, the cartridge was kept 1 day at 4 "C followed by a methanol washing before storingthe cartridge another 20 days at -18 "C (experiment 3A). In the second procedure, the methanol washing step was accomplished before the 1-day storage at 4 "C (experiment 3B). For comparison, the pesticide stability stored in bottled river water samples at 4 "C over 21 days was also evaluated (experiment 3C). Part of the aqueous samples were stored after the addition of 10 mg/L HgC12 for biological inhibition (experiment 3D), according to a procedure followed by the U S . EPA (I). Results are reported in Table 6 . After bottled water storage at 4 "C over 3 weeks without the addition of HgC12,lower percentage recoveries as compared to those at zero-day storage were obtained for 10 of the 34 pesticides studied. The addition of HgC12 to water reached the goal of preserving three carbamate pesticides, that is, carbaryl, ethiofencarb, and methomyl, from bacterial attack or from other unknown mechanisms of degradation. However, negative side effects overwhelmed the beneficial ones. In fact, up to 100% loss for 14 of the 34 analytes stored in HgC12-containingriver water samples was obtained. For diazinon, disulfoton, propyzamide, methyl azinphos, parathion, fenitrothion, fenthion, and malathion, our results are in agreement with those obtained by EPA searchers during the National Pesticide Survey (1). Chelate-forming metals, such as Hg(II), are reported to be capable of increasing the rate of hydrolysis of chlorpyriphos (13).Likely, the same mechanism of degradation took place with the other 13 pesticides. By comparing recovery data of pesticides, the sample storage method on the GCB surface appears definitively more effective than that in refrigerated water. Except for ethiofencarb, the two storage procedures with GCB cartridges succeeded in preserving analytes, as percent recovery data were comparable to those at zero-day storage. The good stability exhibited by carbaryl and methomyl stored on the GCB surface indicated that no bacterial degradation process took place. Likely, bacteria were washed out from the cartridge by passing through it sterilized water, or they were destroyed on washing the GCB bed with methanol. As for ethiofencarb, the device of minimizing residual water in the extraction cartridge by a methanol washing just following the analyte extraction had the effect of increasing its stability. Yet, it appears that the stability problem of e t h i o f e n c a r b is not totally resolved by the device mentioned above. Additional stability
2190 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 9, 1995
measurements of ethiofencarb on the GCB surface conducted after methanol washing and upon 1-day storage at 4 "C made evident that a large part of the loss observed uponthe21-daystorage was due to theinitial 1-daystorage treatment at 4 "C. This result suggests that a remedy for avoiding possible alteration of particularlyunstable analytes occurring during an in situ extraction stage could be that of storing cartridges in bags containing dry ice.
Conclusion This study has shown that, although more efficient than the C-18 material in extracting pesticides, sample storage into a GCB cartridge requires careful storage treatments. By adopting the method of cartridge storage, the problem of the sample alteration occurring in bottled water between the time of sampling and that of the analysis can be eliminated and storage space saved. The main advantage of the cartridge storage method over conventional water storage at 4 "C is that the small-volume tube can be easily maintained at much lower temperatures. An additional advantage attainable by an immediate analyte isolation from the aqueous matrixwith a sorbent material is that the addition of biological inhibitors,which may alter the sample, can be avoided. Even when sample collection is accomplished by bottles, extraction of the analytes by sorbent cartridgesjust upon arrival of field samples to the laboratory and storing the small-volume cartridge at low temperatures until chromatographic analysis could minimize analyte alteration by both bacterial and water attacks and save storage space.
Literature Cited (1) Munch, D. J.; Frebis C. P. Enuiron. Sci. Technol. 1992,26, 921925. (2) Green, D. R.; Le Pape, D. Anal. Chem. 1987, 59, 699-703. (3) Senseman, S A . ;Lavy, T. L.; Mattice, J. D.; Myers, B. M.; Skulman, B. W. Enuiron. Sci. Technol. 1993, 27, 516-519. (4) Di Corcia, A.; Marchetti, M. J. Chromatogr. 1991, 541, 365-373. (5) Di Corcia A.; Marchetti, M. Anal. Chem. 1991, 63, 580-585. (6) Di Corcia A.; Marchetti, M. Enuiron. Sci. Technol. 1992, 26, 6674. (7) Di Corcia, A.; Samperi, R.; Marcomini,A,; Stelluto S.Anal. Chem. 1993, 65, 907-912. (8) Campanella, L.; Di Corcia, A.; Samperi, R.; Gambacorta,A. Mater. Chem. 1982, 7, 429-438. (9) Millard, B.; Caswell, E. G.; Leger, E. E.; Mills, D. R. J. Phys. Chern. 1957, 59, 876-881. (10) Di Corcia, A.; Samperi, R. Anal. Chem. 1990, 62, 1490-1494. (11) Di Corcia, A.; Marchese, S.; Samperi, R; Cecchini, G.; Cirilli, L. J. AOAC Int. 1994, 77, 446-453. (12) Miller, J. C.; Miller, J. N. StatisticsforAnalytical Chemistry;John Wiley & Sons, Inc.: New York, 1984. (13) ThePesticideManual, 9thed.;Whorting, C. R., Hance, R. J., Eds.; The British Crop Protection Council: Farnham, U.K., 1991.
Received for review J u n e 21, 1994. Revised manuscript received January 6, 1995. Accepted May 16, 1995.%
ES9403975 @
Abstract published in Advance ACS Abstracts, July 1, 1995
