Environ. Sci. Technol. 2000, 34, 714-718
First LC/MS Determination of Cyanazine Amide, Cyanazine Acid, and Cyanazine in Groundwater Samples IMMA FERRER,† E. M. THURMAN,‡ AND D A M I AÅ B A R C E L O Ä * ,† Department of Environmental Chemistry, IIQAB-CSIC c/ Jordi Girona, 18-26, 08034 Barcelona, Spain, and U.S. Geological Survey, 4821 Quail Crest Place, Lawrence, Kansas 66049
Cyanazine and two of its major metabolites, cyanazine amide and cyanazine acid, were measured at trace levels in groundwater using liquid chromatography/atmospheric pressure chemical ionization/mass spectrometry (LC/APCI/ MS). Solid-phase extraction was carried out by passing 20 mL of groundwater sample through a cartridge containing a polymeric phase (PLRP-s), with recoveries ranging from 99 to 108% (n ) 5). Using LC/MS detection in positive ion mode, useful structural information was obtained by increasing the fragmentor voltage, thus permitting the unequivocal identification of these compounds in groundwater samples with low sample volumes. The fragmentation of the amide, carboxylic acid, and cyano group was observed for both metabolites and cyanazine, respectively, leading to a diagnostic ion at m/z 214. Method detection limits were in the range of 0.002-0.005 µg/L for the three compounds. Finally, the newly developed method was evaluated for the analysis of groundwater samples from New York containing the compounds under study and presents evidence that the metabolites, cyanazine acid, and cyanazine amide may leach to groundwater and serve as sources for deisopropylatrazine. The combination of on-line SPE and LC/APCI/MS represents an important advance in environmental analysis of herbicide metabolites in groundwater since it demonstrates that trace amounts of polar metabolites may be determined rapidly. Furthermore, the presence of both cyanazine amide and cyanazine acid indicate that another degradation product, deisopropylatrazine, may be occurring at depth because of the subsequent degradation of cyanazine.
Introduction Cyanazine is used on corn to control broadleaf weeds and grasses, especially in the upper Midwestern United States during planting. Because of the faster soil half-life of cyanazine (approximately 30 days, ref 1), it is often preferred in colder climates over atrazine with a 45-60 day half-life. Cyanazine is also used on cotton in the southern United States late in the season to control weeds. For these reasons, large quantities of cyanazine are used annually on crops in the United States, approximately 10 million kg in 1997. * Corresponding author phone: 34-93-4006118; fax: 34-932045904; e-mail:
[email protected]. † IIQAB-CSIC. ‡ U.S. Geological Survey. 714
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However, cyanazine is scheduled to be removed from the market in the U.S. in the year 2000, so that studies to determine its metabolites and their fate in the groundwater environment are important today. Moreover, information on pesticide degradation products is necessary to understand the environmental fate of pesticides and to establish important degradation pathways (2-4). Several cyanazine degradation products have been detected in soil (5, 6) and water samples (2, 7). Cyanazine is known to degrade to cyanazine amide and cyanazine acid through a hydrolytic reaction occurring at the nitrile group (6). Beynon et al. reported cyanazine amide residues in soils 4 weeks after cyanazine application; however, no residues of this metabolite were detected below the 10-cm depth, 1633 weeks after application. In another soil study, Muir and Baker (8) found that cyanazine acid was a more persistent product of degradation than cyanazine amide. Reddy et al. (9) demonstrated that, under soil field conditions, cyanazine degradation products (especially cyanazine amide and cyanazine acid) were more likely to remain in the aqueous phase of soil and had a greater mobility than cyanazine, thus indicating the more favorable transport to deeper soil layers and perhaps to groundwater. Meyer (10) and Thurman et al. (11, 12) reported that cyanazine amide and deisopropylatrazine (DIA) often cooccurs in groundwater with cyanazine suggesting that DIA is transported to groundwater via cyanazine metabolites or from cyanazine degradation. Because DIA degrades rapidly in soil (13) coming from either atrazine or simazine, it is not commonly found in groundwater (2, 10), unless simazine or cyanazine is present (11). Finally, only cyanazine amide has been reported in groundwater samples (2, 10) because of the difficulty of determination of cyanazine acid (10); thus, the determination and presence of cyanazine acid in groundwater is important to the hypothesis of Meyer (10) and Thurman et al. (11) that cyanazine acid may be an important metabolite in the transport of DIA to groundwater. For this reason, the main goal of this study was to develop a sensitive and reliable methodology for the determination of cyanazine acid in groundwater samples and to demonstrate the possibility that cyanazine acid may be a source of deisopropylatrazine in groundwater. To our knowledge, this work represents the first identification of cyanazine acid in groundwater and the first all LC/MS method for cyanazine and its metabolites.
Experimental Section Chemicals. HPLC-grade solvents acetonitrile, methanol, and water were purchased from Merck (Darmstadt, Germany). Hydrochloric acid and acetic acid were also purchased from Merck. Pesticide standards, cyanazine amide and cyanazine acid, were a gift from Dupont (Philadelphia), and cyanazine was obtained from ChemService (Westchester, PA). Stock standard solutions of 1000 µg/mL were prepared by weighing the solutes and dissolving them in methanol. A stock solution of 0.5 µg/mL was used to fortify groundwater samples at the µg/L level for preconcentration through the cartridges and further determination of recoveries and construction of the calibration graphs. The final standard solutions did not contain more than 0.5% of methanol. The SPE cartridges used (Spark Holland, The Netherlands) consisted of 10 µm × 2 mm i.d. disposable precolumns that contained 20 mg of either 40-µm C18-bonded silica or polymeric material. Chromatographic Conditions. (a) LC/DAD. The eluent was delivered by a gradient system Waters 600-MS with a 20 µL injection loop and a Waters 996 photodiode array detector 10.1021/es990462g CCC: $19.00
2000 American Chemical Society Published on Web 01/15/2000
TABLE 1. LC/APCI/MS Operational Parameters Used in the Analysis of Groundwater Samplesa ionization mode operational parameters
positive
negative
drying gas flow drying gas temperature nebulizer gas pressure vaporizer temperature capillary voltage corona current gain
4 L/min 350 °C 60 psi 350 °C 2000 V 8 µA 3
4 L/min 350 °C 60 psi 350 °C 3500 V 70 µA 3
a LC mobile phase: acetonitrile-water (40:60) containing 0.3% acetic acid at a flow rate of 0.8 mL/min.
(Waters, Millipore, MA). The mobile phases used for the elution of the analytes consisted of 30% acetonitrile and 70% water acidified at pH 2.5 with hydrochloric acid at a flowrate of 1 mL/min. The analytical column was a 25 cm × 4.6 mm i.d. C-18 from Supelco. Quantification was carried out with UV detection at 220 nm for all the compounds. (b) LC/APCI/MS. Liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry in both positive and negative modes of operation was used for the determination of the compounds at low levels of concentration. The eluent was delivered by a liquid chromatograph model HP 1090 (Hewlett-Packard, CA). The mobile phases used for the elution of the analytes consisted of 40% acetonitrile, 60% water, and 0.3% acetic acid at a flow-rate of 0.8 mL/min. The analytical column was the same as that used in the LC/DAD analyses. This HPLC system was connected to a HP mass spectrometer, model HP 1100, system equipped with an APCI probe. The different operating parameters included a drying gas (N2), a nebulizing gas, a capillary voltage, a corona current, and a fragmentor voltage setting. The experimental conditions and operational parameters used for the characterization of the analytes are summarized in Table 1. These values were optimized for the compounds under study in both ion modes of operation. The fragmentor voltage was varied between 30 and 200 V in order to study the fragmentation of the three compounds. Generally, a fragmentor voltage of 120 V was used in the analysis of the water samples owing to the enhanced fragmentation as compared to lower fragmentor voltages. The instrument control and data processing utilities included the use of the HP application software installed in a Digital Pentium PC. Chromatograms were recorded under selected ion monitoring (SIM) and full SCAN (from m/z ) 100-400) conditions. Sampling. Groundwater samples were collected from two tile drains in June of 1997 in the Canajoharie Creek watershed near Ithaca, NY. Groundwater samples were collected as grab samples in 3-L Teflon bottles, which were cleaned and rinsed with soap and water and tap water and distilled water prior to use. All the samples were filtered through a 0.45 µm glassfiber filter (Millipore, Bedford, MA) before use in order to remove suspended particles. The fields overlying the sample collection location are farm fields where cyanazine had been applied during the 1997-farming season. Sample Preparation. Preconcentration of the samples was performed online with an automated sample preparation system. The automated SPE device used (Prospekt, Spark Holland, The Netherlands) was connected on-line with the gradient pumps. The general scheme of the system was previously described (14). The C18 and the PLRP-s cartridges were washed sequentially with 6 mL of acetonitrile and 4 mL of LC-grade water (pH ) 2.5). Afterward, a 20-mL aliquot of water sample was passed through the cartridge at a flow rate of 2 mL/min. The compounds trapped on the sorbent were
FIGURE 1. Chemical structures and main fragmentation of the compounds studied.
TABLE 2. Mean Percent Recoveries of Extraction and Relative Standard Deviations (n ) 5) of Cyanazine and Its Metabolites after Passing 20 mL of Groundwater, (Fortified at 5 µg/L) through C18 and PLRP-s Cartridges pH ) 7 compound
C18
PLRP-s
pH ) 2.5 C18
PLRP-s
cyanazine amide 83 ( 1.6 107 ( 2 84 ( 4 100 ( 0.9 cyanazine acid 12 ( 1 22 ( 0.6 101 ( 2.9 108 ( 4.1 cyanazine 105 ( 1 105 ( 1 102 ( 1.9 99 ( 0.9
eluted with the chromatographic mobile phase by switching the valve into the elute position. For recovery studies, 20 mL of pesticide-free groundwater was fortified at 5 µg/L, passed through the C18 and the PLRP-s cartridges, and analyzed by LC/DAD. This experiment was performed in replicate (n ) 5) for all the compounds studied. The external calibration curves were obtained by LC/MS after passing 20 mL of pesticide-free groundwater sample fortified in the trace concentration range of 0.01-1.5 µg/L in order to have a similar matrix as in the environmental water samples. Equipment blanks were analyzed, as well, and no interferences were noted.
Results and Discussion Solid-Phase Extraction. The retention of cyanazine and its two major metabolites, cyanazine amide and cyanazine acid (see Figure 1), was investigated onto two different types of sorbents at neutral and acidic pH (Table 2). Due to the high polarity of cyanazine amide, its retention on the C18 phase is lower than on a polymeric phase (83-84% recovery compared to 100-107%, Table 2). The main drawback of the C18 sorbent appears to be its limited breakthrough volumes for polar (15, 16). For example, according to preliminary experiments (not shown here), a recovery value of 49% for cyanazine amide was obtained after passing 50 mL of groundwater sample through a C18 cartridge, as compared with a value of 76% when preconcentration took place onto a polymeric cartridge. For this reason a lower volume of 20 mL was investigated in order to avoid any breakthrough of the cyanazine amide. Recoveries for cyanazine amide and cyanazine are not dependent on pH (Table 2), and this result is in agreement with previous work regarding the analysis of cyanazine in groundwater samples (2). On the other hand, cyanazine acid is an ionic molecule at neutral pH, so that, acidification increases retention, as shown in Table 2. As the pH of the VOL. 34, NO. 4, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Typical Fragment Ions and Relative Abundances (RA) of the Herbicides in LC/MS Using an APCI Interface, in Either Positive (PI) or Negative (NI) Ion Mode of Operationa APCI in PI 70 V compound
Mn
m/z
RA
APCI in NI
120 V
m/ z
RA
70 V
m/ z
RA
120 V
m/z
RA
cyanazine amide 258 259 100 259 15 257 100 257 90 242 10 242 50 239 20 239 100 214 100 222 15 222 10 194 20 cyanazine acid 259 260 100 260 100 258 100 258 100 214 50 172 20 106 15 cyanazine 240 241 100 241 100 239 100 239 15 214 7 214 90 212 60 194 60 a Fragmentor voltage set at 70 and 120 V. LC mobile phase: acetonitrile-water (40:60) containing 0.3% acetic acid at a flow rate of 0.8 mL/min. Mn ) nominal mass.
sample increases, its ability to sorb decreases until it is not possible to isolate it at neutral pH. At high pH values this compound is ionic, and then it is not isolated by the sorbent. Results of this extraction process show that isolation is possible down to a value of pH ) 4. From the results obtained after the SPE optimization a preconcentration volume of 20 mL, a pH value of 2.5 and the use of polymeric cartridges were chosen for the analysis of the environmental water samples. LC/APCI/MS. The different operational parameters of the APCI interface were optimized for the three compounds under study, and those presenting the maximum sensitivity were chosen for further analysis. In Table 1 the operational parameters used in the analysis of the groundwater samples are summarized. The most relevant parameters were the capillary voltage and the corona current. The capillary voltage is applied to the entrance of the capillary, and it is relative to the nebulizer and spray chamber, which are at ground potential. The corona current parameter controls the current (in µA) from the corona discharge needle to the end plate. A value of 2000 V for the capillary voltage and a value of 8 µA for the corona current were chosen for the analysis under positive ion. A value of 3500 V and a value of 70 µA were chosen for the analysis under negative ion. A comparison of the use of APCI under positive and negative ion mode of operation was also carried out. In this way, the difference in response of the three analytes studied under both positive and negative mode of operation was assessed. Whereas for cyanazine amide and cyanazine parent compound the sensitivity obtained in positive ion mode of operation was higher than in negative ion mode, the sensitivity for cyanazine acid was lower in positive ion mode. According to this, the sensitivity encountered for cyanazine acid was better under negative ion mode than under positive ion mode by three times. This is due to the presence of the carboxylic group in the structural moiety of this compound. This functional group is easily ionized under negative ion conditions, as compared with positive ion conditions. The fragmentor voltage affects the transmission and fragmentation of sample ions. In general, the higher the fragmentor voltage, the more fragmentation will occur. In compounds that do not fragment readily, higher fragmentor voltages often result in better ion transmission. The fragmentor voltage gives the ions a “push” that helps them traverse the relatively high-pressure region between the exit of the capillary and the skimmer. Thus, at higher voltages the maximum structural information is obtained (17). Table 3 reports the typical fragment ions of the three compounds studied in this work in both positive and negative ion mode. 716
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Fragmentor voltages of 70 and 120 V were examined in order to study the main fragment ions formed under LC/APCI/ MS. At the low fragmentor value of 70 V, all the compounds studied gave only the molecular ion plus or minus a proton as a base peak under positive and negative ion mode, respectively. So that this fragmentor value does not provide useful structural information. However, it was observed that, under positive ion conditions, all three compounds presented a major fragmentation at 120 V, which provided both good structural information and enough sensitivity for the compounds studied, being the common fragment ion at m/z 214 the most abundant. On the other hand, under negative ion conditions, scarce fragmentation was observed for cyanazine acid as compared with positive mode of ionization. Moreover, the sensitivity for cyanazine amide and cyanazine was very low in negative ion conditions. For this reason, the positive ion mode was chosen for the construction of calibration curves and the quantitation of environmental groundwater samples. From the results obtained by LC/APCI/MS it is clearly shown that the optimum fragmentor voltage is compound dependent, and, for this reason, an accurate evaluation of a wide range of fragmentor values for each one of the compounds studied in this work was performed. In Figure 2 the different fragmentation patterns for cyanazine and its two metabolites are shown as a function of the fragmentor voltage in positive ion mode. In all three cases, the molecular ion presents its maximum sensitivity at a fragmentor value of 70V. At higher fragmentor values, the sensitivity for the molecular ion decreases with the exception of cyanazine acid, which remains constant till the value of 120 V. In general, the main fragment ion obtained was at m/z 214 which corresponded to the loss of the amide, carboxylic and cyano groups for cyanazine amide, cyanazine acid, and cyanazine, respectively (see Figure 1). The pattern exhibited by this fragment ion was similar for all three compounds, showing a maximum sensitivity for fragmentor voltage values around 130-140 V. As a consequence, a fragmentor voltage value of 120 V was chosen for the analysis of groundwater samples in order to identify the major metabolites of cyanazine. At this value the maximum fragmentation was obtained without decreasing the sensitivity for the molecular ion, which gave us both structural information and high sensitivity. Next, groundwater samples were analyzed either by LC/ APCI/MS with positive mode of ionization under SIM conditions or by LC/DAD in order to compare both techniques in terms of selectivity and sensitivity. Figure 3 shows the comparison between the analysis of a groundwater sample by LC/APCI/MS and LC/DAD after its preconcentration through polymeric cartridges. As it can be seen in this figure, the chromatogram obtained from the groundwater sample analyzed by APCI/MS presented a clear baseline as compared with that corresponding to the analysis by LC/ DAD. Thus, the compounds could be easily identified and detected using LC/APCI/MS detection compared to LC/DAD detection. One of the improvements achieved with mass spectrometric detection as compared to conventional diodearray detection is that a better sensitivity is obtained working under SIM conditions (by 2 orders of magnitude). Furthermore, the broad matrix peak corresponding to the humic and fulvic substances of natural waters is avoided by the use of SIM conditions, which gives a clearer baseline. Therefore, the presence of the two major metabolites of cyanazine was easily confirmed with the LC/APCI/MS technique. Cyanazine acid was measured with great sensitivity, with detection limits of 0.005 µg/L for a 20 mL sample, which means a total mass detected of ∼10 pg. When using only LC/DAD analysis, the humic and fulvic acids constitute a major interference peak in the chromatogram. With APCI using positive ion detection we can “see-through” the humic material and no interference
FIGURE 2. Fragmentation pattern of the compounds studied under LC/APCI/MS in Positive mode of operation. LC mobile phase: 40% acetonitrile, 60% water (0.3% acetic acid). occurs. This result enhances the detection limit of the method. Calibration Curves and LODs. The relative standard deviation of the method was calculated from five independent extractions of the compounds from groundwater samples upon both type of sorbents. The relative standard deviation ranged from 1 to 9% indicating good performance of the method developed in this work. One advantage of automation in an online preconcentration is that more reproducible results can be expected, provided that the manipulation of the samples is avoided as compared with an offline methodology. Calibration curves were constructed by passing 20 mL of groundwater sample, fortified with a solution containing cyanazine and its two metabolites, through a PLRP-s cartridge. Calibration data are summarized in Table 4. The curves were linear in the concentration range studied, from 0.01 to 1.5 µg/L, and the correlation coefficients were higher than 0.98 for all the pesticides studied, thus indicating a good performance of the online methodology developed in this work.
FIGURE 3. Analysis of a groundwater sample from New York after its preconcentration through polymeric cartridges by (a) LC/APCI/ MS with positive ion (PI) mode of operation under SIM conditions and (b) LC/DAD at 220 nm. Peak numbers: (1) cyanazine amide, (2) cyanazine acid, and (3) cyanazine. LC conditions as described in the Experimental Section.
TABLE 4. Calibration Data Obtained with LC/APCI/MS in Time-Scheduled SIM-PI Mode for the Studied Pesticides (Fortified from 0.01 to 1.5 µg/L) after Online Preconcentration of 20 mL of Groundwater through a PLRP-s Cartridge compound
calibration equationa
R2
cyanazine amide Y ) 406763 + 2E+07x 0.9844 cyanazine acid Y ) 139926 + 7E+06x 0.9933 cyanazine Y ) 100388 + 2E+07x 0.9974
LODb (µg/L) 0.002 0.005 0.002
a Least squares regression equation. b LODs were calculated by using a signal-to-noise ratio of 3 (the ratio between the peak intensity and the noise).
The limits of detection were calculated using a signalto-noise ratio of 3 (the ratio between the peak intensity under SIM conditions and the noise). Low detection limits at the ppt level can be obtained due to the high selectivity and sensitivity encountered by the APCI/MS system since few if any interferences are encountered under SIM conditions (3, 18). Sample Analysis. Groundwater samples were analyzed by the methodology developed in this work. The structural VOL. 34, NO. 4, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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development of methods for very polar pesticide analytes. The use of this technique opens up new possibilities for the trace determination of polar pesticide metabolites in water matrices and will probably facilitate a better understanding of fate of pesticides in the aquatic environment. Since the European Community (EEC) Drinking Water Directive (DWD) establishes that individual pesticides and their transformation products (TPs) should be monitored at the 0.1 µg/L level and the total pesticide concentration cannot exceed 0.5 µg/L, this method can be applied to control cyanazine and its metabolites in groundwater (15). On the other hand, this methodology could be useful for the compliance of the U.S. EPA legislation for drinking waters, which requires the monitoring of cyanazine at the 1 µg/L level (Heath Advisory Level, HAL). FIGURE 4. Concentration (µg/L) of the compounds studied, in groundwater samples from New York, after their analysis by LC/ APCI/MS under PI mode of operation. Relative SD (n ) 3) varied between 5 and 10%. confirmation of these compounds in groundwater was possible by using APCI/MS and the main fragmentation ions. Concentrations of cyanazine amide and cyanazine in the ppt level were found in the two groundwater samples collected (see Figure 4) and were similar to those analyzed in a previous study carried out with these samples by gas chromatograph/mass spectrometry (2). On the other hand, as it can be observed in this figure, cyanazine acid was present at high concentrations (ppb level). It is important to mention that cyanazine acid had not been detected in groundwater in any previous studies and is the first unequivocal confirmation of the presence of cyanazine acid by LC/MS in groundwater samples. The concentrations for cyanazine acid were much higher than those corresponding to cyanazine amide or cyanazine, indicating a high mobility and perhaps longer half-life of this compound as compared with the parent compound. Furthermore, these results indicate that the hypothesis of Meyer (10) and Thurman et al. (11) that cyanazine acid may be an important transport molecule for the subsequent formation of DIA in deeper soil horizons or in groundwater is plausible. More samples are being analyzed by the authors to further substantiate these results. Increased knowledge about the degradation of herbicides and the dissipation of their metabolites in the environment could include consideration of herbicide metabolites as part of the basis for the establishment of health advisories and water-quality regulations. For instance, it is estimated that groundwater is the source of drinking water for 90% of rural households and three-quarters of U.S. cities (15). In this sense, if pesticide metabolites are not quantified, the effects of chemical use on groundwater quality will be substantially underestimated. So that continuous research is needed to identify major degradation pathways for all pesticides and to develop analytical methods to determine their concentrations in water and other matrices. The methodology described in this paper can be a useful approach in carrying out the monitoring of cyanazine acid, which is the main metabolite of cyanazine, in environmental waters. The high sensitivity achieved, the capability to analyze complex water matrices, and the selectivity of the method provide a useful tool in the metabolite monitoring of water samples. This area will continue to expand since efforts for pesticide detection in the environment will be directed toward the
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Acknowledgments We thank Pat Philips (USGS, Ithaca, NY) for providing the groundwater samples. This work was partly supported by the Commission for Cultural, Educational and Scientific Exchange between U.S.A. and Spain (Contract Number HNCCT 98148) and by the Commission of the European Communities, MAST-III, ACE (MAS3-CT98-0178), and CICYT (AMB98-913-103). The use of trade, firm, or brand names in this paper is for identification purposes only and does not constitute endorsement by the U.S. Government.
Literature Cited (1) Blumhorst, M. R.; Weber, J. B. J. Agric. Food Chem. 1992, 40, 894-897. (2) Kolpin, D. W.; Thurman, E. M.; Goolsby, D. A. Environ. Sci. Technol. 1996, 30, 335-340. (3) Ferrer, I.; Thurman, E. M.; Barcelo´, D. Anal. Chem. 1997, 69, 4547-4553. (4) Field, J. A.; Thurman, E. M. Environ. Sci. Technol. 1996, 30, 1413-1418. (5) Beynon, K. I.; Bosio, P.; Elgar, K. E. Pestic. Sci. 1972, 3, 401-408. (6) Beynon, K. I.; Stoydin, G.; Wright, A. N. Pestic. Sci. 1972, 3, 293-305. (7) Kolpin, D. W.; Kalkhoff, S. J.; Goolsby, D. A.; Sneckfahrer, D. A.; Thurman, E. M. Ground Water 1997, 35, 679-688. (8) Muir, D. C. G.; Baker, B. E. Weed Res. 1978, 18, 111-120. (9) Reddy, K. N.; Locke, M. A.; Zablotowicz, R. M. Weed Sci. 1997, 45, 727-732. (10) Meyer, M. T. Ph.D. Thesis, Department of Geology, University of Kansas, 1994; 364 p. (11) Thurman, E. M.; Goolsby, D. A.; Aga, D. S.; Pomes, M. L.; Meyer, M. T. Environ. Sci. Technol. 1996, 30, 569. (12) Thurman, E. M.; Meyer, M. T. Herbicide Metabolites in Surface and Groundwater; ACS Symposium Series 630, American Chemical Society: Washington, DC, 1996; Chapter 1. (13) Mills, M. S.; Thurman, E. M. Environ. Sci. Technol. 1994, 28, 600-605. (14) Ferrer, I.; Barcelo´, D. J. Chromatogr. A 1997, 778, 161-170. (15) Barcelo´, D.; Hennion, M.-C. Trace Determination of Pesticides and their Degradation products in Water; Elsevier: 1997; Vol. 19. (16) Thurman, E. M.; Mills, M. S. Solid-Phase Extraction: Principles and Practice; John Wiley and Sons: 1998. (17) Ferrer, I.; Barcelo´, D. Analusis 1998, 26, 118-122. (18) Ferrer, I.; Hennion, M.-C.; Barcelo´ D. Anal. Chem. 1997, 69, 4508-4514.
Received for review April 23, 1999. Revised manuscript received November 2, 1999. Accepted November 23, 1999. ES990462G
