COLY & AARON: JOURNAL OF AOAC INTERNATIONAL VOL. 84, NO. 6, 2001 1745 ENVIRONMENTAL ANALYSIS
Simultaneous Determination of Binary Mixtures of Sulfonylurea Herbicides in Water by First-Derivative Photochemically Induced Spectrofluorimetry ATANASSE COLY1 and JEAN-JACQUES AARON2 Institut de Topologie et de Dynamique des Systèmes de l’Université Denis-DIDEROT-Paris 7, Laboratoire Associé au CNRS, 1 Rue Guy de la Brosse, 75005 Paris, France
First-derivative photochemically induced spectrofluorimetry (PIF-1D) is applied to the simultaneous determination of binary mixtures of 4 sulfonylurea herbicides in aqueous micellar samples. Synthetic binary mixtures of sulfometuronmethyl with chlorsulfuron, metsulfuron-methyl, and 3-rimsulfuron, respectively, are well resolved by using the zero-crossing point procedure. PIF-1D allows the determination of binary mixtures of these herbicides with linear dynamic ranges over about 2 orders of magnitude, limits of detection between 0.5 and 52 ng/mL, and relative standard deviations within 0.3–2.9%. Application to the determination of binary mixtures of these herbicides in spiked tap water samples yielded satisfactory recoveries (90–117%).
ulfonylurea herbicides, a new important class of pesticides used for weed control in crops, are present at low concentrations in agricultural runoff waters (1). Conventional analytical methods for the determination of sulfonylurea herbicides in environmental samples are based on liquid chromatography (LC; 2–8), gas chromatography (GC; 9–14), bioassay (15–17), enzyme immunoassay (18, 19), and capillary electrophoresis (20). Although these techniques are sensitive and selective, they are relatively time-consuming (5–7) and costly (20), and often require lengthy optimization procedures (11) and, in some cases, derivatization of the sulfonylureas (9–13). The derivatives have very similar physicochemical properties, and, therefore, they cannot be determined easily in mixtures of this group of herbicides. On the other hand, fluorimetric methods have been widely used in the analysis for pesticides because of their great sensitivity, simplicity, and versatility (21). Direct fluorimetry can-
S
Received July 27, 2000. Accepted by JS November 2, 2000. Presented, in part, at the IXth International Symposium on Luminescence Spectrometry in Biomedical and Environmental Analysis, May 15–17, 2000, Montpellier, France. 1 On leave from the Faculté des Sciences et Techniques, Département de Chimie, Université Cheikh Anta DIOP, Dakar, Sénégal. 2 Author to whom correspondence should be addressed; e-mail:
[email protected].
not be applied to sulfonylurea herbicides because they are naturally nonfluorescent. However, we recently developed a photochemically induced fluorescence (PIF) method for the determination of sulfonylurea herbicides in aqueous micellar media (22) in a flow injection analysis system (23). Although PIF is very sensitive, it demonstrates poor selectivity in screening mixtures of sulfonylurea herbicides because of the similarity of the excitation and emission wavelengths of their photoproducts and the overlap of the PIF spectra. Therefore, we decided to examine the possibility of improving the selectivity of PIF by recording derivative PIF spectra (24). Indeed, to resolve complex matrixes or spectra, derivative spectroscopic techniques have been shown to be very useful for the reduction of band overlapping errors in quantitative analysis (25–27). In this paper, we report the feasibility of using first-derivative photochemically induced fluorescence (PIF-1D) spectra to detect and resolve synthetic binary mixtures of 4 sulfonylurea herbicides, i.e., chlorsulfuron, metsulfuron-methyl, 3-rimsulfuron, and sulfometuron-methyl (Figure 1) in spiked tap water samples. Experimental
Reagents (a) Chlorsulfuron (99.2%, m/m), metsulfuron-methyl (97.4%, m/m), 3-rimsulfuron (99.1%, m/m), and sulfometuron-methyl (99.2%, m/m).—E.I. Du Pont de Nemours and Co., Inc. (Wilmington, DE). (b) Methanol.—Spectroscopic grade (Merck, Darmstadt, Germany). (c) Cetyltrimethylammonium chloride (CTAC).—Obtained as a 25% (w/w) solution in water (Aldrich, Milwaukee, WI). (d) Sodium dodecyl sulfate (SDS).—99%, m/m (Acros Organics, Geel, Belgium). (e) Sodium hydroxide.—97%, m/m (Prolabo, Rhône Poulenc, Paris, France). (f) 20% (v/v) buffer solution.—pH 8 (Acros Organics); used for analytical measurements. Very alkaline solutions (pH 11.8) were prepared with a convenient concentration of NaOH. (g) Distilled water.—Used for preparing the micellar solutions. (h) Tap water samples.—For analytical applications, drinking water from Paris were freshly collected in our labora-
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and stored in a refrigerator. Stock solutions of SDS (0.1M), CTAC (0.1M), and NaOH (1M) were prepared with distilled water and used for serial dilutions. Herbicide micellar solutions were prepared by transferring 10–50 µL aliquots of the methanolic working standard solutions to 5 mL volumetric flasks, adding the needed volume of SDS or CTAC stock solution, 25 µL 1M NaOH, or 1 mL pH 8 buffer solution, and diluting to volume with distilled water. The solutions were then shaken before irradiation and analytical measurement. All working aqueous and micellar solutions contained 20% occurred only at relatively high concentrations of interferent (>69 ng/mL for 3-rimsulfuron and, reciprocally, >290 ng/mL for sulfometuron-methyl); and (2) for the 2 other mixtures, satisfactory recovery values of sulfometuron-methyl (considered as the analyte) of 103–112% and 87–96% were obtained when metsulfuron-methyl and chlorsulfuron, respectively, were used as interferents. Reciprocally, in the presence of interferent amounts of sulfometuron-methyl, the recovery values obtained were good, ranging between 88 and 108% and between 96 and 118%, respectively, for metsulfuron-methyl and chlorsulfuron considered as analytes. For the latter 2 mixtures, the maximum allowed concentration of each interferent herbicide compound with respect to each analyte was higher than the upper concentration in the LDR of the interferent herbicide. The maximum allowed concentration values were >2600 ng/mL for sulfometuron-methyl and >150 ng/mL for chlorsulfuron, reciprocally, and >1500 ng/mL for sulfometuron-methyl and >60 ng/mL for metsulfuron-methyl, reciprocally. The synthetic binary mixtures were then resolved by using the direct measurement procedure under the optimum analytical conditions.
Resolution of Binary Mixtures Tap water samples spiked with binary mixtures of the herbicides were investigated. Table 3 presents the results of the analyses in the case of the sulfometuron-methyl/3-rimsulfuron mixtures. Recovery values were between 94 and 116% for sulfometuron-methyl and between 99 and 118% for 3-rimsulfuron. Satisfactory recoveries were also obtained for the remaining 2 mixtures (data not shown); they ranged between 90 and 104% for concentration ratios of sulfometuronmethyl/metsulfuron-methyl from 50:1 to 1:1 and between 91 and 117% for concentration ratios of sulfometuron-methyl/chlorsulfuron from 4:1 to 1:1. The method seems more suitable for the determination of 3 sulfonylureas (chlorsulfuron, metsulfuron-methyl, and 3-rimsulfuron) in the presence of sulfometuron-methyl because
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a greater concentration of the last component can be tolerated in the mixture. However, for real environmental water samples in which lower herbicide concentrations (≤0.05 ng/mL) may be found (28), a preconcentration and/or a purification step is needed before analysis. Conclusions The usefulness of the proposed method for quantitating binary mixtures of sulfonylurea herbicides with good sensitivity and selectivity has been well demonstrated. Although the PIF-1D method suffers from some limitations due to the constraining optimum analytical conditions required to perform the analyses, its application is very simple. Future developments for improving the PIF determination of herbicide mixtures, based on multivariate statistical techniques, are expected. Acknowledgments We thank Du Pont de Nemours and Co., Inc. for the kind gift of herbicide samples. A. Coly thanks the University of Paris 7 for financial support during his stay in Paris. References (1) Brown, H.M. (1990) Pestic. Sci. 29, 263–281 (2) Wells, M.J.M., & Michael, J.L. (1987) J. Chromatogr Sci. 25, 345–350 (3) Howard, A.L., & Taylor, L.T. (1992) J. Chromatogr. Sci. 30, 374–382 (4) Nilvé, G., Knutsson, M., & Joensson, J.A. (1994) J. Chromatogr. A 688, 75–82 (5) Zahnow, E.W. (1982) J. Agric. Food Chem. 30, 854–857 (6) Prince, J.L., & Guinivan, R.A. (1988) J. Agric. Food Chem. 36, 63–69 (7) Slates, R.V. (1988) J. Agric. Food Chem. 36, 1207–1211
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