Usefulness of micellar media for the quantitative analysis of phenylurea herbicides in water by photochemically-induced fluorescence

Analusis, 1999, 27, 857-863 © EDP Sciences, Wiley-VCH 1999 Usefulness of micellar media for the quantitative analysis of phenylurea herbicides in water by photochemically-induced fluorescence A. Bautista2, J. -J. Aaron1*, M. C. Mahedero2 and A. Muñoz de la Peña2 1 Institut de Topologie et de Dynamique des Systèmes de l'Université Paris 7 Denis Diderot, CNRS, UPRES-A 7086, 1, rue Guy de la Brosse, 75005 Paris, France 2 Department of Analytical Chemistry, University of Extremadura, Badajoz, Spain Abstract. UV irradiation of four non-fluorescent phenylurea herbicides including linuron, diuron, isoproturon and neburon is shown to yield fluorescent photoproducts. The photochemically-induced fluorescence (PIF) properties of these herbicides in several media (water, 2-propanol and their mixtures) and aqueous micellar solutions of sodium dodecyl sulfate (SDS), and cetyltrimethylammonium chloride (CTAC) are reported. The use of micellar media enhances significantly the PIF signal relative to an aqueous solution. A PIF method is developed for the determination of the four herbicides under study, with linear dynamic ranges over about one order of magnitude, and limits of detection (LOD) between 410 and 640 ng mL-1, according to the compound. Applications to the analysis of tap water and river water samples yield satisfactory recoveries (86-115 %). Keywords. Phenylurea herbicides – micellar media – photochemically-induced fluorescence – water analysis. Introduction tion coupled with FIA has been proposed for the determination of a phenylurea derivative (diflubenzuron) and other aromatic pesticides [15-17]. Therefore, the combination of the PIF technique with dynamic systems such as HPLC and FIA is making progress and a great development is expected in the future. Phenylurea herbicides are widely used for the control on non-crop areas as well as for selective pre- and post-emergence weed control on crops. As a consequence, they constitute important environmental pollutants. Therefore, the development of very sensitive methods for the residue analysis of these compounds is needed. No systematic identification of the exact nature of photoproducts formed during the pesticide photolysis reactions has been reported. However, some authors have found that, in the photodegradation pathways of phenylurea herbicides, one of the photoproducts formed is methylamine or dimethylamine [18,19]. In this respect, it was also reported that aniline and substituted anilines present fluorescence spectra similar to those of the phenylurea photodegradation products, suggesting that similar structures may be responsible for the fluorescence response observed [16]. A number of pesticides can be directly analysed by gas chromatography (GC) with various detectors. However, the urea derivatives undergo generally thermal decomposition during the analysis [1-4]. As a consequence, the direct application of GC to phenylurea herbicides is not possible because these compounds are thermally unstable, and derivatization prior to detection is requested. For this reason, high performance liquid chromatography (HPLC) with UV absorption or fluorescence detection [5-8] is preferred to GC. The proposed HPLC methods, however, suffer from a lack of specificity [9,10]. In other GC studies, the urea derivatives were hydrolysed into the corresponding anilines and then derivatized for sensitive electron capture detection [11-13]. In spite of their great analytical interest, micellar media have been seldom applied to the fluorescence or PIF analysis of pesticides [20-22]. Recently, we have proposed the use of aqueous micellar solutions to enhance the PIF signal and improve the detection of fenvalerate and deltamethrin [21] and some sulfonylurea herbicides [22]. Recently, we have applied photochemically-induced fluorescence (PIF) spectrometry to the determination of some aromatic insecticides and resolution of their binary mixtures [14,15]. In the analysis of a variety of pesticides, including neburon, HPLC postcolumn photolysis and fluorogenic labelling with OPA/2-ME, was also utilized [16]. PIF detec- In this paper, as a continuation of our preliminary PIF study of phenylurea herbicides in various solvents [23], we examine the usefulness of aqueous micellar media for PIF analysis of these compounds and the possible advantages of these media with respect to plain aqueous solution. We describe here a PIF method for the determination of four * Correspondence and reprints. Received June 18, 1999; revised September 9, 1999; accepted September 28, 1999. 857 Article available at http://analusis.edpsciences.org or http://dx.doi.org/10.1051/analusis:1999154 Original articles phenylurea herbicides including linuron, diuron, isoproturon and neburon (Fig. 1), based on their photolysis reaction in anionic and cationic micellar media. The method is applied to the quantitative analysis of these herbicides in natural water samples. software. An unfiltered Osram 200 W HBO high-pressure mercury lamp with an Oriel Model 8500 power supply was utilized for photolysis reactions. The photochemical set-up included a light-box consisting of a fan, the mercury lamp and a quartz lens. A standard Hellma (Mullheim, Germany) 1-cm pathlength quartz fluorescence cuvette was placed on an optical bench at 30 cm from the mercury lamp. The solutions were magnetically stirred during the UV irradiation. All spectrofluorimetric measurements were carried out at 20 ± 1°C, using the thermostated cell holder and a Landa Model K4R thermostatic bath. Procedure Solution preparation Stock solutions of the phenylurea herbicides (5 × 10-3 M) were prepared by dissolving the compound in 2-propanol. Working solutions were obtained by appropriate dilutions with distilled water. All solutions were protected against light with aluminium foil. Stock solutions of CTAC (0.01M) and SDS (0.1M) were prepared with distilled water. Working solutions were obtained by serial dilutions. Micellar solutions of the herbicides were prepared by transferring 25 or 50 µl of 2-propanolic working standard solution into a 5 mL flask, and adjusting to the marker with micellar solution, 1 mL of the optimal pH buffer solution (in some experiments) and distilled water. All working micellar aqueous solutions contained less than 1 % (v:v) of 2-propanol. Analytical measurements and photolysis reaction The herbicides under study did not exhibit any native fluorescence, but when herbicide micellar or aqueous solutions were irradiated with UV light, a fluorescent emission was obtained. Fig. 1. Structure of the phenylurea herbicides under study. For the PIF analytical method, an aliquot of the micellar solution was placed in a quartz cuvette and irradiated at room temperature with UV light for a fixed time. Curves of fluorescence intensity versus UV irradiation time were constructed, at the analytical excitation (λex) and emission (λem) wavelengths of the herbicide photoproduct, using different (...truncated)