Analysis of chlorothalonil and degradation products in soil and water by GC/MS and LC/MS
Introduction
Chlorothalonil (2,4,5,6-tetrachloroisophthalonitrile, Fig. 1), a non-systemic foliar fungicide, is widely applied for the control of a variety of fungal diseases in different commodities, including agricultural crops, turf and ornaments (Tomlin, 1994). In the United States, it accounts for approximately 15% of all the fungicide use (by weight) and is predominantly applied on peanuts (34%), tomatoes (12%) and golf courses (10%) (US EPA, 1999).
Several studies have been published on the degradation of chlorothalonil (Sato and Tanaka, 1987, Rouchaud et al., 1988, Katayama et al., 1992, Katayama et al., 1997, Motonaga et al., 1996, Regitano et al., 2001, Sakkas et al., 2002, Kwon and Armbrust, 2006). The compound 4-hydroxy-2,5,6-trichloroisophthalonitrile (metabolite II, Fig. 1) has been reported as the predominant metabolite in environmental samples (Ballee et al., 1976, Rouchaud et al., 1988, Ukai et al., 2003). This metabolite appears to be more persistent, mobile, and toxic than the parent compound, resulting in suppression of soil microorganisms (van Doorn et al., 1995, Hamish and Jones, 1997, Ukai et al., 2003). Other metabolite products cited in the literature are the result of reactions leading to the substitution of Cl atoms in chlorothalonil and conversion of the CN functional groups to amides, thiazoles and acidic groups (Fig. 1) (Rouchaud et al., 1988, Regitano et al., 2001, Putnam et al., 2003, Kwon and Armbrust, 2006).
Chlorothalonil has been routinely analyzed by gas chromatography after isolation via liquid–liquid extraction or solid-phase extraction (SPE) (Ballee et al., 1976, El Nabarawy and Carey, 1988, Rouchaud et al., 1988, Voulvoulis et al., 1999, Albanis et al., 2002). More recently, there has been an increased interest in the analysis of degradation products, due to an increased awareness of their potential toxic effects. However, due to the polar nature of these compounds, most metabolites cannot be analyzed by GC due to their thermal instability and/or low volatility. In many cases, this results in a need for derivatization, and in more labor-intensive methods.
In this study, we have modified and developed new procedures for the analysis of chlorothalonil and several of its metabolites in water and soil samples using GC and LC techniques in order to identify more suitable alternatives for the simultaneous analysis of chlorothalonil and metabolites. For the gas chromatography–mass spectrometry (GC/MS) procedures, the methods reported by Ballee et al., 1976, Rouchaud et al., 1988, Putnam et al., 2003 were modified and improved. In our methods, off-line SPE using disposable extraction cartridges was substituted for liquid–liquid extraction (LLE). Despite this advance, the analytical methods remained very labor intensive; requiring derivatization and resulting in low recoveries of the very polar 4-hydroxy-2,5,6-trichloroisophthalonitrile, due to unspecific losses during the concentration step.
Liquid chromatography with mass selective detection (LC/MS) is currently used for the analysis of polar pesticides, including carbamates, organophosphates, triazines, and chlorinated phenoxyacids among others, giving adequate detection limits for residue analysis (Wang and Budde, 1981, Jablonska et al., 1993, Niessen, 1995, Baglio et al., 1999, Van der Heeft et al., 2000). LC/MS is especially powerful for the direct analysis of polar compounds, which are subject to thermal decomposition or do not have adequate vapor pressure for GC separations. Based on the type of interface, LC/MS is also capable of providing structural information to confirm identification of analytes in a sample (Barceló et al., 1996).
For the LC/MS methods, ionization of the compounds was studied using both atmospheric pressure chemical ionization (APCI) and electrospray (ESI) in the positive (PI) and negative (NI) ionization modes and found to be suitable for sample analysis. Operational parameters were optimized using flow injection analysis (FIA) and analytes quantified in the selected ion monitoring mode (SIM) using internal standards. For each analyte, diagnostic fragment ions were tentatively identified. The methods developed in this study were used to quantify chlorothalonil and metabolites in soil and channel water samples from a banana plantation as part of a major research project on the fate of chlorothalonil in tropical environments (Chaves et al., 2007).
Section snippets
Experimental section
Details of the chemicals and reagents used, and the sample preparation procedures are provided in Supplementary material. Fig. 2, Fig. 3 summarize the methods for the analysis of chlorothalonil and metabolites in water samples. First, a 100-ml sample is extracted using solid-phase extraction disks (OASIS® HLB) after adjusting the pH of the water sample to acidic, to extract group I compounds (chlorothalonil, pentachloronitrobenzene and metabolites II, 2,5,6-trichloro-4-methoxyisophthalonitrile
LC/MS characteristics
In this study, ionization and fragmentation patterns of each of the analytes in Fig. 1 are obtained using APCI and ESI in the positive and negative ionization modes. The operational parameters of the APCI and ESI interfaces were optimized by flow injection analysis (FIA) of a standard solution of each compound at 100 and 1 μg ml−1, respectively. The mobile phase was a mixture of acetonitrile and water (1:1, v/v). The most relevant parameters were the corona current and the capillary voltage.
Conclusions
This research demonstrates the applicability of liquid chromatography–mass spectrometry for the simultaneous analysis of chlorothalonil and degradation products in environmental samples. OASIS® HLB cartridges proved to be adequate for the recovery of all the compounds, even the more polar ones like metabolite II. The high recoveries together with low detection limits and very good precision, allowed the quantification of chlorothalonil and several of its degradation products at levels that are
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