Simultaneous determination of traces of pyrethroids, organochlorines and other main plant protection agents in agricultural soils by headspace solid-phase microextraction–gas chromatography

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Abstract

A solvent-free and simple method based on headspace solid-phase microextraction (HS-SPME) was developed in order to determine simultaneously 36 common pesticides and breakdown products (mostly pyrethroids and organochlorine compounds) in soil. The analysis was carried out by gas chromatography with micro-electron-capture detection (GC-μECD). As far as we know, this is the first study about the SPME of pyrethroid insecticides from soil. Factors such as extraction temperature, matrix modification by addition of water, salt addition (% NaCl) and fiber coating were considered in the optimization of the HS-SPME. To this end, a 3 × 23−1 fractional factorial design was performed. The results showed that temperature and fiber coating were the most significant variables affecting extraction efficiency. A suitable sensitivity for all investigated compounds was achieved at 100 °C by extracting soil samples wetted with 0.5 mL of ultrapure water (0% NaCl) employing a polyacrylate (PA) coating fiber. Using the recommended extraction conditions with GC-μECD, a linear calibration could be achieved over a range of two orders of magnitude for both groups of analytes. Limits of detection (LODS) at the sub-ng g−1 level were attained and relative standard deviations (RSDs) were found to be lower than 14% for both groups of pesticides. Matrix effects were investigated by the analysis of different soil samples fortified with the target compounds. The method accuracy was assessed and good recovery values (>70%, in most cases) were obtained. The method was also validated with a certified reference material (RTC-CRM818-050), which was quantified using a standard addition protocol. Finally, the proposed HS-SPME-GC-μECD methodology was further applied to the screening of environmental soil samples for the presence of the target pesticides.

Introduction

Of all human activities, agriculture is perhaps the closest to nature. Pesticides are widely used in farming for their economic benefits to control crop pests and reduce competition from weeds, thereby improving yields and ensuring the quality of crop production. However, their use does involve risk because most have inherent properties that can make them dangerous to health and the environment if not used properly. Along with the soil erosion and the physical deterioration, the diffuse pollution is regarded as priority by most of the European countries. Soil is the principal reservoir of these plant protection products, which residues can be released into the atmosphere, plants and living organisms. Some pesticide residues and their metabolites can also be transported to ground and surface waters via leaching and runoff processes [1], [2].

Organochlorine pesticides (OCPs) are known to be one of the most persistent organic pollutants in the environment. Although they have been prohibited in developed countries since 1970s due to their toxicity and tendency to accumulate in living organisms, they have been recently detected in soils [3], [4], [5], [6]. Many of these organochlorine compounds were replaced by organophosphorus insecticides (OPPs), which, in general, are more rapidly transformed in the environment to less toxic species than OCPs. Despite this behaviour, OPPs have been found in real soil samples collected in agricultural areas [3], [7], [8]. At the end of 1990s, pyrethroid insecticides accounted for a fourth of the world insecticide market and they have gained importance due to their low mammalian toxicities and their high toxicities for insects even at low doses. Many of these compounds are strongly adsorbed on soil particles and do not leach from the application point due to their low solubility in water and their high lipophility [9]. The detection of pyrethroid residues in soils has also been recently reported [3].

The regulation of contaminated sites is receiving significant attention in the European Union (EU) and, in September 2006, the European Commission adopted a comprehensive EU strategy dedicated to soil protection. Nevertheless, only nine EU Member States have current specific legislation related to soil protection (especially on diffuse contamination) [10]. Spain is one of these countries in which the government introduced, in 2005, a regulation for assessing and managing the contaminated soils. According to this directive, a soil is declared contaminated when unacceptable risks for human health (or ecosystem) protection will be identified due to the presence of compounds listed in the 9/2005 Spanish Royal Decree (most of them being organochlorine and aromatic compounds). Generic reference levels according to the soil use (industrial, urban or other) were proposed in order to assess the soil pollution [11].

The complete list of the pesticides studied in the present work can be found in Table 1. It mostly includes organochlorine and pyrethroid pesticides, although other main plant protection compounds (organophosphorus, chloroacetanilides) have also been investigated. Generic reference levels in soil established by the Spanish legislation for human health protection are also shown [11].

The classical methodologies used for the determination of pesticides in solid matrices based on agitation procedures and Soxhlet extraction have been replaced by less time- and solvent-consuming techniques [12] involving high-pressure and/or high-temperature processes such as: microwave-assisted extraction (MAE) [13], [14], supercritical fluid extraction (SFE) [3], pressurized liquid extraction (PLE) [15], [16], or ultrasonic extraction (USE) [8], [17]. The complete removal of organic extracting solvents can be achieved by employing solid-phase microextraction (SPME). This technique is based on the use of a coated fiber to extract traces of organic compounds from the matrix, followed by desorption of the retained substances into an analytical instrument [18]. SPME has been successfully employed for the analysis of pesticides in various matrices (water, food, biological fluids). Furthermore, its application to the analysis of soil matrices brings many benefits such as simplification in sample handling and absence of additional clean-up procedures [19]. Several classes of pesticides (carbamates, triazines, organophosphorus and organochlorine compounds) have been then determined in soil samples by SPME coupled both with liquid (HPLC) [20], [21] and gas (GC) [4], [5], [6], [7], [22], [23] chromatography. In some of these studies, the extraction of soil samples was achieved by the preparation of a mixture of the soil with distilled water and subsequent immersion of the SPME fiber on this slurry [20] or by dilution with distilled water of the organic extract obtained by other liquid extraction technique such as USE [7], MAE [4] or microwave-assisted micellar extraction (MAME) [21]. In other studies, easier SPME procedures allowing the determination of pesticides in soil samples by a HS-SPME technique have been developed [5], [22], [23]. Finally, cold activated carbon fiber SPME (CACF-SPME) was also developed to analyze OCPs in soil [6].

The aim of this study was the development of a HS-SPME-GC-μECD procedure for the simultaneous determination of 36 pesticides (see Table 1) in soil. To our knowledge, no previous works dealing with the SPME extraction of pyrethroids from soil have been reported. The main parameters affecting the SPME procedure were investigated and optimized using a fractional factorial design. Finally, the analytical methodology was validated and applied to real soil samples.

Section snippets

Reagents and materials

Tefluthrin, transfluthrin, allethrin (mixture of stereoisomers), tetramethrin, λ-cyhalothrin, cyphenothrin (mixture of cis and trans isomers), permethrin (mixture of cis and trans isomers), cyfluthrin (mixture of isomers), flucythrinate, fenvalerate, chlorpyrifos and acetochlor were of Pestanal grade and were provided by Riedel-de Häen (Seelze, Germany). Fenitrotion and alachlor were purchased from Dr. Ehrenstorfer (Augsburg, Germany). A standard mix solution containing organochlorine

Preliminary experiments

First experiments were conducted to optimize the chromatographic separation of the more than thirty target analytes. A satisfactory chromatographic separation in a short analysis time was achieved after testing several GC temperature programs (see the final selected conditions in Section 2). Some of the investigated pyrethroids (such as cyphenothrin, cyfluthrin and cypermethrin) gave isomeric peak clusters and they were quantified as the sum of isomers.

In the present work, the headspace

Conclusions

HS-SPME was employed to develop a quick and simple multiresidue method for the determination of 36 pesticides (mostly pyrethroids and organochlorines) in soil. As far as we know, there are no reports about SPME of pyrethroids from this matrix. The developed method did not require previous treatment of the samples, implying a drastic reduction of working time and organic solvent consumption. Water addition and sample heating were needed to enhance the analyte release from the soil matrix. To

Acknowledgements

This research was supported by FEDER funds and projects PGIDIT05RAG50302PR (Xunta de Galicia, Spain) and CTQ2006-03334 (CICYT, Ministerio de Ciencia y Tecnologia, Spain). M.F. would like to acknowledge her doctoral grant to the CICYT.

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