a b s t r a c tThe components of seven matrices in the analysis of 11 pesticides by GC-ECD were analysed. The matrix effect was calculated based on the changing of chromatographic response of the analyte in the presence of co-extractives of the matrices in the organic phase obtained by solid-liquid and liquid-liquid extraction with partition at low temperature (ESL-PBT and ELL-PBT), in relation to the response of it in the pure solvent. It was used the Principal Component Analysis (PCA) in evaluating the results obtained for the percentages of the matrix effect. The tomato, grape and pineapple matrices caused greater matrix effect and were grouped. The other matrices such as apple, water and potato caused small matrix effect. For most pesticides the soil matrix caused negative matrix effect. The influence of pH of the samples on the matrix effect was also evaluated showing not to have a direct effect on the phenomenon.

This work describes the optimization and validation of a method employing solid-liquid extraction with low temperature partitioning (SLE/LTP) together with analysis by gas chromatography with electron capture detection (GC/ECD) for the determination of nine pesticides (chlorothalonil, methyl parathion, procymidone, endosulfan, iprodione, λ-cyhalothrin, permethrin, cypermethrin, and deltamethrin) in lettuce. The method was found to be selective, accurate, and precise, with means recovery values in the range of 72.3-103.2%, coefficients of variation ⩽ 12%, and detection limits in the range 0.4-37 μg kg(-1). The matrix components significantly influence the chromatographic response of the analytes (above 10%). The optimized and validated method was applied to determine the residual concentrations of the fungicides iprodione and procymidone that had been applied to field crops of lettuce. The maximum residual concentrations of the pesticides in the lettuce samples were 13.6 ± 0.4 mg kg(-1) (iprodione) and 1.00 ± 0.01 mg kg(-1) (procymidone), on the day after application of the products.

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