Matrix effect in gas chromatographic determination of insecticides and fungicides in vegetables

A study of the matrix-induced effect was performed for 16 common pesticides, most frequently found in monitoring studies in tomato, pepper and cucumber, using a simple multiresidue method with gas chromatography (GC) and electron-capture (ECD) or nitrogen-phosphorus (NPD) detection, without a previous cleanup step. Anomalously high gas chromatography responses and subsequently very high recoveries for several pesticides in the extracts were obtained by a conventional calibration with pesticide solution in ethyl acetate. Sample matrix enhancement varied from little to no effect for some pesticides (e.g. chlorpyrifos, pirimicarb) to >200% in the case of certain susceptible pesticides (captan, procymidone, iprodione). Pronounced matrix effects were observed at low concentration levels of analyte for all the ECD-detected pesticides. The use of matrix-standards solutions was found to reduce the recoveries of most pesticides to the levels of 70-110% acceptable for residue analysis.