Validation of a QuEChERS-Based Gas Chromatographic Method for Multiresidue Pesticide Analysis in Fresh Peppermint Including Studies of Matrix Effects

Abstract

A convenient analytical method for quantitative characterization of 46 pesticides in fresh peppermint (a plant with high chlorophyll content) has been developed. The plant samples were extracted with acetonitrile and then with graphitized carbon black (GCB), and primary–secondary amine (PSA) was added to dispersive solid phase extraction tubes in order to reduce chlorophyll content in the resultant extracts for the purpose of improving analytical performance in terms of recovery, precision, linearity and measurement uncertainty. The analytical method was established by using the technique of gas chromatography coupled with electron capture and nitrogen phosphorus detection (GC/ECD/NPD). Samples spiked with the target pesticides at the concentration level 0.004 mg/kg and higher levels (depending on the compound) yielded average recoveries in the range of 70–120 % with relative standard deviations (RSDs) 0–15 %, except for chlorothalonil, folpet, imazalil and spiroxamine and displayed very good linearity (R 2 > 0.99) for nearly all of the analytes. The matrix effect (ME) was evaluated for 17 compounds through the study of ratios of the slopes obtained in the solvent and peppermint matrix. Percentage values of the ME (% ME) were in a range of −25 % (bifenthrin) to 15 % (azoxystrobin); however, matrix effects for the vast majority of pesticides were small, showing suppression or enhancement in the range from 0 to ±7 %. Expanded measurement uncertainties were estimated, with the employment of a top-down empirical model, as being between 6 and 24 % and yielding an average value of 15 % (coverage factor k = 2, confidence level 95 %). This is significantly less than the maximum threshold value of 50 % recommended by European Union guidelines, which clearly demonstrates suitability for the purpose of the proposed method.