Chapter 4 - Matrix Effect for Determination of Pesticide Residues in Tea

Abstract

To better understand the matrix effects in pesticide multi-residue analysis in tea, matrix effects of tea with different origin and type were studied, and methods using the combination of analyte protectants to overcome matrix effects in GC–MS and GC–MS/MS analysis of pesticide residues in tea matrix were proposed. In this thesis, 28 varietals of tea which are the products of national geography symbol were selected to be studied for the matrix effects in GC–MS and GC–MS/MS analysis. Chemometric methods were used to classify the teas. Tea samples were divided into three clusters: the first cluster consisted of not fermented and slightly fermented tea samples; the second cluster were composed of semi-fermented tea samples; the third cluster consisted of fully fermented and post-fermented teas. Any type of tea can be chosen from each cluster as corresponding representative matrix within this cluster to make matrix-matched solutions. The result provides a good theory basis for the selection of representative matrix for tea. The representative tea matrix could simplify the operation of the experiment while guarantee the accuracy of the experiment. Different analyte protectants (APs) were evaluated to find the most suitable ones for improving chromatographic quality of the signal. The influence of the APs on the peak shape and abundance were studied. The concentration and combination of different APs were also evaluated. A method to compensate for the matrix effects in the determination of 186 pesticide residues in tea by GC–MS using analyte protectant was proposed. A mixture of triglycerol and D-ribonic acid-γ-lactone (both at 2 mg/mL in the injected samples) was found to be the most effective in improving the chromatographic behavior of 186 pesticides. The proposed method was quick and convenient, with good sensitivity and precision and the limit of quantification could reach the residue analysis requirements. The influence of the APs combination on the stability of the instrument was also investigated. The combination of APs resulted in ruggedness improvement and consequently, reduced the maintenance for GC. As compared to matrix-matched standardization, the analyte protectant approach offers a more convenient solution to the problems associated with calibration in routine analysis of pesticide residues and reducing need for frequent maintenance. A GC–MS/MS analytical method was developed for simultaneously determining 205 pesticides in tea matrices using analyte protectants to counteract the matrix-induced effect by the same evaluation method of GC–MS. A mixture of triglycerol and D-ribonic acid-γ-lactone (both at 2 mg/mL in the injected samples) was found to be the most effective APs. The linearity, sensitivity, accuracy, and precision of the method were validated. The combination of APs improved the ruggedness of GC–MS/MS. The category and concentration of the APs in the proposed methods of GC–MS and GC–MS/MS were the same which means that the difference of the detector is not the main influence of the matrix effect. The matrix effects of tea are mainly caused by the gas chromatography system.