| A large number of pesticides were used to increase the production of fruits and vegetables, the harm to people health and natural environment which cased by pesticides residue have attracted more and more attentions, therefore, it was important to establish a rapid, stable and high efficient analysis method to determine pesticide multi-residues.As a high sensitivity, fast and stable technique, LC-MS/MS was widely used to analyze pesticide residues in fruits and vegetables, but the accuracy was affected due to the widespread existence of matrix effects. So it was helpful to find a fast, easy method to reduce matrix effects in LC-MS/MS system. This study was consisted of two parts as following:1. Reducing matrix effect of all pesticides in fruits and vegetablesThe matrix effects in 6 matrices (apple, cucumber, tomato, luffa, cabbage and eggplant) for all pesticides were determined divided into three levels (weak, medium, strong). The influence of injection volume on the peak separation, sensitivity and linear range in an LC-MS/MS method were investigated. Different from traditional liquid phase chromatographs, using a large injection volume (usually 10 μL or larger) does not improve the sensitivity but lead to peak tailing and retention time drift in LC-MS/MS system.According to the dependence of matrix effect in three matrixes (apple, cucumber and cabbage) on the injection volume, four relationships between signal suppression and injection volume were observed. For most analyte/matrix combinations, the matrix effects could be reduced to negligible level (≤20%) if the injection volume was less than a critical value. For samples which were of strong signal suppression (such as cabbage), matrix effects cannot be reduced to negligible level even injection volume was decreased, then 10 dilution factor for matrix extraction was used and we found after dilution, MEs can also be reduced to negligible level by decreasing injection volume. Moreover, the LODs and LOQs of compounds were barely changed after dilution.2. Determination of pesticide multi-residues in vegetables and fruits using LC-MS/MSA simplified QuEChERS sample preparation method coupled with LC-MS/MS were used to determine the residues of multi-pesticides in 6 matrices (apple, cucumber, tomato, luffa, cabbage, and eggplant). The QuEChERS method consisted of sample extraction using acetonitrile-formic acid (99.9:0.1, v/v). Inorganic salt NaCl and MgSO4 were used to remove water and separate organic phase from aqueous. The sample purification did not employ other sorbents (such as PSA, GCB, C18). On the basis of the first study, the injection volume was 1 μL. It was shown that the recoveries at 10,50, and 100 μg/kg levels were in the range of 70%-120% with intraday relative standard deviation (RSD)<5%(n=5), interday RSD<15% (n=5) for the all pesticides. The LOQs of compounds were all lower the maximum residue limits (MRLs) established by China and European Union. |
