Reliability of methods for extraction of vegetables and clean-up for selective ion monitoring of selected organophosphate and N-methylcarbamate insecticides

The reliability of two multiresidue methods (MRMs) for 11 organophosphates and N-methylcarbamate insecticides (INs) in 3 vegetables or VEGs (corn, navy beans and spinach) were determined and compared. MRM 1 involved acetonitrile extraction followed by florisil clean-up and MRM 2, ethyl acetate-acetone extraction and reverse phase C₁₈ clean-up. INs were measured using an internal standard method and GC-MS selective ion monitoring (SIM). Reliability was determined by percent recovery (PR), coefficient of variation (CV) and sensitivity. The PR of each MRM was dependent upon IN, VEG and method. Each MRM was precise (<±15% CV) for each IN in each VEG. Naled could not be analyzed by either MRM in any VEG (PRs <8); carbofuran, carbaryl, fenamiphos and azinphos-methyl could not be measured by MRM 1 (PRs < 21). Six INs: phorate, dyfonate, disulfoton, methyl parathion, malathion and chlorpyrifos in all VEGs could be analyzed accurately because the PRs were between 80 or 110 or by multiplying levels by a correction factor or CF (PR/100). MRM 2 analyzed accurately the same 6 INs as did MRM 1 plus the 4 INs: carbofuran, carbaryl, fenamiphos and azinphos-methyl not measurable by MRM 1. CFs are necessary for some of these latter INs in MRM 2 for accurate results. All of the INS measured accurately by MRMs 1 and 2, except for azinphos-methyl, could be measured by SIM in spinach extracts near or below the allowable levels. Thus, MRM 2 was the best method for the most INs.