Finding and confirming nontargeted pesticides using GC/MS, LC/quadrupole-time-of-flight MS, and databases

Two methods are described for identifying unknowns in complex matrixes. One approach, suitable for nonpolar and thermally stable compounds, uses GC/MS with a deconvolution technique, followed by a spectral library search. The second approach, suitable for polar and thermally labile compounds, uses LC/quadrupole-time-of-flight (LC/Q-TOF) MS with a Molecular Feature Extractor (MFE), followed by a database search on the exact mass of each found compound. Two example analyses are presented to illustrate the GC/MS method. The first example processes data files obtained by a multiresidue analysis of pesticides in surface water. In this example, 71 pesticides were identified using Deconvolution Reporting Software in conjunction with Automated Mass Spectral Deconvolution and Identification System software. The second example illustrates the deconvolution approach in detail using peaches and pears analyzed after extraction by the quick, easy, cheap, effective, rugged, and safe (QuEChERS) protocol. The LC/Q-TOF approach is illustrated through analysis of a grape extract prepared by the QuEChERS protocol. The resulting total ion chromatogram was found by the MFE to contain 510 chemicals (components). The masses of found chemicals were subsequently searched against an Exact Mass Database of hundreds pesticides, and 15 exact mass hits were found. Several of these exact mass hits were selected for further confirmation by MS/MS analysis using the same LC/Q-TOF.