Planar solid phase extraction clean-up and microliter-flow injection analysis–time-of-flight mass spectrometry for multi-residue screening of pesticides in food

For multi-residue analysis of pesticides in food, a sufficient clean-up is essential for avoiding matrix effects in liquid and gas chromatography (LC and GC) analysis coupled to mass spectrometry (MS). In the last two years, high-throughput planar solid phase extraction (HTpSPE) was established as a new clean-up concept for pesticide residue analysis in fruits and vegetables (C. Oellig, W. Schwack, 2011) and tea (C. Oellig, W. Schwack, 2012). HTpSPE results in matrix-free extracts almost free of interferences and matrix effects. In this study, a time-of-flight mass spectrometer (TOFMS) was applied to directly analyze HTpSPE extracts for pesticide residues. This HTpSPE–microliter-flow injection analysis (μL-FIA)–TOFMS approach detects all pesticides at once in a single mass spectrum, without a liquid chromatographic separation step. Complete sample information was obtained after the injection of the cleaned extract within a single peak. Recovery studies for seven representative pesticides in four different matrices (apples, red grapes, cucumbers, tomatoes) provided mean recoveries of 86–116% with relative standard deviations of 1.3–10% (n = 5) using the mass signal intensities under the entire sample peak. Comparing the mass spectra of sample peaks from spiked extracts and solvent standards indicated the efficiency of HTpSPE clean-up. A pesticide database search detected all spiked pesticides with a low incidence of false-positives. HTpSPE of one sample required a few minutes, and numerous samples could be cleaned in parallel at minimal cost with low sample and solvent consumption. The μL-FIA–TOFMS screening then needed an additional 6 min per sample. The novel screening approach was successfully applied to QuEChERS extracts of several real samples, and the pesticides identified by HTpSPE–μL-FIA–TOFMS were identical to the pesticides detected by common target LC–MS/MS analyses. The high degree of concordantly identified pesticides by the new developed HTpSPE–μL-FIA–TOFMS approach and target LC–MS/MS demonstrates the applicability as a routine screening method.