Screening method for the determination at parts per trillion levels of pesticide residues in vegetables combining solid-phase microextraction and gas chromatography-tandem mass spectrometry

A new analytical method is proposed for determining residues of 70 pesticides of different chemical families at parts per trillion levels in fresh vegetables. For that, only 4 g of the vegetable samples were quickly extracted with 10 ml of ethyl acetate. The method is based on a vanguard/rearguard strategy that reduces the average time required per sample when the method is applied to a high number of vegetable samples in a quality control laboratory. At the beginning, an aliquot of the extract is evaporated and re-dissolved in a mixture water: acetone (9: 1 v/v). For screening purposes, the pesticides were extracted for only 10 min by direct immersion of a solid-phase microextraction (SPME) fiber (65 mu m polydimethylsiloxane-divinylbenzene,PDMS-DVB). The SPME device was automated and on-line coupled to a gas chromatograph with an ion trap mass spectrometer (GC-MS) operated in full scan mode for screening in less than 18 min those samples that potentially contain pesticides above 0.01 mg kg(-1) (cut off value). After that, only those potentially non-negative samples were reanalyzed by a sensitive quantifying/confirming method that re-extract by SPME the pesticides in 55 min of absorption and determine them by GC with tandem MS (MS/MS). The method has been validated following EU guidelines and compared with a conventional extraction method based on the use of higher amounts of organic solvents. The limits of detection (LOD), confirmation (LOC) and quantitation (LOQ) as well as the calibration curves obtained allowed the determination of the target pesticides at concentrations clearly below the maximum residue levels (MRL) stated by EU being possible the determination of parts per trillion of the pesticides in ecological (green) vegetables. The method has been applied to the analysis of real samples and the results compared with those obtained by a conventional extraction method accredited by ENAC (Spanish Accreditation Body). The proposed method was also evaluated participating in a proficiency test with adequate results (z-score among +/- 2).