Trace-level determination of pesticide residues using on-line solid-phase extraction-column liquid chromatography with atmospheric pressure ionization mass spectrometric and tandem mass spectrometric detection

Column liquid chromatography (LC) with pneumatically assisted electrospray (PA-ESP) or atmospheric pressure chemical ionization (APCI) followed by (tandem) mass spectrometry (MS or MS-MS) was used for the analysis of a test mixture of 17 pesticides. In order to achieve low-ng/l detection limits, solid-phase extraction (SPE) of a 100-ml aqueous sample on a small cartridge packed with a hydrophobic sorbent was used. The LC set-up was coupled on-line to the MS part of the system. The complete analysis was automated by means of a gradient controller and a Prospekt valve switching, solvent selection and cartridge exchange unit. When using SPE-LC with either APCI or PA-ESP, the detection limits of 15 (out of the 17) pesticides in tap water were 0.007-3 mu g/l in the full-scan and 0.1-200 ng/l in the SIM mode, with an analysis time of 65 min. Fenchlorphos and bromophos-ethyl could not be detected by either ionization method. APCI full-scan spectra showed much less sodium and acetonitrile/water cluster adducts than PA-ESP spectra. Negative ion (NI) operation was less sensitive for the majority of the compounds tested (73 in total), but several organophosphorus pesticides, nitrophenols and chlorophenols only gave a response in the NI mode. PA-ESP-MS-MS and APCI-MS-MS gave similar product-ion spectra from protonated molecules; an MS-MS library was built for more than 60 pesticides and their degradation products, at constant settings of collision gas pressure (argon, 2.0x10(-3) Torr) and collision energy (25 eV). The library was successfully used for searching product-ion spectra from SPE-LC-APCI-MS-MS at low levels (10 ng/l) in tap water and for the identification of atrazine in surface water (estimated concentration 0.25 mu g/l).