Simultaneous determination of acidic and non-acidic pesticides in natural waters by liquid chromatography-mass spectrometry

There is increasing interest and demand for real multi-residue methods able to simultaneously determine pesticides with a broad spectrum of chemical characteristics in environmental and biological matrices. A method based on solid-phase extraction with a Carbograph 4 cartridge and liquid chromatography with electrospray mass spectrometry (LC-ES-MS) enabling simultaneous determination of non-acidic and acidic pesticides in real water samples is described. On repeatedly (n=5) extracting 4 l of drinking water (spike level 50 ng/l), 2 l of ground water (spike level 100 ng/l) and 1 l of river water (spike level 200 ng/l), recovery of 26 base/neutral pesticides and 13 acidic pesticides were equal to or better than 80%, except for carbendazim (67%), butocarboxim (73%), aldicarb (75%) and molinate (77%). Relative standard deviations ranged between 4 and 15%. Final extracts containing acidic and non-acidic pesticides were analyzed in a single chromatographic run while the ES-MS system was operated in both positive and negative ion modes. With the aim of finding the best operating conditions, in terms of sensitivity, the pH of the LC eluent was varied in the 2.9-8.4 range. Altogether, the best results were obtained by using an LC eluent containing 1 mmol/l formic acid. Over the entire pH range considered, well shaped peaks for both basic and acidic analytes were achieved by the use of a new generation LC column. By extracting selected ion current profiles from the total ion current mass chromatogram relative to analysis of 4 l of drinking water spiked with 50 ng/l of each of the 39 analytes, estimated limits of detection ranged between 0.05 and 1.5 ng/l, except for propyzamide (8 ng/l) and 2,4-DB (3 ng/l).