A very straightforward method for the analysis of the very polar organophosphorus pesticides (OPs), acephate, methamidophos, omethoate, oxydemeton-methyl, vamidothion, and monocrotophos in water has been developed. Large volume (1 ml) water samples were directly injected onto a RP18 HPLC column with a polar endcapping. The compounds were ionized using APCI and detected on a tandem mass spectrometer operated in multiple reaction-monitoring mode. The detection limits are in the range of 0.01 0.03 μg/l. The method is applicable for analysis of polar OPs in surface, ground, and drinking water. Compared to conventional GC methods, the developed LC/MS procedure is very straightforward, fast, and more reliable.
The OPs investigated are thermally labile and/or very polar and therefore less GC-amenable. Moreover, especially acephate, methamidophos, monocrotophos, and omethoate are extremely water soluble and, in contrast to most other pesticides that are quoted in literature as highly polar (e.g. oxamyl, acidic pesticides, dichlorvos, mevinphos, dimethoate), not extractable (after the adequate pH adjustment) using the more common LLE or SPE procedures.
Extraction efficiency of polar OPs using SPE:
MS/MS chromatogram of a surface water sample found positive on acephate. Acephate was detected at a level of 0.2 μg/l. The trace of methamidophos (peak indicated with arrow, approx. 0.03 μg/l) can be expected since methamidophos has been identified as a major metabolite of acephate.
For the very polar OPs included in this work, LC/MS/MS proves to be a more favorable alternative to GC analysis. The method is very straightforward, highly sensitive and selective. After filtration and acidification, water samples are dire ctly injected onto a LC-MS system. By direct injection of 1 ml, the LOD (S/N = 3) varied between 0.01 and 0.03 μg/l depending on the analyte and applied resolution. It may be possible to further lower the LOD by injection of even larger volumes, directly on the analytical column or by (on-line) SPE applying a pre-column packed with the same stationary phase.
Journal of Chromatography A, 918 (2001)67-78