Optimisation of a gas chromatographic–mass spectrometric method for the determination of phenoxy acid herbicides in water samples as silyl derivatives

A procedure for the determination of nine phenoxy acid herbicides in water samples using gas chromatography in combination with mass spectrometric detection is presented. Analytes were concentrated from water samples using a solid-phase extraction (SPE) polymeric sorbent, functionalised with N-vinylpyrrolidone. Compounds were eluted from the SPE cartridge using ethyl acetate and silylated with N-methyl-N-(tert-butyldimethylsilyl)trifluoracetamide (MTBSTFA). The influence of several factors on the yield of the silylation reaction and on the stability of the obtained compounds is described. Under optimal conditions derivatisation was performed at room temperature, for 5 min, using 100 (mu)l of MTBSTFA. The obtained derivatives were stable for at least 5 days after preparation. Using a sample intake of 1000 ml, the quantification limits of the proposed method ranged from 1–4 to 4–12 ng l^-1 for surface and sewage water, respectively. Recoveries between 80 and 120% were obtained depending on each particular compound and the kind of water sample considered.