On-fibre silylation following solid-phase microextraction for the determination of acidic herbicides in water samples by gas chromatography Original Research Article

In this work a solid-phase microextraction (SPME) method for the determination of eight phenoxy acid herbicides and dicamba in water samples has been developed. Analytes are concentrated on a SPME fibre, on-fibre silylated using N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide (MTBSTFA), and determined by gas chromatography with mass spectrometry detection (GC–MS). Influence of microextraction and derivatization conditions on the performance of the developed method are systematically evaluated. The highest enrichment factors were achieved using a polar polyacrylate (PA) fibre directly exposed to the stirred sample. Sample acidification and addition of sodium chloride improved the yield of the extraction step; however, the last factor increased the variability of the method. After the microextraction step, derivatization was carried out by exposing the SPME fibre to the headspace of a vial containing 50 μl of MTBSTFA. The on-fibre silylation reaction was completed in 10 min without need of temperature control. Quantification limits from 0.004 to 0.030 ng/ml were obtained for all compounds using an extraction step of 40 min. The proposed method showed and excellent linearity in the concentration range from 0.1 to 10 ng/ml. Repeatability was evaluated using spiked samples at different concentration levels, relative standard deviations from 4 to 12% were achieved. The yield of the whole procedure was scarcely affected by the type of water sample and the presence of relatively high levels of humic acids.