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

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.