Superheated water extraction and phase transfer methylation of phenoxy acid herbicides from solid matrices

Phase transfer catalytic methylation was applied to directly derivatise chlorophenoxy acid herbicides in superheated water extracts from sand and soil samples. The extractions were carried out at 120 °C statically for 5 min and then dynamically for 10 min at 1.0 mL min−1 using water at pH 11.0 for a sand matrix and a flow rate of 0.5 mL min−1 at pH 7.0 for soil samples. The methylation was carried out on-line on the extraction solution with ultrasonication at 80 °C, using either 0.05 mmol tetrabutylammonium bromide (TBAB) or 0.0125 mmol cetyltrimethylammonium bromide (CTAB) as phase transfer catalysts with 0.20 mmol methyl iodide in 2.0 mL dichloromethane trapping solvent. The former catalyst provided a higher yield but the latter gave fewer interfering peaks. The recoveries of most chlorophenoxy acids using the TBAB catalyst ranged from 67 to 105% for sand and from 82 to 114% for soil sample, except phenoxyacetic acid, 2-(2, 4-dichlorophenoxy)propanoic acid and 1-naphthaleneacetic acid, while those by using CTAB were slightly lower. Detection limits of all the analytes extracted from sand using TBAB catalyst were in a range of 5.3–16 μg g−1 analysed by using gas chromatography with flame ionization detection (GC–FID).