Determination of chlorophenoxy acid herbicides in water by in situ esterification followed by in-vial liquid-liquid extraction combined with large-volume on-column injection and gas chromatography-mass spectrometry

A new approach for rapidly analysing chlorophenoxy acid herbicides in water is presented. The chlorinated acids are derivatised with dimethyl sulphate in the water sample itself (800 mu l) and, next, the methyl esters are ext racted with 800 yl of n-hexane. A 200-mu l volume of the extract is injecte d into the GC-MS system. The miniaturisation of both the methylation and ex traction steps could be implemented because of the use of large-volume on-c olumn injection and mass spectrometric detection. The optimisation of the m ethylation reaction for the simultaneous determination of (3,6-dichloro-2-m ethoxy)benzoic acid. (2-methyl-3-chlorophenoxy)- and (2,4-dichlorophenoxy)a cetic acids, (+/-)-2-(4-chloro-2-methylphenoxy)- and 2-(2,4-dichlorophenoxy )propanoic acids and 4-(4-chloro-2-methylphenoxy)- and 4-(2,4-dichloropheno xy)butyric acids showed that tetrabutylammonium salts act as catalysts. Add ition of sodium hydroxide was required to obtain quantitative reaction yiel ds for 4-(4-chloro-2-methylphenoxy)- and 4-(2,4-dichlorophenoxy)butyric aci ds. The methylation-cum-en traction procedure takes only 3 min per sample f or a batch of seven samples. Linear calibration plots were obtained for the complete procedure and the limits of detection were of 10-60 ng/l with a s ignal-to-noise ratio (SIN) of 6. Relative standard deviations ranged from 8 to 15% (n=7) for analyte concentrations of 0.5 mu g/l in surface water. (C ) 2000 Elsevier Science B.V. All rights reserved.