GC-MS Determination of Acidic Herbicides in Water with Prederivatization

Water sample was adjusted to pH<2 by adding H2SO4 (1+2) dropwise and extracted for 3 times with dichloromethane. The extracts were combined, dehydrated with anhydrous acidic sodium sulfate, concentrated to 1 mL and evaporated to near dryness by N2-blowing. The residue was taken up with acetone and derivatized with pentafluorobenzyl bromide with K2CO3 as catalyst. The derivatized product in n-hexane solution was purified on polar silica gel column. Interfering by products were removed by elution with mixture of toluene and n-hexane (1+6) and the objective compound was then eluted from the column with mixture of toluene and n-hexane (9+1), which was used for GC-MS analysis after adding octafluorobiphenyl as internal standard. The capillary chromatographic colume HP-5MS was used for separation, and the negative chemical ionization source with SIM mode was adopted in MS. Values of detection limit (3S/N) for the 14 acidic herbicides were all below 10 ng·L-1. Recovery of the 14 herbicides was tested and the results found were from 66.0% to 117.0%, with values of RSD′s (n=7) less than 9%.