Determination of acetic acid in aqueous samples, by water-phase derivatisation, solid-phase microextraction and gas chromatography

The direct derivatisation of acetic acid with n-hexyl chloroformate and wit h benzyl bromide in water was evaluated. With n-hexyl chloroformate, acetic acid did not give the n-hexyl acetate derivative, but the reaction of acet ic acid with benzyl bromide in aqueous solution resulted in the formation o f benzyl acetate. The derivatisation of acetic acid with benzyl bromide and the headspace solid-phase microextraction (SPME) of benzyl acetate were op timised. Under optimum conditions, the limit of detection for acetic acid w as 260 nM, and the relative standard deviation of the overall procedure at 1.10(-4) M acetic acid was 15.6% (n = 10). A linear response was obtained i n the 1.10(-4) to 5.10(-6) M concentration range (R-2 = 0.993, n = 6). Alth ough Carbowax-divinylbenzene (CW-DVB)-coated fibres exhibited a higher extr action capacity for benzyl acetate, polyacrylate (PA) was selected, because its mechanical stability was better than that of CW-DVB fibres. Moreover, the relative standard deviation of the SPME was better with PA (1.5%. n = 1 0 at 1.10(-5) M) than with CW-DVB-coated fibres (8.0%, n = 10 at 1.10(-5) M ). Thus, a new analytical method for the quantitative determination of micr omolar concentrations of acetic acid in the aqueous phase was developed. Th is method is based on water-phase derivatisation with benzyl bromide, heads pace SPME with PA fibres and GC-FID. It was observed experimentally that be nzyl alcohol formed by hydrolysis of the reagent affected the fibre-gas pha se partitioning of benzyl acetate. (C) 2000 Elsevier Science BN. All rights reserved.