In situ derivatisation and extraction of volatile fatty acids entrapped on anion-exchange resin from aqueous solutions and urine as a test matrix using pentafluorobenzyl bromide in supercritical carbon dioxide

The simultaneous extraction and derivatisation of anion-exchange resin-trapped volatile fatty acids (C-2-C-5) as their pentafluorobenzyl esters has successfully been performed under CO2 supercritical fluid extraction conditions. Volatile fatty acid standards of acetic, propionic and n-butyric acids (at 20 and 100 ppm) as their ester derivatives were recovered at 78.0-101.5% (CN. 3.5-7.5%, n = 6 and 7). Likewise, acrylic acid recoveries were 57.0-61.0% (C.V. 5.5-5.6%, n = 6 and 7). This methodology was applied to the quantitation of acetic, propionic, n-butyric and n-valeric acids in spiked urine as a test matrix. Initial clean-up of phosphate and sulfate in the urine was required prior to anion-exchange application and this was achieved by barium salt precipitation. Recoveries ranged from 36 to 66.5% (C.V. 5.9-14.4%, n = 9 and 6).