Quantitative gas chromatographic determination of low-molecular-weight straight-chain carboxylic acids as their p-bromophenacyl esters after extractive alkylation in acidic medium

Abstract

The measurement of acetate by gas chromatography at low concentrations in biological fluids is a difficult task. A significant improvement can be brought about by derivatization of acids as p-bromophenacyl esters. By such a procedure, the determination of low-molecular-weight straight-chain carboxylic acids (acetic, propionic and butyric acids) was enhanced with respect to sensitivity and specificity. The volatile fatty acids were esterified by means of an extractive alkylation mechanism in acidic conditions from ageous solutions. Tetrahexylammonium was used as counter-ion and and the alkylating agent was a, p-dibromoacetophenone. The phenacylation reaction was studied with respect to pH, concentration of counter-ion and kinetics. The yield of derivatization was ca. 70% after 2 h at 42°C. A mass spectrometry study was performed in order to ascertain the structure of derivatives using electron impact and chemical ionization. The gas-liquid chromatrographic behaviour of the p-bromephenacyl esters of acetic, propionic and butyric acids was studied on an OV-225 packed column and an OV-1 capillary column. Flame ionization detection was shown to be linear between 0.2 and 4 nmol injected, with a quantitative limit of detection of 31 pmol ( i.e. 1.8 ng of acetic acid).