An assay method for the plasma level of oxalic acid (OA) was developed by using high-performance liquid chromatography (HPLC). Low molecular weight carboxylic acids including OA were separated from interfering plasma components by passing through ultrafilter (Centriflow CF 25, Amicon) and a Sep-Pak C18 cartridge (Waters), and OA was extracted with tri-n-butyl phosphate. The OA in the organic layer was converted to a fluorescent substance by the esterification with 9-anthryldiazomethane (ADAM). This reaction mixture was injected into a HPLC apparatus with a fluorophotometric detector. In the experiment using standard OA, a linear relationship was obtained in concentrations ranging from 0.2 to 2.0 micrograms/ml. The detection limit of this method was 0.1 micrograms/ml, and the coefficient of variation was 2.0%. The method developed in the present study is considered to be useful as a routine assay method for the human plasma OA level.
Anthracenes Chromatography, High Pressure Liquid