[A highly sensitive assay method of oxalic acid in human plasma using high performance liquid chromatography with...

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.