Determination of oxalates in plasma and urine using gas chromatography

A gas-chromatographic procedure for the analysis of oxalic acid is described. The procedure requires relatively small quantities of urine (1 ml) or plasma (5 ml). The procedure consists of three steps: (1) extraction of oxalic acid by acidified diethyl ether; (2) esterification by isopropanol; and (3) final analysis by gas chromatography.The oxalic acid was found to have an elution temperature of 107°C and a retention time of 14 min. The standard curve is linear up to 800 nmol. In the described condition the lower limit of detection is 20 nmol.The mean normal plasma and urine oxalate levels ($\text{x}\text{?}\text{± 2}\text{S.D.}$) were found to be 20 μmol/l ± 17.5 and 275 μmol/24 h ± 200 respectively.