[Determination of plasma oxalic acid by high-pressure liquid chromatography]

A new specific and sensitive method for determination of oxalic acid in plasma by High Performance Liquid Chromatography (HPLC) is described. The plasma sample is deproteinized by ultrafiltration. The oxalic acid in the ultrafiltrate is purified by precipitation with CaCl2, new dilution of calcium oxalate precipitate, oxalic acid extraction with diethyl-ether and total dryness of the sample. The losses of oxalic acid during this process are evaluated by the addition of oxalic acid (U-14C) before the precipitation step. The dried samples are redissolved in mobile phase (o-H3PO4, 0.05 M) and injected into a HPLC chromatograph, with reversed phase column (Lichrosorb RP-8, Merck). Oxalate peak is detected spectrophotometrically at 220 nm with a retention time of 3.20 minutes. The method shows a mean recovery value of 82.11, with an intra-run and between-run CV values of 2.54 and 6.95 respectively. The oxalic acid measured in plasma by this method is 291 +/- 89 micrograms/100 ml plasma ultrafiltrate, in 16 normal subjects.