Determination of urinary oxalate by ion chromatography: some modifications

Ion chromatography was used to determine urinary oxalate concentration. The minimal detectable limit in a standard solution was 0.02 microgram/ml, and the regression line for the standard curve from 0.5 to 10 micrograms/ml had a significant correlation coefficient (p less than 0.01), but it was difficult to obtain reproducible results with ion chromatography. There were two obstacles: Urine was always oversaturated with calcium oxalate, and the presence of sulfate, the peak of which is next to the oxalate peak on the chromatogram of urine, seemed to interfere with oxalate conductivity. Comparison of 4 different dilutions of urine showed that the oxalate conductivity was highest in the 100-fold dilution and decreased gradually with increasing concentration. The conductivity of the oxalate standard (1 microgram/ml) also decreased as the concentration of sulfate increased. Therefore, it was concluded that to obtain reproducible results urine should be diluted 100-fold and that a recovery test by adding 0.25 microgram/ml oxalic acid should be performed on all urine samples.