Determination of trace amounts of oxalate in renal calculi and related samples by gas chromatography - Mass spectrometry

A GC-MS method has been developed for determination of trace amounts of oxalate in renal stones. The steps involved are: solubilisation of the oxalate in renal stones with HCl, elimination of cationic species by use of a cation-exchange resin, evaporation of water, reconstitution with ethyl acetate, and formation of trimethylsilyl esters of oxalic acid with chlorotrimethylsilane. Trichloroacetic acid was used as internal standard. With appropriate sample treatment the method has also been applied to urine and food. The main analytical features of the method were: linear range 0.2-5.0 mg oxalate L -1 , limit of detection 0.06 mg oxalate L -1 (concentrations in the solution chromatographed), coefficient of variation of the method 1.3%. Twenty-seven renal stones were analysed by the method. The average oxalic acid content of uric acid calculi was 0.11 mg g -1 (SD = 0.08, n = 13). A larger amount of oxalate was found in struvite calculi (mean value 0.2 mg oxalic acid g -1 , SD = 0.13, n = 8). Hydroxyapatite stones can be classified according to their oxalate content: samples with very low oxalate content (below the LOD of the method) and samples with large amounts of oxalate (0.431 and 0.801 mg oxalic acid g -1 ).