A rapid gas chromatographic procedure for the analysis of oxalate ion in urine

A specific, precise, rapid and sensitive method for the analysis of oxalic acid in urine is described. This microanalytical procedure can detect concentrations of urinary oxalic acid as low as 4 mg/l with a relative standard deviation of replicates of less than 5% (24-h urine collections). 10% BCl3:2-chloroethanol is used to derivatize 0.1 ml of urine after evaporation to dryness or lyophilization. The bis-2-chloroethyl ester of oxalic acid formed is extracted into ethyl acetate/isopropyl ether (1:3, v/v) and is detected by electron capture gas chromatography at the 50 pg level. The total analysis time for 35 samples in 8 h per GC column. A series of determinations of urinary oxalic acid in 21 kidney stone-forming patients resulted in values ranging from 10.6 to 42.0 mg/24 h with a mean recovery of added oxalic acid of 98.2% (range 84.0-111.3%).