Kinetic-enzymatic determination of oxalate in urine by flow-injection analysis with double stopped flow

An enzymatic stopped-flow injection method for the determination of oxalate is proposed. Two sequential haltings of the flow are made, the first while the sample plug is in a reactor packed with controlled-pore glass and immobilized oxalate decarboxylase, and the second on merging of the sample plug with a stream containing an auxiliary enzyme (formate dehydrogenase) as the plug reaches the flow cell, where absorbance-time data are collected. The linear range of the calibration graphs is between 0.1 and 3.0 mM oxalate and their sensitivity depends on the interval over which measurements are made. The proposed method was applied to the determination of oxalate in urine without separation of the analyte by precipitation, with excellent results.