Flow-injection analysis of serum urea using o-phthalaldehyde and naphthylethylenediamine

An analytical procedure is developed for the determination of urea using flow injection analysis. The methodology is based on the color that develops (lambda(max), 517 nm) when urea reacts with o-phthalaldehyde in the presence of naphthylethylenediamine under acidic conditions. Calibration curves are found to be linear up to 51 mg urea N/dl when the FIA manifold is operated at 42 degrees , utilizing 90-mul samples and a flow-rate of 0.44 ml/min. By manual injection up to 40 samples can be analysed per hour. Recovery yields varied between 95-99%. The within day CV was 0.5-2.2% whereas the day to day CV was 2.1-3.9%. When applied to the analysis of urea in serum samples, excellent correlations (0.998) are obtained when the FIA results are compared with those obtained for the same serum samples analysed by the free urease/Berthelot's and the hospital method (employing the Abbot Bichromatic Analyser). Interferences are observed with sulphur compounds such as sulphanilamide (0.76 mg/dl) as well as with many amino acids but at relatively high concentrations (21 mg/dl).