Liquid-chromatographic measurement of amino acids in biological samples after formation of phenylthiohydantoin derivatives.

In this method for measuring amino acids in urine, serum, and tissue, the amino acids to be assayed and the internal standard, L-norleucine, are converted to phenylthiohydantoins , isolated by organic solvent extraction, separated by reversed-phase "high-pressure" liquid chromatography, and detected by ultraviolet absorbance. The analytes are identified by retention times and quantified by comparing peak heights with that of the internal standard. The peak-height ratios vary linearly with concentrations of 50 to 500 mg/L, corresponding to the concentration range usually found in biological samples. Detection limits are 5 to 20 mg/L. Inter- and intra-assay precision (CV) varies between 1 and 26%. Average analytical recoveries range between 67 and 100%.