Determination of amino acids in biological fluids by capillary gas chromatography with nitrogen-phosphorus selective detection.

A selective and sensitive method for the determination of protein and non-protein amino acids in biological fluids by capillary gas chromatography (GC) has been developed. The amino acids in the samples were directly converted into their N(O,S)-isobutoxycarbonyl methyl ester derivatives and measured by GC with nitrogen-phosphorus selective detection (NPD) using a DB-17ht capillary column. Using this method, the derivatives of the 21 protein amino acids and the 25 non-protein amino acids provided excellent NPD responses and were quantitatively and reproducibly resolved within 28 min. The lower detection limits of these amino acids, at a signal-to-noise ratio of 3, were ca. 6-150 pg injected. The calibration curves for each amino acid in the range of 0.02-2 micrograms were linear and sufficiently reproducible for quantitative analysis. This method was successfully applied to small urine and serum samples without prior clean-up; there was no evidence of interference from coexisting substances. Overall recoveries of amino acids added to urine and serum samples were 83-112%. The intra-assay and inter-assay R.S.D. of amino acids in these samples were 0.3-8.9% (n = 3) and 1.9-15.8% (n = 3), respectively.