Assay for plasma free phenylalanine and tyrosine by high-pressure liquid chromatography and field desorption mass spectrometry and its application to anin vivo study of the phenylalanine hydroxylating system in man

Abstract

An analytical assay for the determination of free amino acids in human plasma based on high‐pressure liquid chromatography and field desorption mass spectrometry has been developed. Compared with assays using gas chromatography and mass spectrometry, this method appears to be advantageous with respect to the analysis of polar and/or labile amino acids. Using stable isotope labelled analogues as internal standards quantifications by field desorption can be performed on about 250 pmol of individual amino acids as dansyl derivatives, corresponding to about 10 μl of human plasma. The practicability of the assay is demonstrated by a stable isotope in vivo study of the phenylalanine hydroxylating system in man.