We report on an enzymatic-chemical method for the specific preparation of L-amino acid-derived quinoxalinols suitable for the sensitive estimation of ({ }^{13} \mathrm{C}) - or ({ }^{2} \mathbf{H})-label enrichment by gas chromatography-mass spectrometry: Amino acid fractions are isolated from physiological fluids by ion-exchange chromatography. The compound of interest is converted to the corresponding 2-oxo acid by treatment with an L-amino acid dehydrogenase of the desired specificity. Reaction of the 2-oxo acid with o-phenylenediamine yields the quinoxaline derivative. Isotopic label enrichment is then determined by gas chromatographic-mass spectrometric analysis of the (O)-trimethylsilyl derivative using an ammonia-chemical ionization mode and selected ion monitoring of the quasi-molecular ions ([M H]^{+})and ([\mathrm{MH}+\boldsymbol{n}]^{+})( (\boldsymbol{n}=) number of labeled positions). Details for application of various generally available L-amino acid dehydrogenases (L-ala DH, L-glu DH, Lleu DH, L-phe DH) are presented. The method was used, e.g., for serum analysis in in vivo studies on the decarboxylation rates of branched-chain (\mathrm{L}-\left[1-{ }^{13} \mathrm{C}\right]) amino acids. c 1995 Academic Press, Inc.