Quantitative analysis of beta-phenylpyruvic acid by single ion monitoring. Evaluation of isomeric internal standards.

Quantitative single ion monitoring of beta-phenylpyruvic acid at high sensitivity is possible after derivatization first with omicron-phenylenediamine and then with a silylating reagent. The resulting o-trimethyl-silyl-quinoxalinol (O-TMS-Q) has previously been shown to be highly stable during storage and on chromatography. As an internal standard the isomeric omicron-methylphenylglyoxylic (omicron-toluylformic) acid is introduced. The mass spectra of both O-TMS-Q's are characterized by abundant [M]+. at m/e 308. The concept of "class specific metabolic profiling" is discussed in relation to quantitative gas chromatography--mass spectrometry detection of aliphatic and aromatic alpha-ketoacids.