An LC-MS method was developed for the analysis of four compounds produced in the preparation of L-phenylalanine by enzymatic synthesis procedure. The mass spectrometric conditions were investigated in detail. Under optimal conditions,linear relationships existed between peak areas and concentrations of pyruvic acid,L-aspartic acid,phenylpuruvic acid,and L-phenylalanine with correlation coefficients of 0.9995,0.9984,0.9999,and 0.9991,respectively. The average recoveries were 98.8%~103%,and the RSDs were 4.3%...