The optical stability of chiral 2-hydroxy-2-phenylacetic acid in electron ionization mass spectrometry(EIMS) process has been detected directly by deuterium labelling technique. From the EI mass spectrum of deuterated 2-hydroxy-2-phenylacetic acid, the major characteristic fragment ion at m/z 108(the capture of the carbonyl group) and the corresponding isotopic ion(13C) at m/z 109 can be observed, and the ratio of m/z=109 to m/z=108 is about 8%(the calculated value is 7.8%). As enolization of deuterated 2-hydroxy-2-phenylacetic acid in mass spectrometry can yield the characteristic fragment ion at m/z 109 and results in much higher ratio of 109/108 than 8%; this study shows that no enolization takes place during the EI-MS process of 2-hydroxy-2-phenylacetic acid. This results can be concluded that these compounds are optically stable in the process of instantaneous vaporization at high temperature and electron impact ionization.
