“…Especially the p-bromo-substituted phenylthiohydantoins characterized by a significant double peak of a mass difference of 2 amu for the molecular ion peak facilitated identification and interpretation of the relevant mass spectra. Mass spectrometry is superior to other methods of analysis based on thin-layer chromatography [10,11] or gas-liquid chromatography [12,13], since identification is directly correlated to individual mass numbers of the phenylthiohydantoin under investigation. Electron impact mass spectrometry, however, suffered from the difficulty of cysteine identification.…”