The lactam of N,S-dicarboxymethylcysteine is usually not present inbatch samples. The influence of the solutions pH on the formation of the lactam of N,S-dicarboxymethylcysteine were investigatedby an HPLC-UV method and nuclear magnetic resonance (NMR)spectroscopy. The cyclization of N,S-dicarboxymethylcysteine wasmonitored by the appearance of the additional signals for theN CH2COOH group resonating at ı = 4.51 and 3.22 ppm. Thestructure is supported by the peak of m/z 218 in the mass spectrum.In conclusion, carbocisteine and N,S-dicarboxymethylcysteine arerapidly cyclized in acidic solution (the peak area of the lactam dou-bles within 30 min) and more stable in slightly basic solutions (seeFig. 5). As a consequence the sample solution has to be preparedin dilute ammonia solution and immediately before injection. Acyclization on-column is likely, but would not cause additionalpeaks; the lactam is formed continuously, so that the baselinewould rise slightly until the API is eluted. This was not observedand is therefore not considered to be an issue.