I am working on quantification of Amino acids like L-Lysine HCL & Cysteine in Hairbless tablet using Waters Amino acid analysis method by using Waters Xbridge Column ( 150*4.6mm id, 3.5micron) and mobile phases AccQ tag A eluent concentrate (WAT052890) , Mobile phase B , AccQ tag A eluent concentrate (186003839) and AccQ. Tag Ultra Derivatization (Kit Catalogue No. : 186003836) but i observed that there is continuous decrease observed in area of L-Cystine while L-Lysine area observed very constant with v. good % RSD. Area decrease observed in Cystine is in Mixed standard as well as test sample. If you have experienced this application then please share your experiences.
Since this was waters Validated method that's why we have not tried nitrogen purging. since this degradation occurs inside HPLC Autosampler, so its not possible to provide nitrogen purging to sample solution. Decrease in area of cysteine occurs not only in test samples but it also observed in standard solution peak.
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