We performed high resolution, high mass accuracy measurements of the peptide mixture. The m/z of the a1CT-Y+X peptide was determined to be 2803.1029 ± 0.0103 Da from four replicate measurements. Note that this is the monoisotopic mass, while the earlier number, 2805, was average mass. The difference between the de-tyrosinated peptide, a1CT-Y, and the unidentified peak (a1CT-Y+X), is 106.979 ± 0.0102 Da. Therefore, we considered that the difference of m/z 106.979 represents the mass of the modification being observed. If we were to consider that the signal at 2803.1029 was a result of only a loss from the parent a1CT peptide, the difference in mass is 56.085 ± 0.015 Da which could not be clearly assigned. Therefore, we considered that the m/z 2803.1029 represented a modified version of the de-tyrosinated peptide.