Here the 1xSuccinimide D PTM has a mme of 32.3 ppm. This high mme is a result of the reduced spectral intensity created by the increased MS resolution and its low relative intensity to that of unmodified VP3. Peptide mapping revealed 3 peptides with a Succinimide D modification with an abundance ≥ 1% (Table ), all within the VP3 region, though no individual peptide containing the Succinimide D PTM had a relative percent abundance above 6%. Full-length mass analysis indicated that this PTM had a relative abundance of 16-19% unmodified VP3, depending on the column temperature used during LC separation. The exact reason for this discrepancy is not clear. A potential explanation is that the presence of succinimide PTMs below 1% were excluded by our threshold during peptide mapping and that their abundances could generate a greater presence of succinimide than reported. Additionally, the highly acidic nature of the mobile phases due to the presence of DFA might be initiating on column sample deamidation, but the highly acidic environment is more favourable to the succinimide intermediate than the deamidation product. However, further investigation into this is required.