01Compare residue lists before comparing percentages

The EMA's synthetic-peptide guideline describes deletions arising from incomplete coupling or deprotection. Its terminology also distinguishes truncated products associated with capping of unreacted sites. These are descriptions of chemical process possibilities, not findings about an untested research vial.EMA — Synthetic peptide guideline, section 4.3.2 (opens in a new tab)

These examples describe sequence relationships rather than measured materials. An internal omission and a chain that stopped early can share the broad feature of being shorter while implying different assembly histories. Keep the exact sequence comparison visible.

02An omitted step can be followed by successful additions

In a conceptual assembly, a chain can miss an intended residue yet remain able to accept a later one. The resulting sequence carries the omission forward. This differs from a chain that is permanently terminated and no longer participates in extension.

A 2025 primary investigation of peptide-pool impurities reported traces of deletion peptides and discussed incomplete coupling and deprotection as possible sources. Its observations concern that analysed pool; they do not provide a universal frequency for deletion impurities in all synthetic peptides.Investigation of Impurities in Peptide Pools (opens in a new tab)

Reading a final impurity structure may suggest a plausible process history, but a suggestion is not a direct observation of the manufacturing event. Establishing the cause can require process records or intermediate measurements. Do not turn a sequence assignment into an unsupported claim about a supplier's procedure.

03A matching loss may leave positional ambiguity

For otherwise comparable linear peptides with the same terminal chemistry, removing a residue changes the elemental composition by that residue's contribution within the chain. It is not generally correct to subtract the mass of an isolated free amino acid without accounting for the peptide-bond composition.

Repeated residues create a second ambiguity. In the invented target Ala-Gly-Ser-Gly-Leu, loss of the first glycine and loss of the second glycine give different sequences but the same total elemental change. An intact mass difference alone cannot identify which position was omitted.

Zeng and colleagues investigated LC-HRMS for peptide impurity analysis and reported sequence-related impurities that were not resolved by particular submitted HPLC-UV methods. That study supports checking what a method can distinguish, rather than assuming one chromatographic peak represents one sequence.Zeng and colleagues — LC-HRMS peptide quality analysis (opens in a new tab)

04Keep presence, identity and amount separate

A report may first flag a signal compatible with a deletion, then establish a more detailed identity through additional evidence. Those are different confidence levels. Preserve words such as proposed or confirmed instead of upgrading a tentative assignment in a summary.

Quantifying an identified deletion is another step. A percentage of detector response should retain that measurement basis rather than being silently rewritten as the same percentage of molecules or sample mass.

• ✓Write the target and proposed deletion sequences together.
• ✓Check repeated residues for positional ambiguity.
• ✓Distinguish an identified product from an inferred cause.
• ✓Keep the analytical basis beside any reported percentage.