Intact mass answers an important but bounded question
An observed mass consistent with the theoretical material supports identity, subject to instrument accuracy, charge-state interpretation and supplied-form assumptions. It does not always distinguish every sequence isomer or closely related impurity.
Sequence-level evidence can reduce first-lot risk
For longer, modified or high-risk peptides, tandem mass spectrometry or another sequence-appropriate method may add useful fragment evidence. Method suitability depends on the molecule and the decision; more data is not automatically better data.
- Theoretical mass for the supplied form
- Observed mass and acceptance window
- Modification and counter-ion assumptions
- Fragment coverage where requested
- Reference standard or orthogonal evidence
Write the requirement into the RFQ
If sequence verification is required, state it before quoting. Retrofitting a method after production can change sample needs, time and cost.