Laboratory guides
What ESI-MS establishes about peptide identity
A mass match supports part of identification under the method used. The conclusion must retain the sample link and state ambiguities the measurement cannot resolve, rather than asserting complete identity from one number.
Source editorial review:
Compare compatible quantities
In ESI-MS, an observed m/z value is not directly comparable with molecular mass without considering charge and ion assignment. Check whether the report presents individual ions, deconvoluted mass or both, and retain the tolerance used by the method.
The theoretical reference must correspond to the declared identity, including relevant modifications. Copying mass from a page with a similar name can produce a comparison that is arithmetically correct but chemically wrong.
Case: compatible mass, an unresolved question
If the question requires a complete sequence or distinguishing species the method cannot resolve, a compatible intact mass may be insufficient. Assess whether additional evidence, such as appropriate fragmentation analysis, is needed. Do not expand the technique’s scope merely because the expected result appears.
Finish with a testable statement
State which sample, observation and reference support the conclusion. Avoid turning “compatible mass” into “confirmed purity”: the second question requires its own determination. Catalog pages help locate the declared material; they do not replace the sample spectrum.
A record for this decision
Which identity claim does the spectrum support, and which ambiguities remain?
| Field | What to document |
|---|---|
| Reference | Theoretical mass, declared formula or sequence and modifications. |
| Observation | Ions, charges, adducts and deconvoluted mass when reported. |
| Resolution | Method tolerance and species it cannot distinguish. |
When to leave the decision open
A mass match alone does not demonstrate complete sequence, stereochemistry or purity. State the conclusion at the level supported by the method.
Questions and answers
Does a mass spectrum measure vial purity?
Not automatically. Identity and purity are different questions governed by the analytical method’s scope.
Can I use a spectrum from another lot of the same product?
It can be a comparative reference, but not a result for the sample or lot being evaluated.