Laboratory guides
What ESI-MS establishes about peptide identity
What ESI-MS contributes to peptide characterization, how m/z differs from mass, and why a match does not establish purity or quantity.
ESI-MS measures ions’ mass-to-charge ratios. A mass compatible with a declared structure supports identity assessment, but does not alone establish complete sequence, stereochemistry, purity or amount per vial. Conclusions depend on selectivity and complementary evidence.
Distinguish m/z from molecular mass
The horizontal axis of a raw spectrum is usually m/z. A peptide can produce different charge states, so several peaks can belong to the same species. Deconvolution estimates molecular mass from those signals.
Before comparing values, establish whether the report gives neutral or ionic mass and whether the convention is monoisotopic or average. Retain the assumed chemical form and modifications. No single mass tolerance applies to every instrument and method.
What each type of evidence contributes
| Evidence | What it contributes | What it does not establish alone |
|---|---|---|
| Compatible intact mass | Agreement with an expected mass under the method’s acceptance criterion. | Complete residue order or discrimination among every isomer. |
| LC-MS data | Connection between chromatographic and mass-spectral signals. | That each ion intensity is a mass fraction. |
| MS/MS fragmentation | Additional information supporting sequence assignment or variant localization. | Automatic resolution of every structural ambiguity. |
| Complementary tests | Discrimination of alternatives unresolved by the first method. | A conclusion beyond their established analytical scope. |
Why a matching mass is not the whole identity
Different structures can share formula and exact mass. Increasing mass accuracy does not by itself remove that ambiguity. The method must discriminate the relevant alternatives; otherwise complementary evidence is needed.
ICH Q2(R2) discusses identification using specific properties and appropriate reference materials, including related substances that may interfere. Read a result within that scope rather than as universal confirmation of structure.
What to request from the laboratory
Pureza proposes this checklist to avoid assigning conclusions to a report that it does not contain.
- Sample and batch identification, method, instrument and reference used.
- Expected and observed mass, mass convention and acceptance criterion.
- A readable spectrum and explained assignments; retain original data when deconvolution is used.
- The scope of identification: intact mass, sequence supported by fragments, or additional tests.
- Where related to HPLC, evidence linking the signal to the assigned peak rather than an assumption based on a file name.
Identity is not quantity
An identification spectrum does not convert ion height into milligrams per vial. Quantification requires a method designed for that purpose, with suitable calibration and demonstrated performance.
Seeing only the expected mass also does not establish a purity percentage. Review the chromatographic test and its scope, and request an independent content determination when the protocol requires it.
Scope of the cited methods
ICH Q2(R2), sections 3.1 and 3.1.2, supplies a pharmaceutical analytical framework for selectivity and identification, not approval of these materials. Thermo Fisher’s peptide-mapping documentation describes LC-MS and LC-MS/MS evidence, while its Xtract documentation explains deconvolution and monoisotopic-mass interpretation.
Questions and answers
Do several ESI-MS peaks mean contamination?
Not necessarily. Charge states, isotopes or adducts can generate several signals from one species. Assignment requires the data and method; additional peaks should not automatically be dismissed as harmless either.
Does MS/MS prove the entire sequence unambiguously?
Not always. Support depends on observed fragments, coverage and the alternatives the method can distinguish. Report the assignment’s limitations.
Does a compatible mass prove vial purity?
No. It supports mass agreement for a detected species. Purity and quantity require separate measurements and interpretation.