Mass spectrometry identity for peptides: ESI-MS explained

HPLC tells you how pure a peptide is. It does not tell you which peptide it is. That question is answered by mass spectrometry, and a COA without a mass-spec result is, at best, half a certificate. This note explains what electrospray ionisation mass spectrometry (ESI-MS) measures, how to read the result on a COA, what a mass discrepancy means, and the limits of the technique. All Alphex products are for laboratory and in-vitro research only.

What ESI-MS measures

A dissolved sample is sprayed through a charged needle into the instrument, where the solvent evaporates and the peptide is left as a gas-phase ion carrying one or more protons. The instrument measures the mass-to-charge ratio (m/z) of those ions with high accuracy. Because the peptide picks up protons at basic sites, a peptide of mass M appears at m/z = (M + nH)/n for each charge state n. Software combines the charge states to give a single neutral mass, which is compared to the theoretical mass calculated from the sequence.

The result on a COA is therefore two numbers: theoretical mass (from the sequence) and observed mass (from the instrument), and their difference. A 1 Da tolerance is the usual acceptance criterion where a laboratory reports mass.

What agreement proves

An observed mass matching the theoretical within 1 Da proves that the dominant species in the sample has the elemental composition of the intended peptide. Combined with an HPLC main peak at the reference standard's retention time, that is strong evidence of identity. It rules out the wrong peptide, a truncated peptide, an incompletely deprotected peptide, and gross modifications.

What it does not prove

• Sequence order. Two peptides with the same residues in different order have the same mass. Sequence is established by the synthesis and by comparison to the reference standard; tandem MS (MS/MS fragmentation) can confirm it where needed.
• Stereochemistry. D- and L-amino acids have identical mass. For peptides containing D-residues by design, such as Ipamorelin, mass cannot distinguish the correct isomer from an all-L version; the synthesis route and reference standard do. See the Ipamorelin guide.
• Purity. MS is not quantitative in the way HPLC area is; a small impurity may or may not be visible depending on how it ionises.

Reading a discrepancy

• +16 Da: one oxidation (methionine, tryptophan, cysteine).
• +1 Da: deamidation of Asn or Gln (or, within tolerance, isotope averaging; check average versus monoisotopic).
• +22 Da: a sodium adduct rather than a chemical change; +38 potassium.
• +42 Da: an unexpected acetylation.
• +56, +100: residual tert-butyl or Boc protecting groups.
• Large negative: truncation or a deletion (subtract residue masses to identify which).
• Off by thousands: the wrong compound. The classic case is TB-500 (889 Da) versus full-length thymosin beta-4 (about 4,963 Da); see the TB-500 guide.

Larger peptides

For a peptide such as GLP-3 (RT) at 4,731 Da the spectrum shows a charge-state envelope (3+, 4+, 5+ and so on) rather than a single ion, and the reported mass is the deconvoluted value. A COA that shows a single peak at the full molecular weight for a large peptide, with no envelope, was not produced by electrospray in the usual way. See molecular weight and molar calculations for average versus monoisotopic mass.

What Alphex reports

Alphex certificates do not currently include mass spectrometry. Our testing laboratory reports chromatographic purity by HPLC-UV/VIS, and on some batches an identity result determined by HPLC against a reference standard rather than by mass. Every certificate we hold is published in the COA library, so you can see exactly which assays a given batch carries rather than taking a summary on trust. Method context is in how Alphex tests every batch.

Alphex research peptides are supplied for laboratory and in-vitro research by qualified researchers only. They are not authorised for human or veterinary use.

Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.

Published 11 March 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.

Alphex Research Team

Alphex Limited, UK research peptide supplier

The Alphex research team writes and maintains these notes for UK laboratories buying research peptides. Every batch Alphex lists is verified by reverse-phase HPLC with a batch-matched Certificate of Analysis. Content covers laboratory and in-vitro research only.