When a research peptide vial arrives, purity figures tell you how much of the contents is the intended compound — but they say nothing about whether that compound is actually the right molecule. Mass-spectrometry (MS) identity confirmation answers a different, prior question: is the thing in this vial chemically what the label says it is? Third-party testing labs such as Janoshik and Peptigrity routinely include MS identity data alongside purity measurements, and understanding what that data represents helps researchers evaluate certificates of analysis more critically.
How mass spectrometry identifies a peptide
Every peptide has a molecular weight (mass) determined by the sequence of amino acids in its chain. MS measures that mass with very high precision. In a typical workflow, a sample is ionised — converted into charged particles — and then accelerated through an analyser that separates ions by their mass-to-charge ratio (m/z). The detector records these values and the instrument software calculates the parent molecular weight.
Because each amino acid has a fixed, known mass, a peptide's theoretical molecular weight can be calculated from its sequence alone. If the measured mass matches the theoretical mass within an accepted tolerance (commonly within a few parts per million on high-resolution instruments), the sample passes identity confirmation. A mismatch — even by a single amino acid substitution — produces a detectably different mass, flagging the compound as incorrect.
Common MS techniques applied to peptides include electrospray ionisation (ESI-MS) and matrix-assisted laser desorption ionisation (MALDI-MS). Both are described extensively in the peer-reviewed literature and in pharmacopoeial guidance from bodies such as the United States Pharmacopeia (USP) and the European Medicines Agency (EMA).
Why identity matters independently of purity
A purity figure — typically reported as the percentage of the total HPLC peak area corresponding to the main compound — tells you how clean a sample is. It does not tell you what that main compound is. Research has described cases where a substance can show high HPLC purity yet still be the wrong peptide, a degradation product, or a scrambled-sequence variant. These alternatives would pass a purity threshold while failing an identity check.
The table below contrasts what each test type does and does not confirm:
| Test | What it confirms | What it does NOT confirm |
|---|---|---|
| HPLC purity | Proportion of the dominant peak in the sample | Whether that peak is the correct molecule |
| MS identity | Molecular weight matches the target sequence | Exact sequence order of amino acids; biological activity |
| Amino-acid analysis / sequencing | Full sequence confirmation | Not standard in routine COAs; adds significant cost |
In practice, a certificate of analysis that includes both an HPLC purity figure and an MS identity result provides meaningfully more information than either measurement alone. Third-party labs including Janoshik and Peptigrity publish COA formats that combine both data points.
What to look for in a certificate of analysis
When reviewing a COA that includes MS data, researchers commonly look for:
- Reported m/z values alongside the theoretical value for the stated sequence, allowing independent verification.
- Instrument type and method — high-resolution instruments (e.g. Orbitrap or Q-TOF platforms) provide tighter mass accuracy than lower-resolution single-quadrupole systems.
- Pass/fail language attributed explicitly to the molecular weight match, not conflated with purity.
- Chain of custody — whether the MS was run by an independent third party rather than the supplier's in-house lab.
Sites such as Finnrick and VialAudit aggregate third-party COA data from multiple labs, making it possible to compare identity results across suppliers for a given compound. Peptigrity publishes its own independent test results using similar methodology.
Researchers are encouraged to treat MS identity confirmation as a baseline expectation, not a premium add-on, when evaluating the analytical documentation for any research peptide. As with all analytical data, the value of a COA depends on who ran the test and under what conditions.