A purity figure tells you how much of a vial's contents is the intended compound. It says nothing about whether that compound is the right molecule in the first place. Mass-spectrometry (MS) identity confirmation answers the prior question: is the thing in this vial chemically what the label says it is? Third-party testing labs such as Janoshik and Peptigrity-listed labs routinely include MS identity data alongside purity measurements, and knowing what that data represents helps researchers read certificates of analysis more critically.
How mass spectrometry identifies a peptide
Every peptide has a molecular weight (mass) set 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.
Each amino acid has a fixed, known mass, so 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, the sample passes identity confirmation. On high-resolution instruments that tolerance is tight. Orbitrap analysers routinely deliver mass accuracy around 1 part per million, while time-of-flight (TOF) instruments typically reach roughly 2–5 ppm, according to the analytical-chemistry literature. A mismatch produces a detectably different value, flagging the compound as incorrect. Even a single amino acid substitution that changes the mass will show up.
Common MS techniques applied to peptides include electrospray ionisation (ESI-MS) and matrix-assisted laser desorption ionisation (MALDI-MS). In practice, third-party COAs usually report an intact-mass (LC-MS) measurement, the whole molecule weighed at once, rather than a full sequencing run. Both ionisation methods are described extensively in the peer-reviewed literature and in pharmacopoeial and regulatory guidance, including the United States Pharmacopeia (USP) and the ICH Q6B specification, adopted by the EMA and FDA, which calls for peptide mapping and sequence confirmation as part of characterising peptide products.
Why identity matters independently of purity
Purity is usually reported as the percentage of the total HPLC peak area corresponding to the main compound. It tells you how clean a sample is. It does not tell you what that main compound is. Research has described cases where a substance shows high HPLC purity yet is still the wrong peptide, a degradation product, or a scrambled-sequence variant. Each of those 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 (intact mass) | Molecular weight matches the target sequence | Exact sequence order of amino acids; biological activity |
| Tandem MS / peptide mapping | Sequence-level confirmation via fragmentation | Adds cost and time; not standard in routine COAs |
The sequence-order caveat is a real limitation. Intact mass weighs the whole molecule, so two peptides built from the same amino acids in a different order, or containing isobaric residues such as leucine and isoleucine, can share an identical mass. Telling them apart generally requires tandem MS (MS/MS) fragmentation or peptide mapping, as the proteomics literature notes. A routine COA intact-mass result confirms the molecule is consistent with the target. It does not confirm the sequence has been read end to end.
A certificate of analysis that carries both an HPLC purity figure and an MS identity result gives you meaningfully more than either measurement alone. Third-party labs including Janoshik publish COA formats that combine both data points, and Janoshik assigns each batch a unique key that lets anyone verify the report directly at the lab rather than relying on a vendor-supplied copy.
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 such as 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. Was the MS run by an independent third party rather than the supplier's in-house lab, and can the report be verified at the testing lab directly?
Sites such as Finnrick and VialAudit aggregate third-party test data across multiple suppliers, so you can compare identity and purity results for a given compound. Finnrick reports having published results from several thousand samples across more than two hundred vendors. Peptigrity maintains a directory of independent testing labs and publishes test results drawn from them.
Treat MS identity confirmation as a baseline expectation, not a premium add-on, when evaluating the analytical documentation for any research peptide. Research peptides are sold for laboratory use only and are not approved for human consumption. As with all analytical data, the value of a COA depends on who ran the test and under what conditions.