Mass spectrometry weighs the molecule in the vial and checks that weight against the sequence on the label. That is the identity test on a peptide certificate, and it is the only line that answers what the material actually is. The purity percentage printed beside it answers a different question: how much of the sample came off the column in a single peak. Vendors print both under the same heading, and buyers read them as one number.
The short answer. Mass spectrometry is the identity test on a peptide certificate: it weighs the molecule and checks that mass against the sequence.
- Purity is a separate test. HPLC measures peak area, not what the peak is.
- Equal masses look identical. Leucine and isoleucine, and D- and L-forms, need a further method.
- A mass spectrum says nothing about endotoxin, sterility or fill weight.
Mass spectrometry weighs molecules. It ionizes the sample, then sorts ions by mass against charge. A peptide has one theoretical mass, fixed by its sequence. The instrument reports what it actually measured. Identity is confirmed when the two agree. Purity is a separate number from a separate method.
What is mass spectrometry in peptide QA?
Mass spectrometry weighs molecules. The instrument turns the sample into gas-phase ions, then sorts those ions by mass divided by charge. Every peptide has exactly one theoretical mass, fixed by its amino acid sequence. The measured value either lands on that number or it does not.
That is why mass spectrometry is the identity test on a certificate of analysis. It answers one question: is this molecule the molecule the label names? Nothing else on a routine certificate answers it. HPLC purity describes how the sample separated. It does not describe what separated.
Regulators phrase the same point more coldly. ICH Q6B, published as FDA guidance in August 1999, sets what an identity test has to be.
The identity test(s) should be highly specific for the drug substance and should be based on unique aspects of its molecular structure and/or other specific properties. More than one test (physicochemical, biological, and/or immunochemical) may be necessary to establish identity.
ICH Q6B, section 4.1.2, issued as FDA guidance for industry, 1999Mass is one of those unique aspects. It is not the only one, and Q6B says so in the same breath: one test may not be enough. That sentence is the honest frame for everything below.
What does the mass spec line on a certificate of analysis say?
Most peptide certificates give it one line. A line that is worth reading has four parts.
- The ion form. Peptides are usually detected as charged species, written as [M+H]⁺ or with more charges. A multiply charged ion is not the molecular weight, so the form has to be stated for the number to mean anything.
- The theoretical mass. Calculated from the sequence. It is arithmetic, not a measurement, and it should appear on the page so you can compare.
- The observed mass. What the instrument actually recorded. This is the result. Everything else is context for it.
- The method. ESI-MS, LC-MS, MALDI-TOF or tandem MS. Different methods settle different questions, and the next section is about the difference.
A line that reads only MS: conforms, or verified by mass spec, carries none of that. It is an assertion. It cannot be checked, disputed or reproduced, which is the same position you were in before you read it.
The single most useful thing a vendor can print is the observed mass next to the theoretical mass. It is one line of text. Its absence is the most common defect on peptide certificates that mention mass spectrometry at all.
It also helps to know what the other rows are for. A certificate is a stack of separate tests, and each one answers a narrow question.
| Test | Question it answers | What it cannot tell you |
|---|---|---|
| Mass spectrometry (intact mass) | Is the molecule the one the label names? | Whether two residues of identical mass were swapped |
| Tandem MS or peptide mapping | What order the residues sit in | Leucine from isoleucine, without a specialized method |
| RP-HPLC with UV detection | What share of the eluting material is one peak | What that peak is |
| Content or assay | How much peptide the vial holds | Anything about identity |
| Bacterial endotoxin (LAL) | Whether endotoxin is present | Anything about identity or purity |
| Sterility | Whether viable organisms grow | Anything about identity, purity or amount |
Read across that table and the point is hard to miss. No single row covers another row. A certificate with one row is a certificate with one answer.
Is mass spectrometry the same as HPLC purity?
No, and a 2024 study in the Journal of Medical Internet Research shows how far the two can diverge on the same vial. Ashraf and colleagues bought semaglutide from illegal online pharmacies, then ran the delivered material by liquid chromatography coupled to mass spectrometry.
They evaluated 1,080 search-result links, ordered from six sellers, and received three vials; the three prefilled pens they paid for never arrived. On the vials that did arrive, the measured purity ran between 7.7 percent and 14.37 percent, against the 99 percent claimed on the labels. The same samples measured 28.56 percent to 38.69 percent above their labeled semaglutide content.
Those two findings are not a contradiction. They are two different measurements. Content asks how much of the named peptide is in the vial. Purity asks what fraction of the material in the vial is that peptide. A vial can be over-filled with under-pure powder and read high on one measure and low on the other. The study reported both numbers on the same three vials.
The determination of absolute, as well as relative, purity presents considerable analytical challenges, and the results are highly method dependent.
ICH Q6B, section 2.1.4, issued as FDA guidance for industry, 1999Highly method dependent is the phrase to carry around. A purity percentage with no method attached is not a fact about the powder, it is a fact about an experiment nobody described. The same study also found no viable organisms in the lyophilized samples but endotoxin in every one of them, between 2.1645 and 8.9511 EU/mg. Sterile and clean are not the same word either.
BPC-157
Research-use-only material, sold by the vial with batch documentation. Check the certificate of analysis against the batch you receive.
What can mass spectrometry not detect in a peptide?
Mass is a blunt instrument in one specific way. Two different molecules that weigh the same are, to a mass measurement, the same molecule.
- Swapped isomeric residues. Leucine and isoleucine share a chemical formula, so they share a mass. Edwards and colleagues, writing in Rapid Communications in Mass Spectrometry in 2022, note that tandem methods for telling them apart exist but that all of them carry limitations. Their own work used charge transfer dissociation to get there.
- D-amino acid substitutions. A D-residue weighs exactly what its L-counterpart weighs. Readel and colleagues, in Analytical and Bioanalytical Chemistry in 2023, put it plainly.
- Anything not in the spectrum. Endotoxin, sterility, water content, fill weight and residual solvents are not mass measurements of the peptide, so no mass spectrum reports them.
Because such peptides are indistinguishable by mass alone, selective liquid chromatography tandem mass spectrometry analysis is required to differentiate such peptides.
Readel, Dhaubhadel, Patel & Armstrong, Analytical and Bioanalytical Chemistry, 2023The other half of the limit is that mass spectrometry only reports what the method went looking for. Kopp and colleagues, in Pharmaceutical Research in 2026, compared follow-on and compounded GLP-1 receptor agonists against the originators by LC-MS and found distinct impurity profiles in the copies, including amino acid deletions and additions along with impurities they could not identify at all. Those are exactly the species a single intact-mass check is built to miss: a deletion product has its own mass, and nobody looking only for the target mass will see it.
Degradation is the same story from the other end. Malgave and colleagues, in the European Journal of Pharmaceutics and Biopharmaceutics in 2025, stressed semaglutide across pH, buffer and temperature conditions and resolved thirteen known degradation impurities by high-resolution LC-MS, six of which formed under every condition they tested. A certificate describes the day the lot was tested, not the vial in your fridge months later.
ESI or MALDI: which instrument is behind the number?
Certificates name a method more often than they explain it. These are the four you will actually see, and what each one settles.
| Method | How it appears on a certificate | What it settles | What to watch |
|---|---|---|---|
| ESI-MS, usually as LC-MS | An observed [M+H]⁺ or multiply charged ion, ideally beside the theoretical mass | Intact mass, measured on the same run that produced the chromatography | A multiply charged ion is not the molecular weight; the charge state has to be stated |
| MALDI-TOF | A single mass with the method named | Intact mass, from a fast single-spectrum measurement | Often run separately from the purity chromatography, so the two rows may not describe one run |
| High-resolution MS | A mass quoted to several decimal places, sometimes with an error in ppm | Elemental composition, not just a nominal mass | A tight mass error still cannot separate two isomers of equal mass |
| Tandem MS or peptide mapping | Sequence confirmed by MS/MS, or a map of fragments | The order of the residues | Leucine against isoleucine, and D against L, need methods beyond a routine map |
None of these is the premium option and none is the cheap one. They answer questions of different widths. An intact mass says the molecule weighs right. A peptide map says the residues are in the right order. Neither says the powder is clean, and the rest of the certificate exists because of that.
How do you tell a real identity confirmation from “mass spec verified”?
The FDA's clearest published statement of what confirmation by mass spectrometry requires was written for drug residues in animal tissue, not for peptide vials. Guidance for Industry #118, finalized in 2003, is still the sharpest picture available of where a US regulator sets the bar.
For a full-scan measurement it asks for at least three structurally specific ions, and for the spectrum to match a standard run at the same time. The chromatographic peak has to clear a signal-to-noise threshold of 3:1, and retention time has to fall within 5 percent of the standard for an LC/MS method. Under selected ion monitoring, relative abundances have to match the standard within ±10 percent for three ions and within ±15 percent for four or more. It also rules out the shortcut.
Library-search algorithms should not be used to confirm identity.
FDA Center for Veterinary Medicine, Guidance for Industry #118, 2003No research-peptide vendor is bound by that document, and none should be described as failing it. The value is the shape it gives to the word confirmed: a named method, a contemporaneous standard, several ions and stated tolerances. Set against that, a checkmark in a marketing table is not a weak confirmation. It is a different kind of object.
Ask for the observed mass, the theoretical mass, the ion form, the method, and the lot code the run belongs to. A vendor holding a real report can paste all five in one reply. Matching it to the vial you were sent is the step covered in our COA verification checklist.
- Which laboratory ran it. A name, not a badge. Accreditation scope is worth reading too, and we cover what it does and does not cover in ISO 17025 lab testing explained.
- Whether the MS and the purity number came from one run. Two rows from two dates describe two experiments.
- Whether the report itself is available, rather than a summary line transcribed onto a vendor's own template.
BPC-157
Research-use-only material, sold by the vial with batch documentation. Check the certificate of analysis against the batch you receive.
What to know now
- Mass spectrometry is the identity test on a peptide certificate: it weighs the molecule and checks that mass against the sequence.
- Purity is a separate test. HPLC measures peak area, not what the peak is.
- Equal masses look identical. Leucine and isoleucine, and D- and L-forms, need a further method.
- A mass spectrum says nothing about endotoxin, sterility or fill weight.
- Ask which method produced the identity line. A bare “MS confirmed” with no observed mass is an assertion, not a result.
What we're watching
Whether vendors start printing the observed mass beside the theoretical one instead of the word conforms. It is one line of text, it costs nothing to publish, and it is the whole difference between a result and a claim. A handful of suppliers already do it.
Frequently asked questions
What does mass spectrometry prove about a peptide?
That the molecule in the sample weighs what the named sequence should weigh. That is an identity result. It is not a purity result, a quantity result or a sterility result, and it does not become one because those rows are missing.
Is mass spectrometry better than HPLC for peptide testing?
Neither is better. They answer different questions. HPLC separates the sample and reports what share of it eluted as one peak; mass spectrometry reports what that material weighs. A useful certificate carries both, from a stated method.
Can mass spectrometry tell whether a peptide is fake?
It can show that a mass does not match the compound named on the label, which is decisive. It cannot show the reverse with the same force, because two molecules of equal mass are indistinguishable by mass alone.
Why do certificates quote a theoretical mass as well as an observed one?
The theoretical mass is arithmetic from the sequence and the observed mass is the measurement. Printing only one of them removes the comparison, which is the part that carries the information.
Does a mass spectrometry result say how much peptide is in the vial?
No. Content is a separate assay. In a 2024 study of semaglutide bought from illegal online pharmacies, measured content ran 28.56 percent to 38.69 percent above the labeled amount while measured purity ran between 7.7 percent and 14.37 percent.
What should a vendor's mass spectrometry line include?
The ion form, the theoretical mass, the observed mass, the method, and the lot the run belongs to. Anything less cannot be checked by the person reading it.
References
- International Council for Harmonisation / U.S. Food and Drug Administration. (1999). Q6B Specifications: Test Procedures and Acceptance Criteria for Biotechnological/Biological Products. Guidance for Industry. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/q6b-specifications-test-procedures-and-acceptance-criteria-biotechnologicalbiological-products
- U.S. Food and Drug Administration, Center for Veterinary Medicine. (2003). Guidance for Industry #118: Mass Spectrometry for Confirmation of the Identity of Animal Drug Residues. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/cvm-gfi-118-mass-spectrometry-confirmation-identity-animal-drug-residues
- Ashraf, A. R., Mackey, T. K., Vida, R. G., Kulcsár, G., Schmidt, J., Balázs, O., Domián, B. M., Li, J., Csákó, I., & Fittler, A. (2024). Multifactor quality and safety analysis of semaglutide products sold by online sellers without a prescription: Market surveillance, content analysis, and product purchase evaluation study. Journal of Medical Internet Research, 26, e65440. https://doi.org/10.2196/65440
- Kopp, K. L., Lamberth, K., Schelde, O., Øgendahl, A. K., Wojcieszek, M., Mogensen, J. E., Ramírez-Andersen, H. S., Schneider, C. L., Staby, A., & Hach, M. (2026). Impurities and potential immunogenicity associated with follow-on and compounded glucagon-like peptide-1 receptor agonists. Pharmaceutical Research. https://doi.org/10.1007/s11095-026-04146-9
- Readel, E. R., Dhaubhadel, U., Patel, A., & Armstrong, D. W. (2023). Variable fragmentation and ionization of amyloid-beta epimers and isomers. Analytical and Bioanalytical Chemistry, 415(27), 6799–6807. https://doi.org/10.1007/s00216-023-04958-3
- Edwards, H. M., Wu, H.-T., Julian, R. R., & Jackson, G. P. (2022). Differentiation of leucine and isoleucine residues in peptides using charge transfer dissociation mass spectrometry (CTD-MS). Rapid Communications in Mass Spectrometry, 36(5), e9246. PMID 34927767
- Malgave, A., Akbar, S., Joseph, A., Dande Aishwarya, Peraman, R., & Malayandi, R. (2025). Effect of pH, buffers, molarity, and temperature on solution state degradation of semaglutide using LC-HRMS: A preformulation protocol for peptide drug delivery. European Journal of Pharmaceutics and Biopharmaceutics, 214, 114780. https://doi.org/10.1016/j.ejpb.2025.114780