A peptide certificate usually carries two pieces of raw data: a chromatogram answering how much of the sample is one component, and a mass spectrum answering whether that component is the right molecule. The chromatogram is the one people learn to read. The spectrum tends to get a glance, partly because the largest peak on it is often nowhere near the molecular weight on the same page. That is not an error. It is the instrument working as designed, and the arithmetic that reconciles them takes about a minute to learn.
Quick answer
Why does the peak on a peptide mass spectrum not match the molecular weight? Because a mass spectrometer measures mass-to-charge ratio, not mass. Electrospray ionisation adds protons, often several, so a peptide appears at m/z = (M + n × 1.007) ÷ n, where n is the number of charges. A peptide of mass 1,419.53 appears at about 1,420.54 as a singly charged ion and about 710.77 as a doubly charged one. To recover the mass, multiply the m/z by the charge and subtract one proton mass per charge.
The one formula that does the work
Electrospray is a soft ionisation method: it puts the molecule into the gas phase and gives it charge by adding protons, without breaking it. Each added proton adds roughly 1.007 to the mass and one unit to the charge.
| To find | Formula |
|---|---|
| Observed m/z for charge n | (M + n × 1.007) ÷ n |
| Mass from an observed m/z | M = n × (m/z) − n × 1.007 |
Worked on a real figure, for a peptide with a molecular weight of 1,419.53:
| Ion | Charge | Expected m/z |
|---|---|---|
| [M+H]+ | 1 | 1,420.54 |
| [M+2H]2+ | 2 | 710.77 |
| [M+3H]3+ | 3 | 474.18 |
This is why a certificate can report a molecular weight in the low thousands while the spectrum shows its strongest signal in the hundreds. Nothing is wrong; the ion simply carried more than one charge.
The charge envelope
Longer peptides have more basic sites able to accept a proton, so they generally appear as a series of peaks at different charge states rather than a single one. That series is called a charge envelope, and it is a feature rather than a complication: each peak in it should resolve back to the same mass. A spectrum where the charge states disagree with one another is a far more meaningful red flag than any single peak position.
A short tripeptide may show essentially one ion. A multi-agonist peptide of several thousand daltons will typically show three, four or more. Neither pattern is a quality signal by itself.
Adducts, and the peaks that are not your peptide
Protons are not the only thing that attaches. Sodium and potassium are ubiquitous in glassware, buffers and solvents, and both form adducts that appear at predictable offsets.
| Species | Offset from the protonated ion | Usual origin |
|---|---|---|
| [M+Na]+ | About +21.98 | Glass, buffers, solvents |
| [M+K]+ | About +37.96 | Buffers, salts |
| Oxidised species | About +16 per oxygen | Methionine, cysteine or tryptophan oxidation |
| Deamidated species | About +1 | Asparagine or glutamine deamidation |
The last two rows are worth dwelling on, because they are degradation products rather than handling artefacts. A small peak sitting 16 above the main ion is a plausible oxidation, and one sitting about 1 above it is consistent with deamidation. Peptide degradation explained covers both routes, and buffers and pH covers the conditions that accelerate them.
Average or monoisotopic
A certificate may quote two different masses for the same molecule. Monoisotopic mass uses the lightest isotope of each element and is what high-resolution instruments report for small and mid-sized peptides. Average mass uses the natural isotopic mixture and is the figure used for preparing solutions by weight. For a peptide of a few thousand daltons the two differ by a meaningful amount, and comparing an observed monoisotopic value against a theoretical average value will appear to show a discrepancy that does not exist.
This is the same distinction that matters when converting a solution to molar units, covered in peptide molarity.
How close is close enough
Certificates report an observed mass against a theoretical mass, and the gap between them is the identity evidence. Tolerance depends on the instrument: a high-resolution system may be specified in parts per million, while a lower-resolution one is judged in daltons. What matters for a reader is that the certificate states which, rather than presenting a bare pair of numbers. A result reported to four decimal places on an instrument that cannot support them is a presentation choice, not a measurement.
What a mass result cannot tell you
Mass spectrometry is identity evidence, and it is strong. It is also narrow in three specific ways.
- It does not measure how much. Peak intensity reflects how readily a species ionises, not how much of it is present. Quantity comes from the chromatogram; see how to read an HPLC chromatogram and HPLC vs mass spectrometry.
- It does not distinguish identical masses. Leucine and isoleucine have the same mass, and a sequence with the same composition in a different order has the same mass as the correct one. Mass alone cannot separate them; tandem mass spectrometry, which fragments the molecule, can.
- It does not describe the powder. Identity says nothing about how much of the vial is peptide rather than counter-ion, water and residual solvent; see net peptide content vs purity.
Deletion and truncation impurities do shift mass and are visible, which is one of the useful complements MS provides to a chromatogram; see peptide impurities.
Reading one in order
- Find the stated theoretical mass and note whether it is average or monoisotopic.
- Identify the main ion and its charge state.
- Convert back to mass and compare with the theoretical figure.
- Check that other peaks in the charge envelope resolve to the same mass.
- Account for the remaining peaks: adducts at the expected offsets, plausible oxidation or deamidation, or something unexplained.
- Confirm the method, instrument and ionisation mode are named, and that the lot number matches your vial.
The field-by-field walkthrough of the rest of the document is in how to read a certificate of analysis, and the terminology is defined in the COA glossary. Revoryn Labs publishes available batch documentation in its public COA ledger, matched to the lot printed on the vial.
Key takeaways
- A mass spectrometer reports m/z, not mass; the largest peak is often not the molecular weight.
- Recover mass with M = n × (m/z) − n × 1.007, where n is the charge state.
- Longer peptides show a charge envelope, and every peak in it should resolve to the same mass.
- Sodium and potassium adducts sit at about +21.98 and +37.96 from the protonated ion.
- Peaks about 16 above the main ion suggest oxidation; about 1 above suggests deamidation.
- MS confirms identity but not quantity, and cannot separate species of identical mass without fragmentation.
Frequently asked questions
Why is the peak on my spectrum half the molecular weight?
Because the ion carried two charges. A doubly charged ion appears at roughly half the mass, plus the contribution of the two added protons.
What does [M+H]+ mean?
The intact molecule with one proton added, carrying a single positive charge. Its m/z is the molecular weight plus about 1.007.
What is the difference between average and monoisotopic mass?
Average mass uses the natural isotopic mixture and is used for preparing solutions by weight. Monoisotopic mass uses the lightest isotope of each element and is what high-resolution instruments report. Compare like with like.
Can mass spectrometry prove my peptide is pure?
No. It provides identity evidence. Purity comes from chromatography, because peak intensity in a mass spectrum reflects ionisation efficiency rather than quantity.
What is that peak 22 units above my main peak?
Most often a sodium adduct, which is common and usually originates in glassware, buffers or solvents rather than in the peptide.
Sources
- Mant CT, Chen Y, Yan Z, et al. HPLC analysis and purification of peptides. Methods Mol Biol. 2007;386:3-55. doi:10.1007/978-1-59745-430-8_1
- D Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30. doi:10.1016/j.jpba.2014.06.012
- Bachem. Quality Control of Amino Acids & Peptides: A Guide. bachem.com
For laboratory and in-vitro research use only. Not for human or veterinary use.
For laboratory and in-vitro research use only. Not for human or veterinary use. See our research-use disclaimer.
