A chromatogram with one sharp, symmetrical peak is a satisfying thing to look at. It is also, on its own, insufficient. It establishes that the sample is homogeneous. It says nothing about whether the homogeneous substance is the peptide you ordered.
Mass spectrometry is what closes that gap.
How the measurement works
Electrospray ionisation is the standard technique for peptides. The sample in solution is pushed through a fine capillary held at high voltage, producing a spray of charged droplets. As solvent evaporates, charge density rises until the droplets fragment, eventually releasing gas-phase ions of the peptide itself.
Those ions carry multiple charges — a peptide has several basic sites that can take a proton — so the instrument sees a series of peaks at different mass-to-charge ratios rather than one. Deconvolution software reconstructs the neutral molecular mass from that series. The method is gentle enough to leave the molecule intact, which is why it suits peptides where harsher ionisation would fragment them.
Observed against theoretical
The theoretical mass is calculated from the sequence: sum the residue masses, add water for the free termini, adjust for any modification. The observed mass comes off the instrument. Agreement within tolerance confirms identity.
What makes this powerful is that disagreement is diagnostic. Mass differences map onto specific chemical events, and reading them tells you what went wrong rather than merely that something did.
| Difference | What it indicates |
|---|---|
| +16 Da | Oxidation — most often methionine to the sulfoxide |
| +42 Da | Acetylation, usually at the N-terminus |
| −17 Da | Loss of ammonia; pyroglutamate formation at an N-terminal Gln |
| +1 Da | Deamidation of asparagine or glutamine |
| −2 Da | Disulfide bond formation between two cysteines |
| +80 Da | Phosphorylation |
| Residue-sized gap | A deletion sequence — a residue missing from the chain |
A deletion sequence is worth dwelling on. If a coupling step in solid-phase synthesis fails on a fraction of the resin, the resulting peptide is missing one residue. It is chemically similar to the target, often elutes very close to it, and can be difficult to resolve chromatographically. Its mass, however, is unambiguously different — down by exactly the mass of the missing residue. Mass spectrometry catches what chromatography can miss.
Average against monoisotopic mass
Two mass values are in circulation and they are not interchangeable.
Monoisotopic mass uses the lightest isotope of each element — carbon-12, nitrogen-14. Average mass weights each element by natural isotopic abundance, which is why carbon contributes 12.011 rather than 12. For a small peptide the two are close. For a 39-residue peptide such as retatrutide they differ by several daltons, and comparing a monoisotopic observation against an average theoretical will read as a failure when nothing is wrong.
What it cannot establish
Mass spectrometry has real limits and it is worth being clear about them.
- It cannot distinguish isomers. Leucine and isoleucine have identical masses. A D-amino acid substitution has the same mass as the L form. Sequence-scrambled variants weigh the same as the correct sequence.
- Intact mass alone does not confirm sequence order. Establishing that requires tandem MS with fragmentation, which is not routine on a research-grade certificate.
- It is not straightforwardly quantitative. Different species ionise with different efficiency, so relative peak intensities do not reliably map to relative abundance. Quantitation is chromatography's job.
This is the reverse of chromatography's blind spots, which is exactly why the two appear together. HPLC quantifies but does not identify. MS identifies but does not quantify. Neither is optional.
What to look for on a certificate
- Both observed and theoretical mass, stated as numbers rather than a bare 'conforms'.
- Which convention is in use — average or monoisotopic.
- The ionisation method, normally ESI.
- Ideally the spectrum itself, not only the deconvoluted figure.
A certificate that says 'Identity: confirmed by MS' without reporting the numbers is asking to be taken on trust. The numbers cost nothing to print, and their absence is a choice.
This guide is written for laboratory practitioners and describes analytical and handling practice. It is not medical advice, and it contains no dosing or administration guidance. Material supplied by Ryzen Research Ltd is for research use only and is not for human or veterinary use.