Testing desk · Method explainer
Mass spectrometry: how labs confirm a peptide is what the label says
How LC-MS confirms peptide identity by molecular mass, what charge states and adducts look like on a report, and the limits of mass-only identity checks.
The short answer ·
Mass spectrometry (MS) ionizes molecules and measures their mass-to-charge ratio. Coupled with liquid chromatography (LC-MS), it confirms identity by showing that the main component has the expected molecular mass for the labeled peptide. It is the standard way to catch a wrong or substituted peptide that would look 'pure' on HPLC alone.
A mass match is strong evidence, not proof of sequence.
Why is identity a separate test?
HPLC tells you how clean a sample is. It does not tell you what it is. A vial labeled as one peptide that contains a different, well-made peptide will produce a tidy single peak and a high purity number. Identity testing closes that gap.
How does mass spectrometry work?
- Ionization. Peptides are usually ionized by electrospray (ESI), which gives them one or more extra protons.
- Mass analysis. The instrument measures m/z, mass divided by charge. Analyzers include quadrupoles, ion traps and time-of-flight (TOF). MZ Biolabs lists linear ion trap and QTOF instruments with ESI and APCI sources.
- Deconvolution. Because a peptide appears at several charge states, software or the analyst calculates the underlying neutral mass.
- Comparison. The observed mass is compared with the theoretical mass from the peptide’s formula.
How do you read the MS section of a COA?
| Field | What to check |
|---|---|
| Expected mass | Theoretical monoisotopic or average mass; should match the peptide’s formula |
| Observed mass / m/z | Within the instrument’s stated accuracy |
| Charge states | e.g. [M+2H]2+, [M+3H]3+, consistent with the same neutral mass |
| Adducts | Sodium [M+Na]+ or potassium adducts are common and not a failure by themselves |
| Other masses | Significant unexplained masses may indicate impurities or a mix |
What does mass spectrometry catch?
- Substitution. TitrateLab’s review of archived records for one vendor found vials labeled retatrutide in which the lab reported tirzepatide instead. That is a mass difference that HPLC purity alone would not reveal (source).
- Truncations and deletions with distinct masses.
- Gross contamination with unrelated compounds that ionize.
What are the limits of a mass match?
- Same mass, different structure. Isomers or sequence scrambles with the same composition need tandem MS (MS/MS) fragmentation or a reference standard to distinguish.
- Not quantitative by default. A mass match doesn’t tell you how much is present.
- Not a safety test. It does not measure endotoxin or sterility.
Frequently asked questions
What is LC-MS?
Liquid chromatography coupled to mass spectrometry: the LC separates components, then the MS measures the mass of each as it elutes.
Why does a report show several masses for one peptide?
Peptides pick up different numbers of protons during electrospray ionization, so one molecule appears at several m/z values (charge states such as [M+2H]2+ and [M+3H]3+). Labs deconvolute these to the neutral mass.
Can mass spec detect a swapped GLP-1?
Yes, if the swapped peptide has a different mass, which is the case for semaglutide, tirzepatide and retatrutide. Archived lab records include vials labeled retatrutide in which the lab found tirzepatide instead.
Sources
3 linked records- [1] MZ Biolabs: mass spectrometry techniques MZ Biolabs Primary record
- [2] Freedom Diagnostics Testing: Identity (LC-MS) Freedom Diagnostics Testing Primary record
- [3] TitrateLab: Peptide Sciences archive (identity failures) TitrateLab
This explainer is general information about law and laboratory methods, not legal or medical advice. Amino Wire does not provide dosing information. Spotted an error? See our corrections policy.