Liquid chromatography mass spectrometry
LC-MS
LC-MS separates a peptide sample by liquid chromatography and then weighs the molecules coming off the column with a mass spectrometer, so the observed mass of the main peak can be compared with the theoretical mass of the peptide on the label.
Offered when testing opens
What it answers
- Whether the main chromatographic peak has the molecular mass expected for the labelled peptide
- Whether a sample holds a different peptide with a clearly different mass, such as a swapped compound
- The masses of impurity peaks, which can suggest what they are (for example, a sequence missing one residue)
- With tandem MS (LC-MS/MS), whether fragment masses fit the expected amino acid sequence
What it cannot answer
- Purity or amount on its own: the mass spectrometer's response varies between molecules, so peak heights are not a fair purity measure
- Whether two molecules with the same mass (for example, isomers that differ only in residue order or chirality) are told apart, unless sequence data or a matched reference is used
- Water, counter-ion or salt content of the powder
- Endotoxin, sterility, metals or residual solvents
What it outputs
- A total ion chromatogram, plus usually a UV trace recorded in the same run
- A mass spectrum for each peak, showing ions at different charge states
- A deconvoluted or calculated neutral mass for the main peak
- With tandem MS, fragment ion spectra that can be matched to the sequence
On a COA
Identity by LC-MS is usually reported as the observed mass next to the theoretical mass, with a verdict such as "conforms" or "consistent with reference". Some certificates also list the charge states seen (for example [M+3H]3+) and the mass difference in daltons or parts per million. A bare "identity: pass" with no masses is weak evidence.
What does each half of LC-MS contribute?
The chromatography half is the same kind of separation used for HPLC-UV purity. The mass spectrometer replaces, or sits after, the UV detector: instead of measuring light absorbed, it turns the molecules into charged ions and measures their mass-to-charge ratio (m/z).
Separating first means each mass spectrum describes one peak, so the analyst can say which mass belongs to the main component and which to an impurity.
How do peptide molecules become ions?
Peptide LC-MS normally uses electrospray ionisation (ESI). The liquid leaving the column is sprayed through a charged needle, the droplets shrink as solvent evaporates, and the peptides end up in the gas phase carrying extra protons. A review of LC-MS principles in The Clinical Biochemist Reviews notes that ESI made LC-MS a routine technique by giving a simple and robust interface, and that peptides and proteins with several charge-carrying groups show multiple charging, producing ions written as [M+2H]2+, [M+3H]3+ and so on (Pitt, Clinical Biochemist Reviews, 2009).
So one peptide gives a family of peaks, each at a different m/z, and software works back from that family to the single neutral mass that is compared with the label.
A mass spectrum with a dashed marker at the expected mass and the observed peak sitting on it, labelled as a match within tolerance.
Which mass analysers are used, and does it matter?
| Analyser | How it sorts ions | What to expect on a report |
|---|---|---|
| Single or triple quadrupole | Four parallel rods pass only ions of a chosen m/z at a time | Nominal mass. Pitt's review notes quadrupoles usually work at unit mass resolution, with mass accuracy seldom better than 0.1 m/z |
| Time-of-flight (TOF) or quadrupole TOF | Ions are accelerated and timed down a flight tube; lighter ions arrive first | Finer mass values, often reported to several decimal places |
| Orbitrap or FT-ICR | Ions oscillate in a trap and their frequency is converted to mass | High-resolution, accurate mass; see HRMS |
A quadrupole is usually enough to tell the labelled peptide from one of clearly different size. Small differences, such as a single amino acid change or a modification of a few daltons, need a high-resolution analyser.
What makes an LC-MS identity result convincing?
Regulators treat mass as one strand of identity, not the whole of it. The European Medicines Agency's draft guideline on synthetic peptides recommends at least two orthogonal methods for identification at release, listing mass, relative retention time, LC-MS, LC-MS/MS, peptide mapping, amino acid analysis and NMR among the appropriate options, and says the test or combination of tests must unambiguously confirm the sequence (EMA draft guideline on synthetic peptides, 2023). For characterisation, the same guideline asks for tables of theoretical and observed mass values.
ICH Q2(R2) adds a useful point for reading reports: an identification test should give positive results with samples containing the analyte and negative results with samples that do not, and should be challenged with closely related materials to confirm a positive result is not obtained by mistake (ICH Q2(R2), 2023). A laboratory that runs a reference standard alongside the sample and matches both retention time and mass is giving two pieces of evidence rather than one.
- Observed and theoretical mass both shown. You can check the difference with the mass checker.
- Fragment data (LC-MS/MS). Fragments that fit the expected sequence move the result from "right size" towards "right sequence".
The expected mass marker for the labelled peptide sits empty while the observed peak appears at a different mass belonging to another molecule.
Why is LC-MS not used for the purity number?
How strongly a molecule shows in a mass spectrometer depends on how easily it picks up charge, which varies between molecules, and other components in the same droplet compete for that charge (ion suppression). MS peak size is therefore a poor guide to how much of each component is present, so purity and amount normally come from the UV trace.
Pitt's review lists ion-pairing reagents such as trifluoroacetic acid among the causes of ion suppression and advises keeping their concentration to a minimum. Since trifluoroacetic acid is the usual additive for UV purity work, a laboratory's LC-MS run and its HPLC-UV run may use different chromatography, and their retention times are not directly comparable.
How does Tested.ph use LC-MS?
LC-MS is the identity method in the Tested.ph standard package, run by the partner laboratory when testing opens. If the observed mass does not match the label, the record reports an identity failure and purity and amount are not scored against the label. The lesson Identity: why it comes first explains why.
Sources
- Principles and applications of liquid chromatography-mass spectrometry in clinical biochemistry (Pitt, 2009), The Clinical Biochemist Reviews, via PubMed Central
- Guideline on the Development and Manufacture of Synthetic Peptides (draft, EMA/CHMP/CVMP/QWP/387541/2023), European Medicines Agency
- ICH Q2(R2) Validation of Analytical Procedures, International Council for Harmonisation
Last reviewed 6 Oct 2026.