Reading a Mass Spectrum: How LC-MS Confirms Peptide Identity
LC-MS separates a sample by chromatography, ionises it and measures mass-to-charge ratios. Matching the observed mass to the theoretical mass of the sequence confirms identity; it complements, but does not replace, an HPLC purity result.
From chromatography to ions
The liquid chromatography step separates the sample's components over time. Each eluting component is ionised—most often by electrospray ionisation (ESI)—and the mass spectrometer measures the mass-to-charge ratio (m/z) of the ions.
Charge states and deconvolution
Electrospray typically adds several protons to a peptide, so one molecule appears as a series of peaks: [M+2H]²⁺, [M+3H]³⁺ and so on. Software deconvolutes this series back to a single neutral mass.
For a peptide with a neutral mass of 3,000 Da, the 2+ ion appears near m/z 1,501 ((3,000 + 2) ÷ 2) and the 3+ ion near m/z 1,001 ((3,000 + 3) ÷ 3).
Observed vs theoretical mass
The theoretical mass is calculated from the sequence and modifications. An identity result is meaningful when the observed mass matches within the instrument's stated tolerance. A shift of +16 Da suggests oxidation; −18 Da, loss of water; a missing residue mass, a deletion sequence.
How Peptide Lab Verification Works
What an identity result does not tell you
LC-MS confirms that the expected molecule is present; it is not, on its own, a quantitative purity measurement, and it does not report net peptide content. A complete lot report pairs it with HPLC purity and, where provided, content analysis.
Peptide Purity vs Identity vs Content
Evidence guides
- How Peptide Lab Verification Works — How HPLC, LC-MS, content testing and lot records fit together.
- How to Verify Peptide Authenticity — A checklist for identity, lot matching and report provenance.
- How to Choose a Research Peptide Supplier — Evaluate suppliers on specifications, documentation and handling.
- Peptide Purity vs Identity vs Content — Why purity, identity and net peptide content are separate measurements.
Frequently asked questions
Why does one peptide show several peaks in a mass spectrum?
Electrospray ionisation adds different numbers of protons, creating several charge states. Each appears at a different m/z, and software converts them back to one neutral mass.
Does LC-MS measure peptide purity?
Not quantitatively on its own. LC-MS confirms identity; purity is normally reported from the UV peak area of an HPLC trace.
How do you read peptide mass spectrometry results?
Find the deconvoluted (neutral) mass, compare it with the theoretical mass of the sequence, and check the difference is within the stated tolerance. Shifts such as +16 Da or −18 Da point to oxidation or water loss.
More peptide science
- What Is a Peptide? Amino Acids, Bonds and Sequence
- How Research Peptides Are Made: Solid-Phase Peptide Synthesis
- Why Peptides Degrade: Oxidation, Deamidation and Hydrolysis
Research Use Only. Educational laboratory information; not medical advice.
