LC-MS Identity Confirmation for Peptides
How mass spectrometry confirms identity, what ppm accuracy means, and the one thing a mass match cannot tell you.
How does LC-MS confirm peptide identity?
LC-MS separates the sample chromatographically, then ionises each component and measures its mass-to-charge ratio. For peptides, electrospray ionisation produces a series of multiply charged ions, which deconvolution converts into a single neutral mass. That observed monoisotopic mass is compared against the theoretical mass of the claimed sequence and the difference reported in parts per million.
A match within a few ppm confirms the molecular formula is consistent with the claimed sequence. On a well-calibrated instrument, agreement inside 5 ppm is routine.
What a mass match cannot exclude
Mass is a property of composition, not of order. Two peptides containing the same amino acids arranged differently have identical molecular formulas and therefore identical masses.
Intact mass confirmation cannot distinguish them. Neither can it reliably distinguish isobaric residue substitutions such as leucine for isoleucine, which are exactly equal in mass.
When sequence certainty genuinely matters, the correct assay is peptide mapping: enzymatic digestion followed by tandem mass spectrometry of the resulting fragments, which reconstructs the order rather than only the composition.
Frequently asked questions
- What is a good ppm mass accuracy for peptide identity?
- On a modern QTOF instrument with recent calibration, agreement between observed and theoretical monoisotopic mass within 5 parts per million is routine and within 2 ppm is common. A result outside 10 ppm on such an instrument warrants investigation, either of the sample or of the calibration. Certificates should report the observed mass, the theoretical mass and the difference, rather than only stating that identity was confirmed.
- Can LC-MS tell the difference between two peptides with the same mass?
- Not from intact mass alone. Peptides containing the same amino acids in a different order share a molecular formula and therefore a mass, and substitutions such as leucine for isoleucine are exactly isobaric. Distinguishing them requires peptide mapping, where the peptide is enzymatically digested and the fragments analysed by tandem mass spectrometry to reconstruct the actual sequence.
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REPLACE: Analytical Lead
Head of Analytical Chemistry · PhD Analytical Chemistry
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