Mass Spectrometry for Peptide Characterization

Mass spectrometry (MS) measures molecular mass with enough precision to confirm a peptide's identity: the measured mass must match the theoretical mass calculated from the sequence. No other routine technique makes that statement with comparable specificity, which is why every credible peptide COA carries an MS result. For an instrument-level introduction, see what is mass spectrometry in peptide testing.

Electrospray ionization (ESI) transfers peptides from liquid to gas phase as a series of multiply charged ions — ideal for coupling to LC. MALDI desorbs peptides from a crystalline matrix predominantly as singly charged ions, giving simple spectra from complex mixtures. For synthetic peptide characterization, LC-ESI dominates; MALDI remains useful for mixture screening and high-mass work.

ESI produces an m/z envelope: the same peptide appears at multiple charge states (2+, 3+, 4+...), each peak spaced by a mass that reflects the charge. The neutral molecular weight is deconvoluted from the envelope. A researcher verifying a COA does not need the instrument — the check is whether the deconvoluted reported mass matches the theoretical mass of the labeled sequence, calculated as in our peptide mass calculator article.

Each m/z peak is itself a family of isotope peaks separated by 1/z. At high charge the spacing shrinks accordingly, and the isotope pattern's width and shape scale with molecular size. Adducts (Na+, K+) appear at characteristic mass offsets from the protonated species, and their presence flags sample handling or mobile-phase salts.

Tandem MS fragments selected precursor ions along the backbone, producing b- and y-type fragment ion series that encode the sequence. Matching the fragment ladder to the theoretical fragments confirms the sequence itself — not merely the mass — which is the strongest identity evidence available for a synthetic peptide. Mass-differentiated isobaric impurities survive intact-mass checks but not MS/MS.

On a peptide COA, the MS field should state the calculated (theoretical) and found (measured) molecular weight, the ionization mode, and ideally the mass accuracy. A bare "MS: conforms" without numbers is weak documentation; a spectrum with deconvoluted mass and batch number is strong. How such results are produced by third-party laboratories is part of the vendor research toolkit, and how they interlock with HPLC purity is covered in the characterization pillar.