Understanding mass spectrometry in peptide identity confirmation

The measurement in outline

The sample is ionised — for peptides, usually by electrospray ionisation, which transfers molecules from solution into the gas phase as charged species with minimal fragmentation. The analyser then separates ions by mass-to-charge ratio, and the detector records their abundance.

The result is a spectrum of intensity against mass-to-charge ratio. From it, the neutral molecular mass of the compound is reconstructed and compared with the mass calculated from the intended amino acid sequence.

Observed versus theoretical mass

Theoretical mass is computed from the sequence, accounting for terminal modifications such as amidation or acetylation and for any disulfide bridges, each of which removes two hydrogens. Agreement between observed and theoretical mass within the instrument's expected tolerance supports the identity claim.

A mass difference is often diagnostic rather than merely a failure. Common offsets point directly at a specific defect.

Charge states and why the spectrum looks crowded

Electrospray typically produces multiply charged ions, so one compound appears as a series of peaks at different charge states. Each is a different mass-to-charge ratio for the same molecule; deconvolution software collapses the series into a single neutral mass. A spectrum with many peaks is therefore not evidence of many compounds.

Sodium and potassium adducts add characteristic offsets without indicating that the peptide itself is modified. Reading a spectrum means separating structural differences from ionisation artefacts.

Where identity confirmation stops

Mass confirms composition, not arrangement. Two sequences containing the same residues in different order share a mass, so mass alone cannot distinguish them; that requires tandem MS fragmentation. For routine lot release, molecular mass agreement combined with a clean chromatographic profile is the conventional evidence set.