Definition
A technique that measures molecular mass to confirm a peptide's identity against its theoretical molecular weight.
Mass spectrometry ionises molecules and measures their mass-to-charge ratio (m/z). For peptides, electrospray ionisation (ESI) and matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) are the common methods. ESI typically produces multiply charged ions such as [M+2H]²⁺ and [M+3H]³⁺, which are deconvoluted to give the molecular mass. On a certificate of analysis, the observed mass should be reported alongside the theoretical mass calculated from the sequence; agreement within about ±1 Da is typical for routine identity testing. Mass spectrometry confirms identity, while HPLC measures purity. For blends, each component should have its own confirmed mass.