What is mass-to-charge ratio (m/z)?
Definition
Mass-to-charge ratio (m/z) is the quantity a mass spectrometer measures: an ion's mass divided by its number of charges.
Why m/z, not mass
A mass spectrometer separates ions by how they move in electric and magnetic fields, which depends on mass relative to charge. An ion with twice the mass and twice the charge appears at the same m/z.
Charge states in peptide spectra
Electrospray ionization usually gives a large peptide several protons. The expected m/z for an ion with n protons is (M + n × 1.0073) ÷ n, where M is the molecular weight.
Selank example
With M = 751.87, one proton give m/z 752.9 and two protons give m/z 376.9. Working backward from an observed ion recovers the neutral mass, which is how LC-MS identity is checked.