mass-to-charge ratio
/ mass-to-CHARJ RAY-shee-oh /
Picture pushing two children on swings with the same gust of wind. A heavy child barely moves; a light child swings far. But if the light child also clutches two helium balloons (less to push against), they swing even farther. What the wind 'feels' is not pure weight but weight divided by how much push is acting — and a mass spectrometer feels something just like that about its ions.
The mass-to-charge ratio, written m/z, is exactly what a mass analyzer measures: the mass of an ion divided by the number of charges it carries. An ion of mass 600 carrying a single charge appears at m/z 600; the same mass carrying two charges appears at m/z 300. The symbol z is the number of charges, and m is the mass.
It matters because instruments sort ions by m/z, not by mass alone — so to read a spectrum you must keep charge in mind. This is also a gift: large molecules that carry many charges show up at modest m/z values that ordinary analyzers can handle. The caveat is that a single peak can be ambiguous until you know its charge, which is why multiply charged ions and isotope spacing are read together to deduce the true mass.
A doubly charged peptide ion of mass 1500 does not appear at 1500 on the spectrum; it appears at m/z 750, because the mass is divided by its two charges.
An ion's position depends on mass divided by charge, not mass alone.
m/z is a ratio and is formally dimensionless in many conventions, though it is read informally 'in mass units'. When an ion carries a single charge (z = 1), m/z equals the ion's mass — which is why single-charge spectra are the easiest to interpret.