Mass-to-Charge Ratio (M/z)
Definition
The fundamental MS measurement: the mass of an ion divided by its charge number. A singly charged ion of mass 250 Da appears at m/z 250; a doubly charged ion of the same mass appears at m/z 125. The mass analyser sorts ions by this ratio, not by mass alone.
- Symbol
- m/z
- Definition
- Ion mass divided by its charge number
- Effect of charge state
- A 250 Da ion at charge 2+ reads at m/z 125
- What sorts on it
- The mass analyser, not mass alone
Common questions
Why does a multiply charged ion matter for interpreting a spectrum?+
Two different ions of different mass but the same charge-to-mass ratio can land at the same m/z value, so an analyst must consider isotope spacing or complementary charge states to resolve which mass actually produced a given peak.
How is m/z different from molecular weight on a spectrum?+
Molecular weight is a fixed property of the compound, while m/z is what the instrument actually records, and it shifts with ionisation mode and charge state, which is why the same molecule can appear at more than one position.
Related terms
- Accurate Mass
- A mass measured to four or more decimal places, sufficient to compute a unique molecular formula. High-resolution accurate-mass (HRAM) instruments (Q-TOF, Orbitrap,...
- Base Peak
- The most intense peak in a mass spectrum, normalised to 100 percent relative abundance. All other peaks in the spectrum are reported...
- Molecular Ion (M+)
- The intact ionised molecule with no fragmentation, formed by removing a single electron from the parent. Its m/z gives the molecular weight...
- Resolution
- The ability of a mass analyser to distinguish two ions of nearly equal m/z, defined as m/Δm at a given peak. A...
- Tandem MS (MS/MS)
- Two stages of mass selection in series with a fragmentation step in between. The first stage selects a precursor ion, a collision...