M/z (Mass-to-Charge Ratio)
Definition
The x-axis of every mass spectrum. Ions are separated by their mass divided by the number of charges they carry. For singly-charged ions, m/z equals the ion mass in daltons.
- Axis
- X-axis of a mass spectrum
- Definition
- Ion mass divided by number of charges
- Singly charged ions
- m/z equals ion mass in daltons
- Used by
- Mass analyzers to separate and detect ions
Common questions
Why does m/z matter for multiply charged ions?+
When an ion carries two or more charges, its observed m/z is a fraction of its true mass, so a large biomolecule can appear at a much lower m/z value than its actual mass would suggest. Software must deconvolute the charge state to recover the true molecular mass from such spectra.
How is m/z different from simply reporting an ion's mass?+
Mass spectrometers do not measure mass directly, they measure how ions of a given mass and charge behave in electric or magnetic fields, and that behavior scales with the ratio of mass to charge rather than mass alone. Reporting m/z rather than mass reflects what the instrument actually resolves.
Can two different compounds produce peaks at the same m/z value?+
Yes, this is called isobaric interference, where fragments or molecular ions of different composition happen to share a nominal or even exact m/z. High-resolution instruments and characteristic fragmentation patterns help analysts distinguish such overlapping signals in casework.
Related terms
- Base Peak
- The most intense peak in a mass spectrum, normalised to 100 percent relative abundance. All other peaks in the spectrum are reported...
- EI (Electron Ionisation)
- Hard ionisation at 70 eV. Produces extensive fragmentation, ideal for library search. Standard ion source in GC-MS.
- ESI (Electrospray Ionisation)
- Soft ionisation at atmospheric pressure for polar and thermally fragile analytes in solution. Standard source in LC-MS for drugs and biomolecules.
- Fragmentation
- Breakdown of the molecular ion into smaller charged and neutral pieces. The fragmentation pattern is the chemical fingerprint that identifies the compound.
- MALDI
- Matrix-Assisted Laser Desorption / Ionisation. A soft method that uses a UV-absorbing matrix to ionise large biomolecules. Almost always paired with a...
- 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...
- Nitrogen Rule
- An organic molecule with an odd-numbered molecular weight contains an odd number of nitrogen atoms. A staple trick for narrowing down a...
- TOF (Time-of-Flight)
- Mass analyser that separates ions by their flight time down a field-free drift tube. Theoretically unlimited mass range, very high speed.