Accurate Mass and Mass Defect
Definition
On a Q-TOF or Orbitrap, the measured m/z is reported to four decimal places at parts-per-million accuracy. The deviation from the nominal integer mass (the mass defect) constrains the molecular formula because each element contributes a characteristic decimal.
- Reporting precision
- Four decimal places, ppm-level accuracy
- Instruments
- Q-TOF, Orbitrap
- Mass defect
- Deviation from the nominal integer mass
- Use
- Constrains the candidate molecular formula
Common questions
What causes the mass defect to differ between elements?+
Each element's isotope mass deviates from a whole number by an amount tied to nuclear binding energy, so the decimal portion of a measured mass reflects which elements and how many of each are present.
Why is mass defect useful beyond the accurate mass itself?+
It lets an analyst rule out formula candidates that would require an implausible number of hydrogens, nitrogens or halogens, tightening the identification alongside the accurate mass value.
Is mass defect analysis limited to one instrument type?+
No, any high-resolution accurate-mass instrument such as a Q-TOF or Orbitrap can measure it, though resolving power affects how confidently very close masses can be distinguished.
Related terms
- CID and HCD
- Collision-induced dissociation. The precursor is accelerated and collides with a neutral gas (nitrogen, argon, helium) in the collision cell. Internal energy converts...
- Deuterated Internal Standard
- An isotopologue of the analyte where two to five hydrogens are replaced by deuterium. Same chromatographic and ionisation behaviour, slightly higher mass....
- MRM (Multiple Reaction Monitoring)
- A targeted MS/MS scan mode on a triple quadrupole that monitors specific precursor-to-product ion transitions, with two transitions per analyte (quantifier and...
- Quantifier and Qualifier Transitions
- The most intense MRM transition is the quantifier and is used for the concentration calculation. The second (and sometimes third) transition is...
- 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...