Mass-accuracy and charge-state calculator
Theoretical against observed monoisotopic mass in ppm, mDa and absolute Da, plus the m/z ladder for charge states 1+ to 6+.
| z | Theoretical m/z | Observed m/z | Δ |
|---|---|---|---|
| enter a theoretical mass | |||
Defaults are semaglutide's theoretical monoisotopic mass and a plausible observation. The proton mass used is 1.007276 Da, the mass of a proton rather than of a hydrogen atom — using 1.00794 instead is the commonest arithmetic error in this calculation and produces an offset that grows with charge state.
The arithmetic
ppm error = (observed − theoretical) ÷ theoretical × 1 000 000 m/z = (M + z × 1.007276) ÷ z
ppm is a relative unit and that is the whole point
A 10 ppm error is 0.003 Da on a 340 Da tripeptide and 0.041 Da on a 4,113 Da incretin. Instruments do not achieve a fixed absolute accuracy; they achieve a fixed relative one, so the specification has to be relative too. This is also why the index sets its mass-accuracy limit by compound class: the classes differ in mass by more than an order of magnitude, and a single global ppm limit would be far too loose at the bottom and unachievable at the top.
Monoisotopic, not average
All ppm figures on this site are computed against the monoisotopic mass — the mass calculated using the lightest isotope of each element. For a 4,000 Da peptide the average mass is about 2.5 Da higher than the monoisotopic mass, so mixing the two produces an apparent error of roughly 600 ppm and a certificate that looks catastrophically wrong when nothing is. This confusion is common enough that it has its own method page.
What a mass measurement establishes
That a species of the measured mass is present. Not that it is the only species present, not that the sequence is correct, and not that stereochemistry is correct. Every diastereomer of a peptide has exactly the same mass as the parent, and no mass spectrometer of any resolution will ever separate them — only chromatography does. A compound page that lists a mass-identical impurity is telling you that its purity figure, not its identity figure, is the one carrying the risk.