A mass-spectrum coordinate does not become an uncharged molecular mass merely because the axis is labeled m/z.
State the nominal coordinate convention
The cited IUPAC entry describes the long-used m/z notation as a dimensionless ion mass-number divided by charge-number quantity. This guide uses that nominal convention explicitly. Charge number is an integer count, not a charge in coulombs inserted into the same numerical division. Primary reference: IUPAC: mass-to-charge ratio in mass spectrometry.
Exact-mass reporting requires its appropriate mass and ion convention in addition to this simple nominal example. Even when an ion coordinate is established, it does not by itself identify the neutral molecule: the ion species, adducts or losses can also matter.
Compare two original charge assignments
For a fictional ion with stipulated nominal mass number 600 and positive charge number 2, the nominal m/z coordinate is 300. An ion with stipulated nominal mass number 300 and charge number 1 has the same coordinate. The one coordinate therefore does not choose between those two charge assignments.
Multiplying the first coordinate by its stated charge number recovers nominal ion mass number 600. Labeling 300 as that ion mass would omit the charge. Neither invented ion is a formula assignment, and neither example specifies the mass of a neutral precursor or an exact isotopic mass.
Preserve charge and ion identity separately
Make fields for observed coordinate, nominal or exact convention, charge sign and magnitude, ion species and the evidence for assigning them. Reconstruct the ion quantity only with its declared convention. Keep a proposed neutral-molecule calculation in a separate field that states any adduct or loss treatment.
Before comparing spectra, check whether the ion species and charge basis match. A matching m/z coordinate alone does not prove chemical identity. This worksheet addresses coordinate interpretation and avoids claims about sensitivity, ionization efficiency or a mass spectrometer offered by HM.
Customer Questions
What is the fictional 600/2 coordinate?
It is nominal m/z 300.
Does m/z 300 alone establish nominal ion mass 600?
The charge assignment is also needed.
Does the example calculate neutral mass?
Only the stipulated ion convention is calculated.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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