01One molecular mass can appear in several places

Fenn and colleagues described coherent sequences of multiply charged ions in their original electrospray work. For a peptide measured as protonated ions, increasing the charge places the same neutral mass at a lower m/z. This is a redistribution along the mass-to-charge axis, not a change in the peptide's residue count.Fenn and colleagues — Electrospray ionisation of large biomolecules (opens in a new tab)

These values use a proton mass rounded to 1.0073 Da. The series is hypothetical and does not predict the relative intensities or which charge states a particular peptide will produce. The separate m/z guide derives the underlying calculation.NIST — 2022 CODATA constants (opens in a new tab)

02Test whether the proposed charges give one mass

For each assigned protonated ion, multiply its m/z minus the proton mass by its charge number. The three example signals all return 1,800 Da. This agreement is the useful pattern: several independent positions fit one proposed molecular mass with different integer charges.

Notice that adjacent gaps in the example are not equal. The difference between the 2+ and 3+ positions is 300, while the difference between 3+ and 4+ is 150. Treating charge states as a uniformly spaced ladder would misread the spectrum.

Now imagine an additional signal at m/z 700. A claim that it belongs to this same proton-only series must produce a supported integer charge and compatible mass. It cannot be included merely because it sits between two assigned peaks. It may represent another ion form, another component or a different feature requiring investigation.

03Distinguish charge clusters from isotope peaks

Within a single charge state, isotopic variants can produce a close cluster of signals. That cluster is a different pattern from the widely spaced charge states in the table. The isotope-envelope guide explains how the small spacings can help assess charge when the data resolve them.

For a hypothetical well-resolved cluster, a near-half-unit isotope spacing is compatible with a doubly charged ion. It is supporting evidence to combine with mass and the expected envelope, rather than a rule to apply to any arbitrary pair of peaks separated by roughly that distance.

04Use the whole analytical record

Where chromatography is available, matching elution profiles can support grouping candidate charge states from the same peptide. Waters' primer illustrates this relationship. However, distinct compounds can co-elute, so time agreement alone does not replace compatible mass assignments.Waters — Mass-directed peptide analysis considerations (opens in a new tab)

Relative charge-state intensity is also not a fixed sequence barcode. Xu and colleagues' peptide analysis demonstrates that observed charging behaviour is richer than a simple count of basic sites. Preserve the measured pattern and conditions rather than requiring one predicted charge to dominate.Xu and colleagues — Distinct peptide charging behaviours (opens in a new tab)

A useful annotation links each proposed charge to the inferred mass and any isotope or chromatographic evidence. Unassigned features should remain unassigned until there is enough information to classify them. This avoids both overcounting peptide species and incorrectly absorbing a real additional component into the target's series.