How do I compute the +5 charge state m/z for a peptide of 4731.3 Da?

Concretely: +5 · 4731.3 Da.

Please show the division. I want to check my own against yours.

I would like the general form as well as the specific number, so I can apply it again.

Is my approach right even if my number is wrong?

m/z = 947.27 at 5+. Electrospray charges a peptide by adding protons, so the observed ion is the neutral mass plus 5 protons, all divided by the charge: (4731.3 + 5 × 1.00728) ÷ 5 = 4736.336 ÷ 5 = 947.27. The proton term is the one people drop, and because it is z protons over z charges it shifts m/z by 1.007 at every charge state — small, and far larger than the mass accuracy of the instrument. The neighbouring charge state sits at 789.56, and seeing the two of them where they belong is better identity evidence than either one alone. Use the average mass against an average-mass calculation and the monoisotopic mass against a monoisotopic one; mixing them costs you a couple of daltons on a peptide this size.

A mass shift of exactly zero with a shifted retention time points to an isomer — a scrambled disulfide or a racemised residue — which mass spectrometry alone cannot identify.

A mass shift of plus one usually means deamidation at asparagine or glutamine, which creates a secondary amine instead of an amide and changes the mass by exactly one.

Worth being precise here: a monoisotopic mass includes only the lightest isotope of each element, while the average mass weights by natural isotope abundance, and small peptides use monoisotopic mass.

A correct mass is necessary for identity but not sufficient — you also need the chromatography to confirm it.

Specifically, identity confirmation from mass spectrometry means matching the observed m/z to the calculated m/z for your peptide at its known charge states.

The baseline noise on a mass spectrum sets the limit of detection, and a weak signal close to the noise is not reliable evidence for the presence of a species.

On the detail: a mass shift of minus one hundred and twenty-eight usually means a missing Gln or Lys residue from a synthesis deletion sequence.

Peptide mapping — enzymatic digestion followed by tandem mass spectrometry — can confirm the primary sequence and is the method of choice when identity is ambiguous.

Always run a blank between samples and check for carry-over.

Stated carefully, the single most important fact about mass spectrometry for peptides is that it measures only the molecular weight and tells you almost nothing about whether the peak is actually your target.

A mass shift of minus eighteen usually means dehydration or a succinimide intermediate, which is pH-dependent and can be reversible.

High-resolution mass spectrometry can distinguish a Lys-containing peptide from an Arg-containing peptide of similar mass because of the isotope difference.

The practical summary: use mass spectrometry for identity, not for purity.

Start from what electrospray ionisation does: it ionises the peptide without fragmenting it, creating singly or multiply charged species that the mass analyser then separates by their mass-to-charge ratio.

A mass shift of plus sixteen usually means oxidation at methionine or tryptophan, which is common in peptides and often comes from sample handling rather than synthesis failure.

False positives from contamination are common in mass spectrometry work, and running a blank between every sample and a solvent background are standard practice.

One qualification: high-resolution mass spectrometry gives high mass accuracy but low speed, and the reverse is true for low-resolution instruments.

If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.

edited 22 Mar 2025 by RP_C18 — tightened the wording; no substantive change

In practice, tandem mass spectrometry fragments the ions and measures the fragment masses, which provides sequence information and is the best tool for confirming identity.

For a large peptide with multiple peaks in the mass spectrum, comparing the observed isotope pattern to the calculated pattern is a quick check that the formula matches.

Electrospray ionisation soft-ionisation behaviour is well-characterised and standards exist for m/z calibration and mass accuracy assessment.

In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.