How do I compute the +3 charge state m/z for a peptide of 4731.3 Da?

What I am working with: +3 · 4731.3 Da.

The units are where I keep going wrong, so please be explicit about them.

I have sanity-checked the order of magnitude and it seems right, which is not the same as being right.

How many significant figures are actually justified here?

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

The part that matters: 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.

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

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

A D-amino-acid substitution has the same molecular weight as the L-form, so mass spectrometry cannot distinguish them without fragmenting the peptide.

The charge state distribution depends on the solution pH, the structure of the peptide and the source conditions, so the same peptide can look different under different conditions.

Concretely, electrospray ionisation creates multiple charge states of the same peptide — a 4 kDa peptide might appear at +2, +3 and +4 — and all of them must be accounted for in the spectrum.

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

edited 14 Mar 2026 by Dr_Signe_Baldursdottir — fixed an arithmetic slip in the third paragraph

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

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

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

The limitation is that mass spectrometry tells you the mass and almost nothing else, so it needs to be paired with chromatography or other identity information.

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

edited 13 Apr 2026 by mz_4113 — added the placebo-arm figures