What does [M+4H]4+ mean?

It describes a peptide molecule (M) that has picked up four protons (4H) and so carries four positive charges (4+). Mass spectrometers measure mass divided by charge, so the value reported is the molecule’s mass plus four proton masses, divided by four.

Does a matching mass prove the product is right?

It shows a molecule of that mass is present. It doesn’t show how much, whether other things are also present, or that two different molecules with the same mass can’t be confused, so it is read alongside purity and quantity.

Reading a mass-spec result on a peptide COA: what [M+4H]4+ means

By PeptideProof · Updated 13 October 2026 · 5 min read

The identity line on a peptide COA often reads something like “Confirmed, [M+4H]4+ = 1029.6”. That is shorthand you can check yourself with a little arithmetic — and checking is the best way to know whether “confirmed” means anything.

What the notation means

M is the neutral peptide molecule and its mass in daltons (Da).
[M+4H] is that molecule with four protons added.
4+ is the resulting charge. A mass spectrometer reports mass divided by charge (m/z).

The calculation

For a positively charged ion with n extra protons, each proton adding about 1.007 Da:

m/z = (M + n × 1.00728) ÷ n

A worked example (a made-up peptide of mass 4,000.00 Da)

[M+3H]3+ = (4000.00 + 3.02) ÷ 3 ≈ 1334.34

[M+4H]4+ = (4000.00 + 4.03) ÷ 4 ≈ 1001.01

[M+5H]5+ = (4000.00 + 5.04) ÷ 5 ≈ 801.01

Run it the other way and you can check a report: a stated [M+4H]4+ = 1001.0 implies a mass of about 4 × 1001.0 − 4 × 1.007 ≈ 4,000 Da. If that doesn’t match the mass expected for the named peptide, “confirmed” isn’t.

Why there are several values

A peptide can pick up different numbers of protons, so one molecule shows up at several m/z values, one per charge state. Software combines them into a single molecular mass. A good report shows the spectrum or lists several charge states, not just a single number.

Things to check

The measured value is stated, not just the word “confirmed”.
It agrees with the expected mass for the named peptide, within the instrument’s tolerance.
Which mass is used: monoisotopic (the lightest isotopes) or average. They differ by a fraction of a dalton for small molecules and by more for large ones, so the report should be comparing like with like.
It is the same sample and batch as the purity result.

For how this sits with the other numbers on a report, see purity, quantity and identity.

Frequently asked questions

What does [M+4H]4+ mean?: It describes a peptide molecule (M) that has picked up four protons (4H) and so carries four positive charges (4+). Mass spectrometers measure mass divided by charge, so the value reported is the molecule’s mass plus four proton masses, divided by four.
Why does a peptide show several mass-to-charge values?: Larger molecules can pick up different numbers of charges, so the same peptide appears at several m/z values, one per charge state. Software can combine them to calculate the molecular mass, which should agree with the expected mass.
Does a matching mass prove the product is right?: It shows a molecule of that mass is present. It doesn’t show how much, whether other things are also present, or that two different molecules with the same mass can’t be confused, so it is read alongside purity and quantity.

See it on a real document

PeptideProof reads a COA for you and shows what is only reported, what has no evidence and what looks inconsistent. See an example first, or check your own — new accounts include free analyses.

See an example report Verify a COA

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Educational information about reading laboratory documents. It is not medical, dosing or purchasing advice, and it doesn’t say that any product is safe, legal or suitable for any use. See the medical and research-use disclaimer.