Numbers first: +2 · 3751.0 Da.
This should be a straightforward calculation and I keep getting two different answers.
The numbers are arbitrary; the method is what I am after.
Can someone show the working rather than just the answer?
m/z = 1876.51 at 2+. Electrospray charges a peptide by adding protons, so the observed ion is the neutral mass plus 2 protons, all divided by the charge: (3751 + 2 × 1.00728) ÷ 2 = 3753.015 ÷ 2 = 1876.51. 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 1251.34, 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.
Two ions with the same nominal mass but different molecular formulae have different exact masses, and only high-resolution mass spectrometry can distinguish them.
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.
Stated carefully, high-resolution mass spectrometry can distinguish a Lys-containing peptide from an Arg-containing peptide of similar mass because of the isotope difference.
False positives from contamination are common in mass spectrometry work, and running a blank between every sample and a solvent background are standard practice.
The practical summary: use mass spectrometry for identity, not for purity.
edited 31 Jul 2026 by kwn_analytical — clarified the distinction between purity and content
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Browse resultsOn the detail: identity confirmation from mass spectrometry means matching the observed m/z to the calculated m/z for your peptide at its known charge states.
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.
The m/z accuracy achievable depends on the mass analyser type — quadrupole gives low accuracy, time-of-flight gives moderate accuracy, and Orbitrap gives high accuracy.
The caveat is that a correct mass does not mean the peak is correct — isomers and co-eluting species can have the same m/z.
Always run a blank between samples and check for carry-over.
edited 11 Jul 2026 by plate_count_9k — added the citation requested in comments
Concretely, mass spectrometry is an identity technique, not a purity technique, and conflating the two is a common source of false confidence.
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.
More usefully, 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.
Peptide mapping — enzymatic digestion followed by tandem mass spectrometry — can confirm the primary sequence and is the method of choice when identity is ambiguous.
A correct mass is necessary for identity but not sufficient — you also need the chromatography to confirm it.
More usefully, coupling HPLC to a mass spectrometer adds identity information to the chromatographic separation, but the mass spectrometer's ionisation conditions can distort the HPLC peak shape.
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.
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.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.