The specifics, since they change the answer: +4 · 4207.2 Da.
I would like the arithmetic checked rather than the conclusion asserted.
I have deliberately not used an online calculator because I want to be able to check the result.
Can someone walk through the arithmetic step by step?
m/z = 1052.81 at 4+. Electrospray charges a peptide by adding protons, so the observed ion is the neutral mass plus 4 protons, all divided by the charge: (4207.2 + 4 × 1.00728) ÷ 4 = 4211.229 ÷ 4 = 1052.81. 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 842.45, 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.
Mass spectrometry is an identity technique, not a purity technique, and conflating the two is a common source of false confidence.
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.
| Component | Typical share | Counted in purity? | Counted in content? |
|---|---|---|---|
| Target peptide | 88–94 % | Yes, as main peak | Yes |
| Related impurities | 1–3 % | Yes, as other peaks | No |
| Counter-ion (TFA or acetate) | 2–8 % | No | No |
| Residual water | 2–6 % | No | No |
| Bulking agent, if present | 0–40 % | No | No |
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.
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 caveat is that a correct mass does not mean the peak is correct — isomers and co-eluting species can have the same m/z.
The practical summary: use mass spectrometry for identity, not for purity.
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Browse resultsThe part that matters: source contamination and carry-over between samples are the failure modes most specific to mass spectrometry, and they are invisible without a blank injection between samples.
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.
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.
I would not trust a mass result without a good baseline and a blank injection check.
A correct mass is necessary for identity but not sufficient — you also need the chromatography to confirm it.
Scrambled disulfides have the same mass as correctly formed ones, so mass spectrometry alone cannot detect a scrambling failure.
High-resolution mass spectrometry can distinguish a Lys-containing peptide from an Arg-containing peptide of similar mass because of the isotope difference.
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.
One qualification: high-resolution mass spectrometry gives high mass accuracy but low speed, and the reverse is true for low-resolution instruments.
Always run a blank between samples and check for carry-over.
edited 11 Oct 2024 by tandem_gradient — clarified the distinction between purity and content
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.
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.
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.
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 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.
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.
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.