Details up front: +2 · 4113.6 Da.
I can do the algebra. I am not confident about the conversion factors.
If there is a standard way to lay this out, I would rather learn that than invent one.
Can someone show the working rather than just the answer?
m/z = 2057.81 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: (4113.6 + 2 × 1.00728) ÷ 2 = 4115.615 ÷ 2 = 2057.81. 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 1372.21, 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.
Scrambled disulfides have the same mass as correctly formed ones, so mass spectrometry alone cannot detect a scrambling failure.
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.
| 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 |
More usefully, 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.
Electrospray ionisation soft-ionisation behaviour is well-characterised and standards exist for m/z calibration and mass accuracy assessment.
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.
edited 4 Aug 2025 by nine_point_nine — updated for the 2026 guidance change
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsTandem mass spectrometry fragments the ions and measures the fragment masses, which provides sequence information and is the best tool for confirming identity.
Deconvolution of a mass spectrum with multiple charge states produces a reconstructed neutral mass, and errors in the deconvolution produce errors in the inferred mass.
Put another way, 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.
Always run a blank between samples and check for carry-over.
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.
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.
In practice, a D-amino-acid substitution has the same molecular weight as the L-form, so mass spectrometry cannot distinguish them without fragmenting the peptide.
A mass shift of minus one hundred and twenty-eight usually means a missing Gln or Lys residue from a synthesis deletion sequence.
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.
It helps to be literal here: two ions with the same nominal mass but different molecular formulae have different exact masses, and only high-resolution mass spectrometry can distinguish them.
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.
Worth noting that source contamination is common and silent, so a result that looks too good to be true often is.
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.
edited 3 Oct 2025 by nine_point_nine — clarified the distinction between purity and content
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.