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?
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 tyndall_haze — updated for the 2026 guidance change
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsTandem 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.
Worth being precise 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 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.
The practical summary: use mass spectrometry for identity, not for purity.
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.
Always run a blank between samples and check for carry-over.
edited 3 Oct 2025 by zainab_mustafa — 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.