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?
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.
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemThe 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 cap_the_luer — 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.
The practical summary: use mass spectrometry for identity, not for purity.
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.
A correct mass is necessary for identity but not sufficient — you also need the chromatography to confirm it.
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.