The specifics, since they change the answer: +4 · 4813.5 Da.
I would rather understand the derivation than memorise the outcome.
Two people I asked gave two answers that differ by a factor of ten, which is suggestive.
Can someone walk through the arithmetic step by step?
It helps to be literal here: a mass shift of exactly zero with a shifted retention time points to an isomer — a scrambled disulfide or a racemised residue — which mass spectrometry alone cannot identify.
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.
In practice, 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.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsA D-amino-acid substitution has the same molecular weight as the L-form, so mass spectrometry cannot distinguish them without fragmenting the peptide.
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.
Worth being precise here: 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.
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.
edited 29 Apr 2025 by dead_volume — corrected a unit error in the worked example
Scrambled disulfides have the same mass as correctly formed ones, so mass spectrometry alone cannot detect a scrambling failure.
A mass shift of minus eighteen usually means dehydration or a succinimide intermediate, which is pH-dependent and can be reversible.
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.
Electrospray ionisation soft-ionisation behaviour is well-characterised and standards exist for m/z calibration and mass accuracy assessment.
I would not trust a mass result without a good baseline and a blank injection check.
The practical summary: use mass spectrometry for identity, not for purity.
Identity confirmation from mass spectrometry means matching the observed m/z to the calculated m/z for your peptide at its known charge states.
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.
False positives from contamination are common in mass spectrometry work, and running a blank between every sample and a solvent background are standard practice.
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.
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.
High-resolution mass spectrometry can distinguish a Lys-containing peptide from an Arg-containing peptide of similar mass because of the isotope difference.
Peptide mapping — enzymatic digestion followed by tandem mass spectrometry — can confirm the primary sequence and is the method of choice when identity is ambiguous.
Worth noting that source contamination is common and silent, so a result that looks too good to be true often is.
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.