This is my second independent submission on material from the same supplier.
I am trying to choose between two options that are usually discussed as though only one exists.
I am not optimising for price, but I am not indifferent to it either.
So which one, and on what grounds?
The honest answer is that an intact mass matching the theoretical value rules out a great deal and confirms less than people assume.
Tandem mass spectrometry fragments a selected precursor and reads the b and y ion series, which is what localises a substitution to a specific residue rather than merely detecting it.
| Δ mass (Da) | Most likely cause | Distinguishing feature |
|---|---|---|
| +1 | Deamidation (Asn or Gln) | New peak, slightly earlier retention |
| −17 | Loss of ammonia | Often with deamidation |
| −18 | Dehydration / succinimide | pH-dependent, reversible |
| +16 | Oxidation (Met, Trp) | Earlier retention, light-related |
| −128 | Missing Gln or Lys | Deletion sequence from synthesis |
| 0 | Isomer: racemisation or scrambling | Same mass, shifted retention |
Worked example: a peptide of monoisotopic mass 4113.6 daltons appears at m/z 1372.2 for the triply charged species and 1029.4 for the quadruply charged. Two charge states agreeing on the deconvoluted mass is a much stronger identity claim than one.
Differential response between UV and mass spectrometric detection is well characterised and is why purity figures from the two are not interchangeable.
Ask for the deconvoluted mass and at least two charge states.
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Browse resultsAnswering this needs the ionisation mode and the mass accuracy, because a low-resolution instrument cannot distinguish several of the shifts that matter.
The UV trace and the total ion chromatogram do not agree, and they should not. UV response depends on the chromophore; MS response depends on ionisation efficiency. A small UV peak can be a large MS peak and vice versa.
Because of that, purity by LC-MS peak area is not comparable to purity by UV area, and quoting one as though it were the other is a recurring source of confusion.
The caveat is that intact mass alone cannot detect a mass-neutral substitution, and several of the substitutions that matter most are mass-neutral.
Plus one, plus sixteen, minus eighteen. Learn those three shifts.
Put another way, charge-state envelopes on electrospray are how a multiply charged peptide gives you a molecular weight, and misreading them is a common error.
Mass accuracy decides what the result means. A high-resolution instrument at five parts per million distinguishes a plus-one deamidation from noise; a unit-resolution instrument does not.
The part that matters: ion suppression from co-eluting matrix components can hide a species entirely. A clean-looking total ion chromatogram is weaker evidence than a clean UV trace at the same gradient.
Tandem fragmentation producing b and y ion series is the basis of peptide sequencing by mass spectrometry.
Nothing here is medical advice, and research-use compounds are not approved for human use.
Mass-neutral substitutions need a digest. Ask for peptide mapping if identity is the question.
edited 4 May 2026 by charge_state_3 — fixed an arithmetic slip in the third paragraph
Start with what you want confirmed. Intact mass confirms the molecular formula and nothing about the order of the residues; tandem fragmentation confirms sequence.
Electrospray ionisation produces a series of multiply protonated species. The observed mass-to-charge ratio for charge state n is (M + n×1.00728) ÷ n, and deconvolution across several charge states is what gives the neutral monoisotopic mass.
Charge-state deconvolution from electrospray is standard practice and the arithmetic is the same for every peptide; agreement across charge states is the internal check.
Intact mass rules things out. Fragmentation confirms sequence.
edited 5 Apr 2026 by olu_babatunde — reworded for clarity after a comment
The relevant limitation is that mass spectrometric response is compound-dependent, so peak areas are not proportional to amount across different species.
Diagnostic shifts worth memorising: plus one is deamidation, plus sixteen is oxidation, minus eighteen is dehydration or a succinimide, and an unchanged mass with a shifted retention time is an isomer.
Do not compare MS purity with UV purity. Different detectors, different weightings.
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.