I am comparing a supplier certificate against an independent result on the same lot.
I would like help reading this properly rather than being told what conclusion to reach.
I have the full report including the method section, so I can quote specifics if that helps.
What would I need in addition before this supported a decision?
Start with what you want confirmed. Intact mass confirms the molecular formula and nothing about the order of the residues; tandem fragmentation confirms sequence.
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.
| Test | Answers | Does NOT answer |
|---|---|---|
| RP-HPLC, area % | What fraction of detected material is the target | How much target is present |
| Quantified content | Milligrams of peptide per vial | What the impurities are |
| ESI-MS identity | Whether the molecular weight matches | Purity, or isomeric substitution |
| Peptide mapping | Sequence, localised to a fragment | Quantity |
| Karl Fischer | Water content of the solid | Solvent content |
| LAL endotoxin | Pyrogen load in EU/mg | Sterility |
| Sterility test | Growth in defined media over 14 days | Endotoxin, or bioburden count |
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.
High-resolution accurate-mass instruments routinely achieve better than five parts per million, which is what makes single-dalton discrimination possible on a four-kilodalton peptide.
Peak areas in mass spectrometry are not proportional to amount across different species and should not be read as if they were.
Intact mass rules things out. Fragmentation confirms sequence.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsThe short version: LC-MS gives you identity and a purity estimate weighted very differently from the UV trace, and the difference between the two is informative.
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.
Stated carefully, 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 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.
edited 1 Jul 2025 by plate_count_9k — removed a claim I could not source
Answering this needs the ionisation mode and the mass accuracy, because a low-resolution instrument cannot distinguish several of the shifts that matter.
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 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.
Tandem fragmentation producing b and y ion series is the basis of peptide sequencing by mass spectrometry.
Do not compare MS purity with UV purity. Different detectors, different weightings.
Answer first: liquid chromatography separates, mass spectrometry identifies, and the two together answer a question that neither answers alone.
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.
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.
To be exact about it, this is the technique that settles identity arguments, and it settles them only if the acquisition parameters are reported.
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.
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.
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.
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.