I have both a purity figure and a content figure, which is why the discrepancy is visible.
I can predict the outcome but I cannot explain it, which means I will get the next case wrong.
I would like to know how confident the field actually is about this.
So what is the mechanism, and how well established is it?
The honest answer is that an intact mass matching the theoretical value rules out a great deal and confirms less than people assume.
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.
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.
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.
The caveat is that intact mass alone cannot detect a mass-neutral substitution, and several of the substitutions that matter most are mass-neutral.
Ask for the deconvoluted mass and at least two charge states.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsThe relevant limitation is that mass spectrometric response is compound-dependent, so peak areas are not proportional to amount across different species.
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.
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.
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.
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.
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.
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.
Intact mass rules things out. Fragmentation confirms sequence.
In practice, a mass-neutral substitution — a D-amino acid, a scrambled disulfide — is invisible to intact mass and visible on a digest.
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.
Differential response between UV and mass spectrometric detection is well characterised and is why purity figures from the two are not interchangeable.
Mass-neutral substitutions need a digest. Ask for peptide mapping if identity is the question.
It helps to be literal here: this is the technique that settles identity arguments, and it settles them only if the acquisition parameters are reported.
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.
Peak areas in mass spectrometry are not proportional to amount across different species and should not be read as if they were.
Plus one, plus sixteen, minus eighteen. Learn those three shifts.
edited 20 Aug 2026 by tri_gly_ala — fixed an arithmetic slip in the third paragraph
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