shallow gradients hide related substances behind the main peak. a steeper one separates and looks worse, honestly
#hplc-massspec 2026-04-13
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within inter-lab range
nobody runs enough blanks, including me
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relative retention time against a standard is comparable between labs. absolute minutes are not
integration choices move the number by a few tenths easily. that is most of your inter-lab variance, someone check my working
semaglutide is about 4113.6 Da. [M+2H]2+ lands near 2057.8, [M+3H]3+ near 1372.2
i asked VendorInvestigate for the raw trace and they sent it. that is the right answer to that question
why do two labs integrate the same peak differently
right so why does integration change the number so much
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plus 16 is oxidation
need more sample
is uv at 214 or 280 better for peptides
do you trust an area percent from a shallow gradient
update on the earlier thing plus sixteen, is that always oxidation
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oxidation is about plus 15.99 Da and is usually methionine or tryptophan
has anyone ever had a result where the mass was right and the purity was wrong
coming back to this two labs a point or two apart on the same lot is normal and gets mistaken for one of them being wrong constantly
integration difference
run a blank
Reminder set. I will post here in 7 days.
show me the spectrum
peptidemeter-summary-sg-1177.pdf
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can you tell a dimer from the chromatogram alone
i was wrong about tailing factor. it is usually the injection or the pH, not the column
ms/ms sequencing is the only thing i would call identity. everything else is circumstantial
update from 4 months ago: switched to asking for the chromatogram rather than the number and it changed everything
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the honest position is that hobby-level interpretation of a chromatogram is worth something but not much, ymmv
thats a charge state
how do you spot a deletion in a sequence from mass alone
ms/ms or it isnt identity
deamidation is about plus 0.98 Da. it is not a rounding error, it is a degradant
baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column
how do you tell co-elution from a clean single peak
unrelated but a certificate with a number and no chromatogram is asking you to trust the integration you cannot see
214nm sees the amide backbone so it sees everything. 280 only sees aromatics, i could be wrong
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do the arithmetic
whats a plus one dalton shift usually
if somebody says they ran a mass spec and does not show you a spectrum, that is a claim, not a result, happy to be corrected
i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer
is a tailing factor of anything specific a problem
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you need more sample for identity than for purity. plan for that when you post it
is monoisotopic or average mass the one on most certificates
retention time is a hint
plus 0.98 is deamidation
[edited]thats the solvent peak
[edited]does retention time on its own tell you what the compound is
a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you
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if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic