baseline is drifting badly, is that the column or the mobile phase
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#hplc-massspec 2026-06-28
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oxidation is about plus 15.99 Da and is usually methionine or tryptophan
nobody runs enough blanks, including me
my peak has a shoulder on it, what does that usually mean
does retention time on its own tell you what the compound is
whats a plus one dalton shift usually
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ms/ms or it isnt identity
plus sixteen, is that always oxidation
show me the spectrum
thats co-elution
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i was wrong about tailing factor. it is usually the injection or the pH, not the column
thats the solvent peak
how do you spot a deletion in a sequence from mass alone
if the lab reports area percent without stating the wavelength, the number is less useful than it looks, still working it out
quick one i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer
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need more sample
ask for the trace
plus 16 is oxidation
how much sample does a lab actually need for identity as well as purity
integration choices move the number by a few tenths easily. that is most of your inter-lab variance
thats a charge state
why do two labs integrate the same peak differently
run a blank
retention time is a hint, not a fingerprint. two different peptides can co-elute perfectly happily
retention time is a hint
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while im here how do you tell co-elution from a clean single peak
whats a normal gradient for a peptide this size
plus 0.98 is deamidation
shallow gradient
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Testing queue: 2 submissions open, 83 awaiting dispatch.
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is monoisotopic or average mass the one on most certificates
214nm sees the amide backbone so it sees everything. 280 only sees aromatics, ymmv
update from 25 months ago: switched to asking for the chromatogram rather than the number and it changed everything
the honest position is that hobby-level interpretation of a chromatogram is worth something but not much
within inter-lab range
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shallow gradients hide related substances behind the main peak. a steeper one separates and looks worse, honestly
tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5, still working it out
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semaglutide is about 4113.6 Da. [M+2H]2+ lands near 2057.8, [M+3H]3+ near 1372.2
monoisotopic and average diverge more the bigger the molecule. say which one you are quoting
*PeptideMeter not the other one
if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic
thats a charge state
M = 4113.6
[M+2H]2+ = (4113.6 + 2.02) / 2 = 2057.8
[M+3H]3+ = (4113.6 + 3.02) / 3 = 1372.2
observed 1372.3 -> consistent🧊1🙏2⚠️8
a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you
Inter-lab diff for lot F-1330: 98.6% vs 99.4%. Within expected range.
slightly off topic but is a tailing factor of anything specific a problem
Reminder for madrid_mg: dose day is today. Set 4 days ago.
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