monoisotopic and average diverge more the bigger the molecule. say which one you are quoting
#hplc-massspec 2025-06-26
Thursday25 messages5 participantstimes are UTC
Highlights from this day
- vial_ledger — if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic 19:26
- vial_ledger — if the lab reports area percent without stating the wavelength, the number is less useful than it looks, not advice obviously 20:12
- tail_factor — two labs a point or two apart on the same lot is normal and gets mistaken for one of them being wrong constantly nobody runs enough blanks, including me 20:32
- vial_ledger — whats a normal gradient for a peptide this size 20:48
- vial_ledger — sorry to jump in oxidation is about plus 15.99 Da and is usually methionine or tryptophan 21:19
if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic
while im here is a tailing factor of anything specific a problem
thats co-elution
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plus 0.98 is deamidation
i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer
is there any point running a blank if the sample is the question
how would a truncation sequence show up
tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5
co-elution is the failure mode that a beautiful single peak is most likely to be hiding
slightly off topic but does retention time on its own tell you what the compound is
unrelated but why does integration change the number so much
if the lab reports area percent without stating the wavelength, the number is less useful than it looks, not advice obviously
ms/ms sequencing is the only thing i would call identity. everything else is circumstantial, anyway
baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column
two labs a point or two apart on the same lot is normal and gets mistaken for one of them being wrong constantly
nobody runs enough blanks, including me
how do you tell co-elution from a clean single peak
whats a normal gradient for a peptide this size
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i asked PeptideMeter for the raw trace and they sent it. that is the right answer to that question
integration choices move the number by a few tenths easily. that is most of your inter-lab variance
ms/ms or it isnt identity
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i have a number at about a third of what i expected, is that a charge state
sorry to jump in oxidation is about plus 15.99 Da and is usually methionine or tryptophan
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need more sample
my peak has a shoulder on it, what does that usually mean