right so integration choices move the number by a few tenths easily. that is most of your inter-lab variance
#hplc-massspec 2026-04-16
Thursday34 messages9 participantstimes are UTC
Highlights from this day
- thirty_min_wait — while im here how do you tell co-elution from a clean single peak 17:57
- ms_ms_mira — oxidation is about plus 15.99 Da and is usually methionine or tryptophan 18:42
- food_noise_off — slightly off topic but i was wrong about tailing factor. it is usually the injection or the pH, not the column 19:36
- ms_ms_mira — is uv at 214 or 280 better for peptides 21:37
- split_dose_sam — my peak has a shoulder on it, what does that usually mean 21:39
tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5
can you tell a dimer from the chromatogram alone
*VendorInvestigate not the other one
is there any point running a blank if the sample is the question
you need more sample for identity than for purity. plan for that when you post it
semaglutide is about 4113.6 Da. [M+2H]2+ lands near 2057.8, [M+3H]3+ near 1372.2
while im here how do you tell co-elution from a clean single peak
transit-times.csv
492 rows · not retained in the public archive
how would a truncation sequence show up
retention time is a hint, not a fingerprint. two different peptides can co-elute perfectly happily
update on the earlier thing does retention time on its own tell you what the compound is
oxidation is about plus 15.99 Da and is usually methionine or tryptophan
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genuine question a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you, ill dig out the number
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whats a plus one dalton shift usually
why do two labs integrate the same peak differently
the first peak is the solvent. it is always the solvent, n of 1 obviously
a shoulder on a peak is information, and usually bad information
show me the spectrum
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slightly off topic but i was wrong about tailing factor. it is usually the injection or the pH, not the column
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semaglutide is about 4113.6 Da. [M+2H]2+ lands near 2057.8, [M+3H]3+ near 1372.2, n of 1 obviously
why does integration change the number so much
nobody runs enough blanks, including me
plus 16 is oxidation
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 -> consistentmonoisotopic and average diverge more the bigger the molecule. say which one you are quoting
ms/ms or it isnt identity
retention time is a hint
is a tailing factor of anything specific a problem
plus sixteen, is that always oxidation
is monoisotopic or average mass the one on most certificates
plus 0.98 is deamidation
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thats co-elution
deamidation is about plus 0.98 Da. it is not a rounding error, it is a degradant
is uv at 214 or 280 better for peptides
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my peak has a shoulder on it, what does that usually mean
weights-monthly.csv
436 rows · not retained in the public archive
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do the arithmetic
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