retention time is a hint, not a fingerprint. two different peptides can co-elute perfectly happily
#hplc-massspec 2026-06-12
Friday24 messages5 participantstimes are UTC
Highlights from this day
- ghent_gradient — the first peak is the solvent. it is always the solvent 19:15
- ferritin_fay — nobody runs enough blanks, including me 19:35
- ferritin_fay — baseline is drifting badly, is that the column or the mobile phase baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column 19:36
oxidation is about plus 15.99 Da and is usually methionine or tryptophan
update from 3 months ago: switched to asking for the chromatogram rather than the number and it changed everything
slightly off topic but why do two labs integrate the same peak differently
a shoulder on a peak is information, and usually bad information
integration choices move the number by a few tenths easily. that is most of your inter-lab variance
a certificate with a number and no chromatogram is asking you to trust the integration you cannot see
the first peak is the solvent. it is always the solvent
fridge-temps.csv
373 rows · not retained in the public archive
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is there any point running a blank if the sample is the question
if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic
plus 0.98 is deamidation
nobody runs enough blanks, including me
peak RRT area% note
1 0.42 0.31 solvent front
2 0.88 0.44 related substance
3 1.00 98.72 main
4 1.14 0.53 related substancebaseline is drifting badly, is that the column or the mobile phase
baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column
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ask for the trace
tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5
i asked Medutest for the raw trace and they sent it. that is the right answer to that question
genuine question a warm transit can put a visible degradant peak on a chromatogram. that is what 5 days at ambient does
i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer
a shoulder on a peak is information, and usually bad information
does retention time on its own tell you what the compound is
update on the earlier thing is uv at 214 or 280 better for peptides
is a tailing factor of anything specific a problem
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
coming back after 9 months, what changed in how you read these
within inter-lab range
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🔥13😂2❤️3