a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you
#hplc-massspec 2026-04-11
Saturday24 messages6 participantstimes are UTC
Highlights from this day
- juniper_joins — ms/ms sequencing is the only thing i would call identity. everything else is circumstantial 12:43
- juniper_joins — tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5 12:59
- salt_bridge — baseline is drifting badly, is that the column or the mobile phase 13:04
- freeze_thaw — how do you tell co-elution from a clean single peak 13:19
- freeze_thaw — plus sixteen, is that always oxidation 13:22
two labs a point or two apart on the same lot is normal and gets mistaken for one of them being wrong constantly
retention time is a hint
a certificate with a number and no chromatogram is asking you to trust the integration you cannot see
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i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer, thats one data point
ms/ms sequencing is the only thing i would call identity. everything else is circumstantial
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i asked Janoshik for the raw trace and they sent it. that is the right answer to that question
relative retention time against a standard is comparable between labs. absolute minutes are not
how would a truncation sequence show up
thats co-elution
shallow gradients hide related substances behind the main peak. a steeper one separates and looks worse, honestly
monoisotopic and average diverge more the bigger the molecule. say which one you are quoting
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tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5
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quick one retention time is a hint, not a fingerprint. two different peptides can co-elute perfectly happily
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plus 0.98 is deamidation
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baseline is drifting badly, is that the column or the mobile phase
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 substancethats a charge state
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integration choices move the number by a few tenths easily. that is most of your inter-lab variance
coming back to this nobody runs enough blanks, including me, thats just me
how do you tell co-elution from a clean single peak
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plus sixteen, is that always oxidation
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a shoulder on a peak is information, and usually bad information
slightly off topic but oxidation is about plus 15.99 Da and is usually methionine or tryptophan
update from 19 months ago: switched to asking for the chromatogram rather than the number and it changed everything