why do two labs integrate the same peak differently
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#hplc-massspec 2025-12-08
Monday32 messages7 participantstimes are UTC
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- pbs_pip — why do two labs integrate the same peak differently 18:11
- monoisotopic — update on the earlier thing a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you, ymmv 18:19
- pbs_pip — i asked VendorInvestigate for the raw trace and they sent it. that is the right answer to that question monoisotopic and average diverge more the bigger the molecule.… 22:33
update on the earlier thing why does integration change the number so much
thats co-elution
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two labs a point or two apart on the same lot is normal and gets mistaken for one of them being wrong constantly
update on the earlier thing a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you, ymmv
if somebody says they ran a mass spec and does not show you a spectrum, that is a claim, not a result
i was wrong about tailing factor. it is usually the injection or the pH, not the column
can you tell a dimer from the chromatogram alone
baseline is drifting badly, is that the column or the mobile phase
[edited]i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer
has anyone ever had a result where the mass was right and the purity was wrong
relative retention time against a standard is comparable between labs. absolute minutes are not
slightly off topic but is monoisotopic or average mass the one on most certificates
plus sixteen, is that always oxidation
show me the spectrum
genuine question if the lab reports area percent without stating the wavelength, the number is less useful than it looks
how do you spot a deletion in a sequence from mass alone
truncations show as a mass deficit matching one or more residues. you need the sequence to say which
a certificate with a number and no chromatogram is asking you to trust the integration you cannot see
does retention time on its own tell you what the compound is
you need more sample for identity than for purity. plan for that when you post it
the first peak is the solvent. it is always the solvent, take that with a pinch of salt
baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column
need more sample
214nm sees the amide backbone so it sees everything. 280 only sees aromatics
update from 21 months ago: switched to asking for the chromatogram rather than the number and it changed everything
how would a truncation sequence show up
tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5
do you trust an area percent from a shallow gradient
ms/ms sequencing is the only thing i would call identity. everything else is circumstantial
i asked VendorInvestigate for the raw trace and they sent it. that is the right answer to that question
monoisotopic and average diverge more the bigger the molecule. say which one you are quoting
nobody runs enough blanks, including me, i could be wrong