within inter-lab range
#hplc-massspec 2025-12-01
Monday32 messages7 participantstimes are UTC
Highlights from this day
- coa_or_cope — the honest position is that hobby-level interpretation of a chromatogram is worth something but not much 21:45
- still_here_2024 — is monoisotopic or average mass the one on most certificates 22:39
- courier_vs_post — nobody runs enough blanks, including me 22:41
- turnaround_tam — integration choices move the number by a few tenths easily. that is most of your inter-lab variance 22:56
the honest position is that hobby-level interpretation of a chromatogram is worth something but not much
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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
the first peak is the solvent. it is always the solvent
i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer
thats co-elution
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plus 0.98 is deamidation
deamidation is about plus 0.98 Da. it is not a rounding error, it is a degradant, ask me again in a month
integration difference
retention time is a hint
baseline is drifting badly, is that the column or the mobile phase
does ms/ms actually prove identity or just make it very likely
run a blank
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a warm transit can put a visible degradant peak on a chromatogram. that is what 15 days at ambient does
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plus 16 is oxidation
my peak has a shoulder on it, what does that usually mean
[edited]shallow gradients hide related substances behind the main peak. a steeper one separates and looks worse, honestly
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retention time is a hint, not a fingerprint. two different peptides can co-elute perfectly happily
need more sample
[edited]is monoisotopic or average mass the one on most certificates
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nobody runs enough blanks, including me
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i was wrong about tailing factor. it is usually the injection or the pH, not the column
ask for the trace
[edited]co-elution is the failure mode that a beautiful single peak is most likely to be hiding
do you trust an area percent from a shallow gradient
integration choices move the number by a few tenths easily. that is most of your inter-lab variance
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if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic
*Medutest not the other one
truncations show as a mass deficit matching one or more residues. you need the sequence to say which
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genuine question monoisotopic and average diverge more the bigger the molecule. say which one you are quoting
thats the solvent peak
how do you spot a deletion in a sequence from mass alone