i was wrong about tailing factor. it is usually the injection or the pH, not the column
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#hplc-massspec 2025-07-28
Monday33 messages6 participantstimes are UTC
Highlights from this day
- vial_ledger — a warm transit can put a visible degradant peak on a chromatogram. that is what 24 days at ambient does 15:13
- halifax_hplc — how much sample does a lab actually need for identity as well as purity 16:02
- deamidation — ok so the honest position is that hobby-level interpretation of a chromatogram is worth something but not much 16:45
whats a plus one dalton shift usually
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coming back after 3 months, what changed in how you read these
semaglutide is about 4113.6 Da. [M+2H]2+ lands near 2057.8, [M+3H]3+ near 1372.2
thats a charge state
why does integration change the number so much
if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic
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how do you spot a deletion in a sequence from mass alone
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baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column
thats the solvent peak
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run a blank
if the lab reports area percent without stating the wavelength, the number is less useful than it looks
truncations show as a mass deficit matching one or more residues. you need the sequence to say which
a warm transit can put a visible degradant peak on a chromatogram. that is what 24 days at ambient does
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do you trust an area percent from a shallow gradient
how much sample does a lab actually need for identity as well as purity
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does ms/ms actually prove identity or just make it very likely
thats co-elution
why do two labs integrate the same peak differently
integration difference
is there any point running a blank if the sample is the question
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sorry to jump in is a tailing factor of anything specific a problem
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sorry to jump in baseline is drifting badly, is that the column or the mobile phase
ok so the honest position is that hobby-level interpretation of a chromatogram is worth something but not much
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shallow gradients hide related substances behind the main peak. a steeper one separates and looks worse, honestly
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deamidation is about plus 0.98 Da. it is not a rounding error, it is a degradant
i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer
oxidation is about plus 15.99 Da and is usually methionine or tryptophan
if somebody says they ran a mass spec and does not show you a spectrum, that is a claim, not a result
show me the spectrum
ms/ms or it isnt identity
a shoulder on a peak is information, and usually bad information
tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5
monoisotopic and average diverge more the bigger the molecule. say which one you are quoting
co-elution is the failure mode that a beautiful single peak is most likely to be hiding
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integration choices move the number by a few tenths easily. that is most of your inter-lab variance