ask for the trace
#hplc-massspec 2025-11-29
Saturday36 messages9 participantstimes are UTC
Highlights from this day
- deamidation — a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you 20:51
- fridge_thermo — baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column 21:00
- fridge_thermo — is there any point running a blank if the sample is the question 21:01
- karl_fischer — how do you tell co-elution from a clean single peak 21:04
- LC_MS_Lena — you need more sample for identity than for purity. plan for that when you post it 22:39
need more sample
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sorry to jump in the honest position is that hobby-level interpretation of a chromatogram is worth something but not much
214nm sees the amide backbone so it sees everything. 280 only sees aromatics, still working it out
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i asked VendorInvestigate for the raw trace and they sent it. that is the right answer to that question, still working it out
retention time is a hint
plus 16 is oxidation
tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5, thats just me
a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you
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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
relative retention time against a standard is comparable between labs. absolute minutes are not
baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column
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is there any point running a blank if the sample is the question
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coming back after 11 months, what changed in how you read these
how do you tell co-elution from a clean single peak
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do you trust an area percent from a shallow gradient
why does integration change the number so much
my peak has a shoulder on it, what does that usually mean
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how do you spot a deletion in a sequence from mass alone
thats a charge state
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ms/ms sequencing is the only thing i would call identity. everything else is circumstantial
plus 0.98 is deamidation
within inter-lab range
i was wrong about tailing factor. it is usually the injection or the pH, not the column
shallow gradients hide related substances behind the main peak. a steeper one separates and looks worse, honestly
integration difference
Digest for the week of 2025-06-11 has been published.
a warm transit can put a visible degradant peak on a chromatogram. that is what 11 days at ambient does
how much sample does a lab actually need for identity as well as purity
[edited]you need more sample for identity than for purity. plan for that when you post it
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show me the spectrum
ms/ms or it isnt identity
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monoisotopic and average diverge more the bigger the molecule. say which one you are quoting
thats co-elution