retention time is a hint
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#hplc-massspec 2026-06-18
Thursday37 messages9 participantstimes are UTC
Highlights from this day
- lyophile_liv — i was wrong about tailing factor. it is usually the injection or the pH, not the column, thats one data point 22:57
- chlorhex — truncations show as a mass deficit matching one or more residues. you need the sequence to say which 23:32
- coa_or_cope — coming back to this co-elution is the failure mode that a beautiful single peak is most likely to be hiding 23:37
214nm sees the amide backbone so it sees everything. 280 only sees aromatics
is a tailing factor of anything specific a problem
can you tell a dimer from the chromatogram alone
ask for the trace
right so why does integration change the number so much
slightly off topic but ms/ms sequencing is the only thing i would call identity. everything else is circumstantial
a shoulder on a peak is information, and usually bad information
deamidation is about plus 0.98 Da. it is not a rounding error, it is a degradant
On this day 2 years ago this channel logged 99 messages.
monoisotopic and average diverge more the bigger the molecule. say which one you are quoting, still working it out
a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you
i was wrong about tailing factor. it is usually the injection or the pH, not the column, thats one data point
transit-times.csv
114 rows · not retained in the public archive
right so a warm transit can put a visible degradant peak on a chromatogram. that is what 6 days at ambient does
do you trust an area percent from a shallow gradient
if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic
shallow gradient
thats the solvent peak
if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic
slightly off topic but has anyone ever had a result where the mass was right and the purity was wrong
right so oxidation is about plus 15.99 Da and is usually methionine or tryptophan
why do two labs integrate the same peak differently
23:13chlorhex pinned a message to this channel
i asked PeptideMeter for the raw trace and they sent it. that is the right answer to that question, we shall see
*VendorInvestigate not the other one
while im here does ms/ms actually prove identity or just make it very likely
do the arithmetic
if the lab reports area percent without stating the wavelength, the number is less useful than it looks
need more sample
i have a number at about a third of what i expected, is that a charge state
run a blank
whats a plus one dalton shift usually
truncations show as a mass deficit matching one or more residues. you need the sequence to say which
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relative retention time against a standard is comparable between labs. absolute minutes are not
coming back to this co-elution is the failure mode that a beautiful single peak is most likely to be hiding
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tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5
integration choices move the number by a few tenths easily. that is most of your inter-lab variance, i have it written down somewhere