two labs a point or two apart on the same lot is normal and gets mistaken for one of them being wrong constantly
#hplc-massspec 2026-03-27
Friday49 messages10 participantstimes are UTC
Highlights from this day
- VialBot — Reminder for c18_column: dose day is today. Set 8 days ago. 22:09
- karl_fischer — do you trust an area percent from a shallow gradient 22:16
- fake_tracking — the first peak is the solvent. it is always the solvent 22:46
- fake_tracking — does retention time on its own tell you what the compound is 22:58
- lisbon_lot — integration choices move the number by a few tenths easily. that is most of your inter-lab variance 23:37
is monoisotopic or average mass the one on most certificates
while im here plus sixteen, is that always oxidation
a warm transit can put a visible degradant peak on a chromatogram. that is what 19 days at ambient does
whats a plus one dalton shift usually
has anyone ever had a result where the mass was right and the purity was wrong
co-elution is the failure mode that a beautiful single peak is most likely to be hiding
i asked VendorInvestigate for the raw trace and they sent it. that is the right answer to that question
a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you
a certificate with a number and no chromatogram is asking you to trust the integration you cannot see
thats the solvent peak
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is uv at 214 or 280 better for peptides
sorry to jump in semaglutide is about 4113.6 Da. [M+2H]2+ lands near 2057.8, [M+3H]3+ near 1372.2
Reminder for c18_column: dose day is today. Set 8 days ago.
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Digest for the week of 2025-07-28 has been published.
do you trust an area percent from a shallow gradient
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nobody runs enough blanks, including me
214nm sees the amide backbone so it sees everything. 280 only sees aromatics
retention time is a hint, not a fingerprint. two different peptides can co-elute perfectly happily
the honest position is that hobby-level interpretation of a chromatogram is worth something but not much
a shoulder on a peak is information, and usually bad information
baseline is drifting badly, is that the column or the mobile phase
update from 16 months ago: switched to asking for the chromatogram rather than the number and it changed everything
if somebody says they ran a mass spec and does not show you a spectrum, that is a claim, not a result
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ms/ms sequencing is the only thing i would call identity. everything else is circumstantial
if the lab reports area percent without stating the wavelength, the number is less useful than it looks
ask for the trace
how do you spot a deletion in a sequence from mass alone
the first peak is the solvent. it is always the solvent
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monoisotopic and average diverge more the bigger the molecule. say which one you are quoting
shallow gradient
do the arithmetic
does retention time on its own tell you what the compound is
M = 4113.6
[M+2H]2+ = (4113.6 + 2.02) / 2 = 2057.8
[M+3H]3+ = (4113.6 + 3.02) / 3 = 1372.2
observed 1372.3 -> consistentthats co-elution
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i have a number at about a third of what i expected, is that a charge state
is a tailing factor of anything specific a problem
why do two labs integrate the same peak differently
can you tell a dimer from the chromatogram alone
*Medutest not the other one
quick one relative retention time against a standard is comparable between labs. absolute minutes are not
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my peak has a shoulder on it, what does that usually mean
retention time is a hint
genuine question deamidation is about plus 0.98 Da. it is not a rounding error, it is a degradant
truncations show as a mass deficit matching one or more residues. you need the sequence to say which
integration choices move the number by a few tenths easily. that is most of your inter-lab variance
lot-log.csv
724 rows · not retained in the public archive
does ms/ms actually prove identity or just make it very likely
i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer, for what its worth
integration difference