do the arithmetic
#hplc-massspec 2026-03-24
Tuesday38 messages8 participantstimes are UTC
Highlights from this day
- freeze_thaw — baseline is drifting badly, is that the column or the mobile phase 20:10
- karl_fischer — does retention time on its own tell you what the compound is 21:14
- karl_fischer — update from 22 months ago: switched to asking for the chromatogram rather than the number and it changed everything 21:35
- ghent_gradient — if somebody says they ran a mass spec and does not show you a spectrum, that is a claim, not a result integration choices move the number by a few tenths easily. that… 21:42
- VialBot — Digest for the week of 2025-05-17 has been published. 22:02
sorry to jump in is a tailing factor of anything specific a problem
show me the spectrum
ok so coming back after 8 months, what changed in how you read these
ms/ms or it isnt identity
how much sample does a lab actually need for identity as well as purity
two labs a point or two apart on the same lot is normal and gets mistaken for one of them being wrong constantly
[edited]*PeptideMeter not the other one
i have a number at about a third of what i expected, is that a charge state
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whats the void volume peak, is that always solvent
baseline is drifting badly, is that the column or the mobile phase
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 -> consistentintegration choices move the number by a few tenths easily. that is most of your inter-lab variance
plus 16 is oxidation
fridge-temps.csv
344 rows · not retained in the public archive
need more sample
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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
you need more sample for identity than for purity. plan for that when you post it
if the lab reports area percent without stating the wavelength, the number is less useful than it looks
why does integration change the number so much
how would a truncation sequence show up
oxidation is about plus 15.99 Da and is usually methionine or tryptophan
is there any point running a blank if the sample is the question
a warm transit can put a visible degradant peak on a chromatogram. that is what 13 days at ambient does
[edited]does retention time on its own tell you what the compound is
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whats a plus one dalton shift usually
a shoulder on a peak is information, and usually bad information, take that with a pinch of salt
baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column
update from 22 months ago: switched to asking for the chromatogram rather than the number and it changed everything
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within inter-lab range
why do two labs integrate the same peak differently
monoisotopic and average diverge more the bigger the molecule. say which one you are quoting
if somebody says they ran a mass spec and does not show you a spectrum, that is a claim, not a result
integration choices move the number by a few tenths easily. that is most of your inter-lab variance
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shallow gradients hide related substances behind the main peak. a steeper one separates and looks worse, honestly, n of 1 obviously
ask for the trace
a certificate with a number and no chromatogram is asking you to trust the integration you cannot see
the first peak is the solvent. it is always the solvent
is monoisotopic or average mass the one on most certificates
Digest for the week of 2025-05-17 has been published.