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 2025-10-15
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is uv at 214 or 280 better for peptides
thats the solvent peak
baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column, thats one data point
whats the void volume peak, is that always solvent
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has anyone ever had a result where the mass was right and the purity was wrong
how do you spot a deletion in a sequence from mass alone
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if the lab reports area percent without stating the wavelength, the number is less useful than it looks, i think
if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic
214nm sees the amide backbone so it sees everything. 280 only sees aromatics
[edited]deamidation is about plus 0.98 Da. it is not a rounding error, it is a degradant
thats a charge state
the honest position is that hobby-level interpretation of a chromatogram is worth something but not much, anyway
can you tell a dimer from the chromatogram alone
does ms/ms actually prove identity or just make it very likely
thats co-elution
i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer, n of 1 obviously
a shoulder on a peak is information, and usually bad information
janoshik-kp-0925.pdf
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monoisotopic and average diverge more the bigger the molecule. say which one you are quoting
shallow gradients hide related substances behind the main peak. a steeper one separates and looks worse, honestly
baseline is drifting badly, is that the column or the mobile phase
[edited]retention time is a hint
i was wrong about tailing factor. it is usually the injection or the pH, not the column, ask me again in a month
ask for the trace
quick one a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you
plus sixteen, is that always oxidation
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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
why do two labs integrate the same peak differently
semaglutide is about 4113.6 Da. [M+2H]2+ lands near 2057.8, [M+3H]3+ near 1372.2
while im here is monoisotopic or average mass the one on most certificates
update from 17 months ago: switched to asking for the chromatogram rather than the number and it changed everything, we shall see
plus 16 is oxidation
oxidation is about plus 15.99 Da and is usually methionine or tryptophan
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integration difference
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i have a number at about a third of what i expected, is that a charge state
nobody runs enough blanks, including me
how much sample does a lab actually need for identity as well as purity
you need more sample for identity than for purity. plan for that when you post it
within inter-lab range