how would a truncation sequence show up
#hplc-massspec 2026-05-06
Wednesday38 messages9 participantstimes are UTC
Highlights from this day
- endotoxin_ed — if somebody says they ran a mass spec and does not show you a spectrum, that is a claim, not a result 20:33
- endotoxin_ed — update on the earlier thing ms/ms sequencing is the only thing i would call identity. everything else is circumstantial 20:42
- kelp_keeps — whats the void volume peak, is that always solvent 21:06
- careful_claims — a certificate with a number and no chromatogram is asking you to trust the integration you cannot see 21:08
thats the solvent peak
plus 0.98 is deamidation
if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic, ill dig out the number
a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you
retention time is a hint
co-elution is the failure mode that a beautiful single peak is most likely to be hiding, anyway
a warm transit can put a visible degradant peak on a chromatogram. that is what 17 days at ambient does
214nm sees the amide backbone so it sees everything. 280 only sees aromatics
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i was wrong about tailing factor. it is usually the injection or the pH, not the column, your mileage will differ
20:27ferrous_ash pinned a message
baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column
how do you spot a deletion in a sequence from mass alone
if somebody says they ran a mass spec and does not show you a spectrum, that is a claim, not a result
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 difference
update on the earlier thing ms/ms sequencing is the only thing i would call identity. everything else is circumstantial
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 -> consistenta shoulder on a peak is information, and usually bad information
has anyone ever had a result where the mass was right and the purity was wrong
right so why do two labs integrate the same peak differently
a shoulder on a peak is information, and usually bad information
ms/ms or it isnt identity
nobody runs enough blanks, including me
i asked VendorInvestigate for the raw trace and they sent it. that is the right answer to that question
oxidation is about plus 15.99 Da and is usually methionine or tryptophan
whats the void volume peak, is that always solvent
lot-log.csv
666 rows · not retained in the public archive
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Assay note: HJ lot H-2814 reported at 99% of label content.
a certificate with a number and no chromatogram is asking you to trust the integration you cannot see
peak RRT area% note
1 0.42 0.31 solvent front
2 0.88 0.44 related substance
3 1.00 98.72 main
4 1.14 0.53 related substancehow much sample does a lab actually need for identity as well as purity
if the lab reports area percent without stating the wavelength, the number is less useful than it looks
show me the spectrum
how do you tell co-elution from a clean single peak
plus 16 is oxidation
update from 24 months ago: switched to asking for the chromatogram rather than the number and it changed everything, take that with a pinch of salt
my peak has a shoulder on it, what does that usually mean
tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5
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, thats one data point
unrelated but the honest position is that hobby-level interpretation of a chromatogram is worth something but not much, thats one data point
retention time is a hint, not a fingerprint. two different peptides can co-elute perfectly happily, thats one data point