Reminder set. I will post here in 9 days.
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#hplc-massspec 2025-12-17
Wednesday37 messages8 participantstimes are UTC
Highlights from this day
- monoisotopic — a C18 column separates by hydrophobicity under your gradient. anything with similar hydrophobicity comes off at a similar time. that includes the peptide you wanted,… 15:49
- monoisotopic — still not proof. but much stronger than two chromatograms sitting next to each other 15:53
- swirl_not_shake — which is why we ask for the trace and not just the table 16:41
you need more sample for identity than for purity. plan for that when you post it
On this day 5 years ago this channel logged 111 messages.
monoisotopic and average diverge more the bigger the molecule. say which one you are quoting, ymmv
fridge-temps.csv
714 rows · not retained in the public archive
how do you tell co-elution from a clean single peak
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the first peak is the solvent. it is always the solvent
the honest position is that hobby-level interpretation of a chromatogram is worth something but not much
update on the earlier thing ms/ms sequencing is the only thing i would call identity. everything else is circumstantial
the report says retention time matches the standard so it is confirmed as tirzepatide. that seems solid to me?
it is consistent with tirzepatide. it is not confirmation of tirzepatide
whats the difference
a C18 column separates by hydrophobicity under your gradient. anything with similar hydrophobicity comes off at a similar time.
that includes the peptide you wanted, a close analogue, a truncated version missing a residue that happens to balance out, and things that are not peptides at all.
RT narrows the field. it does not pick a winner
the classic demonstration is that a co-injection tells you more than two separate runs
co-injection?
spike your sample with the reference standard and run them together. if they are the same compound you get one peak that got taller. if they are different you often get a shoulder or two peaks
still not proof. but much stronger than two chromatograms sitting next to each other
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and RT drifts on its own anyway. new column, different lot of acetonitrile, column temperature two degrees out, pump seal wearing. you will see half a minute of movement across a year in a working lab
so how does anyone report RT usefully
you report it alongside a standard run on the same system the same day. that is what the injection sequence on a real report is for
like this
what a usable peak table looks like
sample: TZP lot 24-0611, 0.5 mg/mL in 50:50 A/B
column C18 4.6 x 150 mm 3.5 um, 40 C, 214 nm
# RT(min) area(uV*s) area% assignment
1 8.41 11 204 0.24 early polar, unassigned
2 11.86 23 990 0.51 deletion sequence (susp.)
3 12.44 4 512 700 96.18 tirzepatide
4 12.71 67 130 1.43 shoulder, see notes
5 14.02 31 880 0.68 late/hydrophobic, unassigned
6 16.55 44 210 0.94 unassigned
total 4 691 114 99.98
standard injected same sequence: RT 12.42 min
coinjection: single peak, slight front shoulder at 12.36📌7🙏5
peak 4 is 1.43 and its right next to the main one
yes, and that is where the real argument always is. 12.71 against 12.44 on a 150mm column is close enough that the integrator is guessing where one ends and the other starts
and how it guesses changes the headline number
by how much
drop a perpendicular versus a tangent skim on that shoulder and you can move the main peak between roughly 96 and 97.5 without touching the sample
so the purity number is partly an opinion
the purity number is a measurement plus a decision. the decision is not usually documented
which is why we ask for the trace and not just the table
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WuXi reports i have seen include the raw trace image. most resellers send a table only
a table with no trace is a summary of something you are not allowed to check
asking for the trace from now on
ask for the trace, the gradient, and the injection sequence. if you get all three from a reseller, be pleasantly surprised
*and the wavelength. 214 vs 280 changes what you even see, 280 only picks up aromatics
good catch, yes. a 280nm trace on a peptide with few aromatic residues is nearly blind
CPC sheets are 220nm in my experience, which is fine, it is the peptide bond region either way
ok noted. does any of this change what i should buy
reading_the_pinsthe report says retention time matches the standard so it is confirmed as tirzepatide.…
it changes what you believe about what you bought. that is different and more useful