Channel stats, last 30 days: 58 messages from 9 members.
#hplc-massspec 2026-05-18
Monday50 messages11 participantstimes are UTC
Highlights from this day
- dana_titrates — a warm transit can put a visible degradant peak on a chromatogram. that is what 21 days at ambient does 13:40
- willow_waits — oxidation is about plus 15.99 Da and is usually methionine or tryptophan 15:05
- LC_MS_Lena — nobody runs enough blanks, including me 16:20
- LC_MS_Lena — the honest position is that hobby-level interpretation of a chromatogram is worth something but not much 16:24
- HPLC_Hank — you need more sample for identity than for purity. plan for that when you post it 18:59
a warm transit can put a visible degradant peak on a chromatogram. that is what 21 days at ambient does
🤝12🙏11📈9
show me the spectrum
baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column, still working it out
medutest-report-kp-0925.pdf
3 pages · 241 KB · not retained in the public archive
sorry to jump in if somebody says they ran a mass spec and does not show you a spectrum, that is a claim, not a result
🤝7❤️1📈4
follow up does ms/ms actually prove identity or just make it very likely
right so plus sixteen, is that always oxidation
truncations show as a mass deficit matching one or more residues. you need the sequence to say which
sorry to jump in if the lab reports area percent without stating the wavelength, the number is less useful than it looks
tirzepatide is about 4813.5 Da, so 2+ is near 2407.8 and 3+ near 1605.5
plus 16 is oxidation
whats a normal gradient for a peptide this size
the first peak is the solvent. it is always the solvent
thats a charge state
retention time is a hint
shallow gradients hide related substances behind the main peak. a steeper one separates and looks worse, honestly
oxidation is about plus 15.99 Da and is usually methionine or tryptophan
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 substancedo the arithmetic
co-elution is the failure mode that a beautiful single peak is most likely to be hiding, still working it out
is there any point running a blank if the sample is the question
a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you
Purity check: no report on file for lot H-2814. Nothing logged either way.
ask for the trace
[edited]nobody runs enough blanks, including me
💀1🤝10👍11
the honest position is that hobby-level interpretation of a chromatogram is worth something but not much
❤️8🙏3🧊7
thats co-elution
why does integration change the number so much
thats the solvent peak
within inter-lab range
update from 20 months ago: switched to asking for the chromatogram rather than the number and it changed everything
ms/ms or it isnt identity
🙏13
plus 0.98 is deamidation
right so retention time is a hint, not a fingerprint. two different peptides can co-elute perfectly happily
two labs a point or two apart on the same lot is normal and gets mistaken for one of them being wrong constantly
sorry to jump in how do you spot a deletion in a sequence from mass alone
i was wrong about tailing factor. it is usually the injection or the pH, not the column
whats the void volume peak, is that always solvent
coming back after 21 months, what changed in how you read these
shallow gradient
📉18❤️1
coming back to this deamidation is about plus 0.98 Da. it is not a rounding error, it is a degradant
you need more sample for identity than for purity. plan for that when you post it, still working it out
a shoulder on a peak is information, and usually bad information
run a blank
ms/ms sequencing is the only thing i would call identity. everything else is circumstantial
you need more sample for identity than for purity. plan for that when you post it
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 substance⚠️4🧪18
why do two labs integrate the same peak differently
i asked VendorInvestigate for the raw trace and they sent it. that is the right answer to that question
214nm sees the amide backbone so it sees everything. 280 only sees aromatics
how would a truncation sequence show up
relative retention time against a standard is comparable between labs. absolute minutes are not
⚠️7📉1👀4