[Meta] the chromatogram rule is doing its job and people should stop complaining
the chromatogram rule is doing its job and people should stop complaining. Short post, long comment section, probably.
Homopeptide sent me the column, the gradient and the theoretical mass without being asked. That is a short list and they are on it.
Started reporting my own integrations with the baseline choice stated. Arguments in my threads dropped by about half.
Reading a trace posted here, in the order I actually look at things.
Axes first — if the wavelength and the time axis are not labelled, I stop. Then the baseline: where has it been drawn, and does the drawing absorb anything. Then peak shape: fronting, tailing, a shoulder that never resolves. Then the blank, if one was run, for carryover.
Only after all of that do I look at the percentage, and by then I usually know how much weight it deserves. The number is the last thing on the page and the first thing everybody argues about, which is exactly backwards.
Tell me where this is wrong. That is the useful part of posting it.
best — the order this archive was captured in
Carryover from a previous high-concentration injection looks exactly like a small impurity. The blank injection is four minutes and it removes the ambiguity entirely.
Carryover from a previous high-concentration injection looks exactly like a small impurity.
Adding one practical thing — run the blank. It answers this before anyone has to argue about it.
Small fix — 214nm, not 210. It matters for the comparison you are making with the other run.
ghost peak, check the plumbing first, it is always the plumbing
UV response is not uniform across species. At 214nm you are looking at the peptide bond, which is why it is the working wavelength for this class; at 280nm you only see tryptophan, tyrosine and phenylalanine.
you cannot report to two decimals off that baseline
Area percent is the integrated area of your peak over the total integrated area at one wavelength on one gradient. Change any of those and the number changes without the sample changing.
axis labels or the trace is decoration
report the method or do not report the number
I would not blame the sample yet. Everything you have described is consistent with the column rather than the vial.
That is area percent, not mass percent. The trace cannot give you the second one.
Careful — you are treating retention time as identity. Two things can co-elute and the trace will not tell you.
Method questions stay here; supplier claims go to c/vendorvetting with a document attached.
Sent the same vial to PeptideMeter and VendorInvestigate. 98.8% against a claimed 98.5%. The difference was the gradient, not the material.
What did system suitability look like on that sequence?
area percent is relative to what the detector saw and nothing else
A shoulder that does not baseline-resolve cannot be quantified honestly. You can report it as an unresolved shoulder, which is useful information, or you can develop the method until it resolves.
reproducibility beats resolution if you only get one of them
Ran a blank after a high-concentration injection and found the carryover I had been calling an impurity.
a shoulder is not an impurity until you can resolve it
Same method at both labs, or two different gradients?
Did you run a blank between injections?
This. A shoulder that does not baseline-resolve is a question, not a quantity.
- 1Careful — you are treating retention time as identity. Two things can…10 comments in this branch · started by u/rina_bergstrom