two labs, same vial, 98.1 and 96.4. both right?
Yes, probably, and this thread exists so I can link it instead of retyping it.
Sources of legitimate disagreement on the same physical sample, roughly in order of how much they contribute:
- Integration. Where the analyst draws the baseline and where they cut a shoulder. On a trace with a des-amido form eluting 0.4 min early this alone is worth half a point, sometimes more.
- Gradient. A shallower gradient resolves more, which usually lowers your reported purity because impurities that were hiding under the main peak now have their own.
- Wavelength. 214nm sees the peptide backbone. 220nm and 280nm see different things with different relative response.
- Column. Different chemistry, different age, different selectivity.
A 1.7 point gap is agreement. A four point gap is a finding. Learn the difference before you post an accusation.
best — the order this archive was captured in
Integration being first on that list is correct and under-appreciated. Two competent analysts given the same raw trace will produce different numbers, and the difference is not error, it is judgement.
Somebody should run a round-robin: one trace, twenty people, publish the spread. I suspect it would be wider than most of the vendor disputes this site has had.
i said the same thing in a c/coa thread yesterday. i will post a raw trace this week if you organise it.
Organising it. Post the trace with the method and no interpretation, I will collect submissions by modmail and publish the distribution with nobody named.
put a deliberately ambiguous shoulder in it. that is where the variance lives and a clean trace will just show everyone agreeing.
i have exactly the trace for that. 97.9 with an enormous shoulder that i still cannot confidently call.
that is the one from the ERP thread? i have opinions about that shoulder and i want to submit them anonymously so i can be wrong in peace.
and none of the four items on that list touch identity. all of them are arguments about how much of what the detector saw was the main species. whether the main species is the right molecule is a different instrument.
the gradient point is the counterintuitive one. better method, lower number. people read a lower purity as worse material and sometimes it is a better analysis.
which is a genuine perverse incentive for anyone commissioning their own COA. a lazy method flatters you.
i have been treating the higher number as the true one this whole time. edit: and specifically i once emailed a vendor about a 1.2 point gap. i would like that email back.
if labs cannot agree the whole testing thing is pointless
They agree to within a point or two, which is enough to catch a 94% sold as 99% — which is the actual thing we use them for. Precision and usefulness are different requirements.
- 1Integration being first on that list is correct and under-appreciated. Two…6 comments in this branch · started by u/baseline_drifter