[Lab] LC-MS confirmed identity but the UV purity is 94%. now what
LC-MS confirmed identity but the UV purity is 94%. now what, and before anyone asks: same batch throughout, single submission, no cherry-picking between services.
The relevant figures are 94%, and they come from the same log I have kept the whole time.
Started reporting my own integrations with the baseline choice stated. Arguments in my threads dropped by about half.
Why two honest labs report different numbers on the same vial.
Start with the gradient. A shallower slope holds compounds on the column longer and usually separates close-eluting species better. A steeper one gets you a faster run and a fatter peak. If a related substance elutes near the main peak, one method resolves it and reports it separately, the other absorbs part of it into the main peak.
Then integration. Where the baseline is drawn under a shoulder is a decision made by a person or by a piece of software configured by a person. It moves the number.
A point or two of spread between services on this assay is ordinary. Treating one lab as ground truth is how people end up in arguments with suppliers that neither side can win.
Sceptical readings welcome. The confident ones are the ones I distrust.
best — the order this archive was captured in
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.
a shoulder is not an impurity until you can resolve it
Agreed, and it is why GL Biochem printing the column and gradient on the certificate is genuinely useful rather than decorative.
you cannot report to two decimals off that baseline
That is area percent, not mass percent. The trace cannot give you the second one.
Small fix — 214nm, not 210. It matters for the comparison you are making with the other run.
axis labels or the trace is decoration
Cosigning the wavelength point. Half the disagreements in this board are two people looking at different detectors.
Is that baseline drawn by the software or by hand?
Method questions stay here; supplier claims go to c/vendorvetting with a document attached.
Correction: that is a tailing factor, not a plate count. Different diagnostics for different problems.
Push back: a longer run is not automatically better resolution. You are trading peak width for time and the ratio is what matters.
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.
That figure cannot be quoted to two decimals off that baseline. The precision is not in the data.
Can you post the trace with the axes labelled?
Same method at both labs, or two different gradients?
Can you post the trace with the axes labelled?
Adding one practical thing — run the blank. It answers this before anyone has to argue about it.
What wavelength, and what was the gradient?
Same view — reporting the gradient is what makes a result checkable rather than merely stated.
I would not blame the sample yet. Everything you have described is consistent with the column rather than the vial.
Careful — you are treating retention time as identity. Two things can co-elute and the trace will not tell you.
Resolution between two peaks depends on retention, selectivity and efficiency. A shallower gradient buys retention and usually resolution, at the cost of peak width and run time.
Correcting myself upthread: I said the gradient was linear and looking again it has a hold in the middle.
ghost peak, check the plumbing first, it is always the plumbing
area percent is relative to what the detector saw and nothing else
What did system suitability look like on that sequence?
214nm sees the peptide bond, 280nm sees the aromatics
area percent is relative to what the detector saw and nothing else
ahmed_rasmussen is right — the integration choice is a decision and it should be stated alongside the result.
Right. And a blank between injections settles the carryover argument before it starts.
- 1Area percent is the integrated area of your peak over the total integrated…8 comments in this branch · started by u/rina_bergstrom
- 2Resolution between two peaks depends on retention, selectivity and…8 comments in this branch · started by u/coa_janitor