help me understand LC-MS, I have read the wiki twice
The title is the whole question — help me understand LC-MS, I have read the wiki twice — but here is why I am asking.
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.
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.
The system suitability argument, since it comes up whenever somebody posts a number without one.
Before the sample result means anything, the instrument has to be shown to be fit that day: replicate injections with an acceptable RSD on area, a tailing factor inside limits, adequate plate count, and a resolution check between the pair you care about.
None of that is exotic and all of it is routine in a laboratory that reports for a living. Its absence does not mean a number is wrong; it means the number is unanchored, and unanchored numbers should not be quoted to two decimal places on this board.
Corrections welcome, especially the pedantic ones. Pedantry is how this board earns its reputation.
best — the order this archive was captured in
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.
a shoulder is not an impurity until you can resolve it
Gave the same trace to two people I trust and got two integrations about a point apart. That was the most educational afternoon I have had here.
Yes — system suitability first. Without it the number is an assertion about the instrument, not the sample.
That is area percent, not mass percent. The trace cannot give you the second one.
Chased a ghost peak for three weeks. It was the plumbing. It is always the plumbing.
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.
A shoulder that does not baseline-resolve cannot be quantified honestly.
Disagreeing with this bit: that spread is ordinary inter-lab variance, not a disagreement about the material.
That figure cannot be quoted to two decimals off that baseline. The precision is not in the data.
Ran a blank after a high-concentration injection and found the carryover I had been calling an impurity.
Mass spectrometry answers identity. UV purity answers relative quantity under the run conditions. A document with one and not the other is answering half the question, and the half it answers should be stated.
Retention time shifted 0.4 minutes and I assumed the worst. It was the column temperature.
Method questions stay here; supplier claims go to c/vendorvetting with a document attached.
Cosigning the wavelength point. Half the disagreements in this board are two people looking at different detectors.
I would not blame the sample yet. Everything you have described is consistent with the column rather than the vial.
214nm sees the peptide bond, 280nm sees the aromatics
Sent the same vial to Medutest and PeptideMeter. 97.7% against a claimed 97.5%. The difference was the gradient, not the material.
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