genuine question about LC-MS that I am slightly embarrassed to ask
genuine question about LC-MS that I am slightly embarrassed to ask. Searched first, found three threads that contradict each other, hence the post. 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…
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.
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.
Chased a ghost peak for three weeks. It was the plumbing. It is always the plumbing.
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.