by
lmh » Wed Jul 28, 2021 2:55 pm
... yup! Adding a couple of comments:
(1) Since the quantifier is probably the most abundant ion, and the qualifier is something smaller, by definition the qualifier will have a higher LOQ (if you calculate it, which most people won't, because they're not quantifying using that ion). That means that if you were to reject anything where the qualifier doesn't meet its LOQ, there would be no point in having a LOQ for the quantifier!
(2) But it does mean there is an interaction between LOD values and the precision you require for the ion ratio between quantifier and qualifier(s). If you set this precision too narrow, you can actually create a situation where your true LOD is bigger than you LOQ! You can get to a situation where you can measure the compound to a high level of precision based on the quantifier, but because the ratio of a much less abundant qualifier to quantifier is much more variable (because of the low abundance of qualifier), the peak is getting rejected as the wrong thing! And if you're rejecting the peak, it's not being detected, and therefore whatever the statistics say, you can't claim your LOD is as low as that peak. It's fairly unlikely you'll want to be in this situation, but I can think of one or two scenarios where it could happen intentionally, in real life!
(3) And of course it means that the LOD of the qualifier sets a threshold for all limits on the quantifier; if the qualifier isn't detected, the peak will be rejected, so the precision with which the qualifier is measured becomes irrelevant at that point. Practically this one is quite common: people complaining that the peak is obvious, why hasn't the stupid software labelled it? Yeah, the main peak's obvious, but the qualifier peak isn't...