by
lmh » Fri Aug 11, 2017 10:21 am
completely agreed: in the end RSD is what matters, because it's directly what interests you for a LOQ (is my error bigger than an acceptable value) and indirectly what interests you for a LOD (is my value statistically significantly different from zero).
The point about qualifiers and quantifiers is an interesting one and worth thinking about (it's good someone quite rightly brought it up). It's true that merely taking the error on the quantified result, which uses only the quantifier, could make it possible to calculate a very low LOQ, but it's not the right approach. The qualifier is proof that the peak is the correct thing. If you are using qualifiers, you have taken a decision that you do not believe the peak is the right thing unless the qualifier is present at the correct intensity ratio. The consequence is that a tiny peak of the quantifier ion without the correct qualifier is, from your perspective, not the right chemical and therefore meaningless, and should not be included in the calculation.
When calculating a LOQ based on RSD values from real measurements of dilute standards or samples (ideally spiked control samples/spiked matrix blank etc.), the LOQ is always the higher of two values: (1) the concentration where you get an acceptable RSD; (2) the lowest concentration for which you have real data. Since you cannot have data below the point where you can unequivocally prove detection of the compound, based on acceptable qualifier ion ratio, if the qualifier has a large RSD, your LOQ is limited by (2), the lowest concentration where you were able to detect the peak with the right qualifier ion ratio. Thus LOQ isn't lower than LOD, instead they become equal (with the side-effect that at the LOQ, you actually have a much better RSD than the LOQ was designed to produce).
Think about it this way: an athlete with a failed drugs test wants to know: (a) are you sure that I had that drug in my urine? (b) are you sure it exceeded the limit? There is no value in being absolutely sure that the level exceeded the limit, if you can't be equally sure that the drug was what you think it was.