Pesticide MRM analysis - transitions 1 & 2 required for LOD?

[DB_UK]

Hi all,

I am currently working on a pesticide method and am trying to determine LODs and LOQs as I would like to assess sensitivity. I am using a QQQ with at least 2 transitions for each compound.

I have read the guidelines on how to determine LOD and LOQ (e.g. 40 CFR Part 136 appendix B) and I understand the required number of injections, precision etc..

But what is not clear; do I need both transitions to be visible at the LOD or just the primary transition? I understand both transitions are required for the LOQ along with the necessary precision, ion ratio etc. but for LOD I am not so sure.

Thank you all for your help.

[lmh]

Put it this way: the point of the 2nd transition is to confirm the identity of the target compound.

If you accept a LOD based on the more abundant transition but ignoring the less abundant transition, you are accepting a lower level of proof that it's the right compound. So the big question is what level of proof do you need? I think that if you need a 2nd transition above the LOQ, you also need one at the LOD. Otherwise your LOQ is saying "we can quantify compound X at or above 3ug/mL", while your LOD is saying "we can detect something that might be compound X at or above 0.8ug/mL" (and what's the point of being able to detect something if you aren't absolutely sure what you're detecting??)

In the end, it really depends on what use you will make of measurements between the LOD and the LOQ. Is it guidance that "X" might be present, or evidence that "X" is definitely present?

[Camisotro]

My lab is working on a pesticide method too, but I'm pretty sure our standard procedure anyway is to reanalyze any positives. Often at reanalysis you can make changes to improve detection of those compounds (changes which you would not have made for the general method as they may have caused problems with other compounds). So as long as the quantifier ion is able to flag the sample as a possible positive above LOD, you'll at least have a signal that it's time to collect some extra data.

[lmh]

... exactly, if you use the data as guidance that X might be present, and these samples are deserving of more detailed investigation, then you're quite correct to reduce the level of proof at the 1st stage.

[Camisotro]

Indeed - for example if there are only 3 possible positive pesticides out of your list of 80, or 300, or however many, for confirmation you can cut everything else out of the method and get a much higher dwell time for better S/N on both channels.

Or if they're all later-eluting pesticides, you may be able to crank up the injection volume without any resulting peak distortion (but this might not have been possible for your earlier eluters).

Or run a faster LC method that gives you narrower and taller peaks (which would have sacrificed needed resolution in the full method).

Et cetera.

[DB_UK]

Thanks Camisotro and lmh for taking the time to reply. Your help is much appreciated and I will take your advice.