LC/MS/MS QC Question....

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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Hi everyone,

Question:

If the LOQ of an analyte is 10 ppb, the Quantifier ion result is 11 ppb on a real sample, but the Qualifier ion concentration is 8 ppb (below the LOQ), is this grounds to disqualify the detected analyte? OR does the Quantifier ion's result is the only concentration that matters? I have not seen this topic addressed on LCMSMS QA/QC protocols that I have read so far and I would like to get feedback from any of you that does lcmsms work.

Thx
Is there a requirement for the ion ratio and if so does it meet that requirement?
Steve,

This question is assuming ion ratios meet criteria....
-zwitterion+ wrote:
Steve,

This question is assuming ion ratios meet criteria....


If the qualifier ion can have a ration of -20% versus the quantifier and pass, then when spiked at the LOQ of 10, the quantifier would then be 8 but passing. We normally only look at the ratio of a qualifier and don't quantify them.
The past is there to guide us into the future, not to dwell in.
We call them qualifiers because we don't quantify them.
... 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...
Hi everyone,

Great explanation, thanks to all that replied and everybody else that wanted to and did not. It all absolutely makes sense. Very grateful for sharing your knowledge and experience. thx ! ! !
HI EVERY ONE .I AM NEWLY WORKED ON LC/MS/MS ORBITRAP EXPLORIS 120 MANY ISSUE I FACED ON DATA PROCESSING AND INSTRUMENT METHODS
FIRST/when i make apatch unknown screening and instrument method fullscan-ddms2 the analyst appear in full scan but not undergo fragmentation **why
(program trac finder give hram.isotopic pattern.database score ,of the analyst but no fragment appear )
HI EVERY ONE
.I AM NEWLY WORKED ON LC/MS/MS ORBITRAP EXPLORIS 120 MANY ISSUE I FACED ON DATA PROCESSING AND INSTRUMENT METHODS
FIRST/when i make a patch unknown screening and instrument method fullscan-ddms2 the analyst appear in full scan but not undergo fragmentation **why
(program trac finder give hram.isotopic pattern.database score ,of the analyst but no fragment appear )
Moham@ed,

Your question has nothing to do with the topic discussed here. Please don't highjack others' posts but create your own post.
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