Chromatography Forum

Dear,

I want to use two MRMs for identification of a compound (qualifier and quantifier). Fragmentation is however not optimal, so it is not easy to find two adequate MRMs.

Two clear MRMs are, let's say, 500-50.0 and 500-50.2.
I am using a 3200QTrap with unit resolution (this means 0.7 amu full width at half height).

Are 500-50.0 and 500-50.2 two different, independent MRMs?
Or if 50.0 is measured, 50.2 is automatically measured too?
Can I use these two MRMs with only 0.2 amu difference in daughter ion as decent MRMs for quantification?

I hope someone can help me with this!

Thanks!

The answer is no.

Even if you use enahnced resolution of the trap, it's still no.

I see someone use, in your case, 500-500 as the qualifier MRMs.

Thanks for your response!

The problem is that 500-500 is not a good MRM. Even with the highest possible collision energy (so that molecular ion is not fragmented, but co-eluting interferences do) there is too much noise to use the surviving-ion-transition as reliable identification component...

In my opinion you need two different product ions, one for qualification and one for confirmation. 50.0 en 50.2 are probably the same ions. Then the resolution of the 3200-Qtrap is not that high that 50.0 en 50.2 is different.

For the 500-500 transition, you typically want to lower your collision energy - you just want a pass through - no fragmentation.

Do you have a reason to believe that you have a co-eluting interference of with a mass of 500? If so, using a 500 - 500 is not suitable.