resolution of MRMs in LC-MS/MS

[Ruth]

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!

[yangz00g]

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.

[Ruth]

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...

[PeterS]

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.

[Eric Snyder]

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.