m/z drift

[jameslizh]

I am using multiple reaction monitoring (MRM) to quantify compounds with Agilent LC/ion trap XCT Plus MS. I found that if the sample's concentration is very low and I set a couple of transitions (not single one), sometimes the obtained m/z in the acquired spectra changed. For example, I set transition m/z 309-->180, the resulting data got m/z 179. From the retention time, this peak is consistent with my desired compound. But why not showing m/z 180 as supposed? It this normal? Thanks.

[sassman]

Have you done a mass calibration of the detector?

[jameslizh]

yes, the mass detector was calibrated two weeks ago. Such drift only occurred for a few particular transitions.

[Kostas Petritis]

Have you used another transition before this that has about the same parent or daughter ion? I am trying to imply somekind of cross-talk which is very common in triple quadrupoles but it might not occuring at all in the case of ion traps...

[jameslizh]

Hi, Kostas, take the above m/z 309-->180 as example, I also set m/z 309-->179, the retention time is same for both transitions. I can not recall the peak intensity for these two transitions, but think they seem to be close. For standards with higher concentration, I always got m/z 309-->180 which should be. That is why I am wondering if low concentration sample can result in this ("cross-talk" as you mentioned). I don't have experience to use triple Q.

[Kostas Petritis]

Hi James,

I think I talked too fast, cross talk is inherent to quadrupoles (it happens when you follow transitions one after another where the daughter ion is the same and the fragmentation cell didn't have the time to get rid of the previous daughter ions) and not the ion traps. It is also independent the abundance of the ions...

At this time I do not see any good explanation why this is happening... if you have already tuned the isolation and fragmentation elements of your ion trap...