MRM - Mass Resolution Window

[Travisdog2]

Hi,

In the past, I have created MRM methods in which I have always setup both MS1 and MS2 for 'unit' mass resolution - which on Agilent systems is supposed to be 0.7AMU.

It seems that a fair number of MRM methods (at least those in Agilent application notes) tend to use 'wide' (1.2AMU) mass resolution which I have seen from own work does increase signal - not insignificantly.

I was curious as to your thoughts on the best approach for MRM in terms of mass resolution windows and whether there are any downsides from using a 1.2AMU window as opposed to a 0.7AMU window.

Many Thanks

TD

[bcd_GCLCMSMS]

We traditionally use 'unit' mass for both quads with good results. I agree that opening opening the quads to 1.2 does increase raw signal counts, but I haven't looked at actual MRM signal-to-noise ratios. I believe it also depends on how accurately one optimizes and sets the center of the MRM mass windows (decimals vs. no decimals in the MRM table) and how accurately the instrument is tuned. If the value of the quad mass setting is off from the actual center peak of the ion distribution that is passed by the quad, a lower signal will be observed.

As a matter of practice, we nearly always tune the instrument immediately before optimizing the MRMs of any new compounds.

(The question I always ponder is whether or not MS1 should be set differently than MS2. For example, if MS1 is set to unit (~0.7 amu) for the parent mass, then may the MS2 be set at 1.2 amu to get maximum signal for the resulting fragments?)

[per_oxid]

I have also been thinking about this. When we calibrate at unit resolution for waters 0.75 da this will pass between 0.6-0.9 da width. This is a quite large difference that can influence s/n for your mrm and loq for your method.In many ways the wider resolution is an advantage since mass calibration will be more robust and signal often better.

[Gaetan Glauser]

I agree with all of above, however in practice I have very rarely seen any improvement of S/N ratios by increasing or decreasing the quad resolution. Typically the signal increases when the resolution is decreased, as does the noise.

Thus, in the end, I think this is mostly a matter of robustness (wider quad resolution = bigger tolerance for mass deviation over time) versus potential selectivity issues (wider transmission = more potential interfering signals).

[Travisdog2]

Hi all,

Many thanks for your thoughts on this...
I had a bit of a play over the past few days running a few calibration series using 3 MRM methods as below:

MS1 - Unit, MS2 - Unit
MS1 - Unit, MS2 - Wide
MS1 - Wide, MS2 - Wide

Of the 3 methods, the second (Unit/Wide) seemed to provide the best overall performance, with better RSQ and significantly lower LOD/LOQ values than any of the other methods. No idea why as of yet, but thought worth mentioning.

R