Chromatography Forum

I am trying developing a GC MS MRM method using Waters Quattro GC MS. When doing MS1 scan people set up "low resolution" and "high resolution" all to 15. In my experiment I found the lower resolution the better sensitivity. In order to achieve high sensitivity I could low the resolution parameters.

Having said so, does anyone here know if it is OK for me to set up those resolution instrument parameters as low as 1 or 2? By the way I was suggest to set up like 12 or 10.

Thanks

15/15 low and high resolution on the Quattro Micro corresponds to approx. 0.7 Da FWHM. Decreasing the resolution to 13/13 will give approx. 1.0 Da FWHM and about 20% higher signal but also slightly higher noise.
Decreasing the resolution further will only give little increase in signal height but will generate much broader signals and the noise will dramatically increase as you will loose the specificity.

The resolution setting also depends on your sample matrix. In some cases it can be helpful to increase the resolution in order to eliminate noise that is coming from the matrix. I'd quickly check the S/N ratios at different resolution settings by running some experiments at 0.5, 0.7 and 1.0 FWHM.
Of course you can also play with different settings for Q1 and Q3. For example low resolution on Q1 to get a lot of ions into the collision cell and higher resolution on Q3 to maintain the specificity.

A lot of choices but I think 1.0 Da FWHM is a good starting point for MRM experiments on the QM. You can then further optimize the settings to squeeze out more sensitivity.

Be aware that the mass scale is shifting when you change the resolution settings. You may need to readjust the mass scale (or your target mass) to avoid sitting on the edge of the signal.

Hope this helps!

hema

Hema, I really appreciate your insight.

But could I have a bit more of your precious time to shed more light for me?

What I found is when Rs=12 S/N 105(all manually), peak area 833; When Rs 5 S/N 214 peak area 3453; when Rs =2 S/N 454 , peak area 10021. It appears to me that sensitivity was increased at lower Rs.

Unfortunately I do not see how I could test FWHM for MRM function in my MassLynx. Then how much the FWHM will be estimated to be when RS=2 or 5?

If FWHM=3 at Rs=2 the signal picks M(the target ion) and M+1 (isotope) and M+2 (isotope) including other possible impurities, which means the method lacks specificity. But if all the precision, linearity and accuracy pass will it be OK for a quantitation method?

And How could I measure the MS scale shifting caused by Rs setting change?

Indeed, you won't see the FWHM in MRM mode. But you can run a scan experiment (in profile or MCA mode) to quickly check the FWHM of your signal. You will then also see the mass position drifting. By experience, the drift is around -0.2 to 0.1 Da when you change from 15/15 to 13/13. This may not be too important in most cases but can cause some reproducibility problems when the drift is just on the side of the signal.

You can also get a pretty good estimate of the FWHM directly from the signal in the tune page. You could even use the water/air signal for this: Select MS Scan as function in the tune page, set the mass to 18, the span to 5 and adjust the gain to see a decent signal. Then adjust the resolution settings to see how this affects the mass position and the peak width.