UW CHEMISTRY NMR

Dear Users

I mentioned last week that a couple of hardware related issues need addressing and this is an update about the first and foremost of them :  getting consistent results from Topshim method of shimming for all Lock Solvents.

•   The root cause for the sporadic failure of Topshim, especially for weak lock signals like CDCl3 has been traced to a poor 2H NMR resonance signal that is used under the hood for both locking and shimming.   I have applied an optimized parameter set to mitigate this so that Topshim should work now with all 100% deuterated solvents such as CDCl3.
• There is still this question of why the 2H resonance signal is weak to begin with. This is a hardware related issue which is being looked into.  The remedy applied will be seamless and you won’t notice much other than my loitering near the spectrometer a few times in the coming days.

IMPORTANCE OF TOUCHING UP THE X AND Y SHIMS :

Topshim will ONLY optimize the Z-axis shims ! To get a quick and best final spectrum, it is for us to optimize the so called ‘transverse’ or XY plane shims.  There are just two shims for you to take care of : X and Y.    Here is what you do :

• After Topshim completes its job,  type ‘bsmsdisp‘  on the command line.  You will get a graphical window which has buttons like the one shown here :
• You click on the X button to select it and then use either the Step+ or Step- buttons to increase the lock level monotonically,  as far as you can go.
• Repeat the operation for Y button.  
• That’s it !  You have touched up the X and Y shims.

I will leave you with the following comparative spectrum recorded on AV500 to give you an idea, how much of a difference this simple step above will make.   In the spectrum shown below,  the GREEN trace is after just Topshim completion.  The RED trace is recorded with the sample spun at 20 Hz.  But the BLUE trace is recorded on the same sample, after touching up your X and Y shims and without any spinning ! You can see that the BLUE trace is almost as good as a sample spun to get the best homogeneity.

 

NMR Facility Manager

8/27/21