UW CHEMISTRY NMR

The majority of lock solvents used in our facility are fully deuterated i.e. 99.0% or more 2H enriched.  This means that the Topshim system performs what is called ‘1D gradient shimming’  or ‘Z-axis only’ shimming.  This leaves the job half-done when it comes to shim optimization.  The XY-plane or ‘transverse plane’ shims also need adjusting, albeit for a very small subset of shim gradients :  [X, Y ] as an absolute minimum and ideally [X, XZ, Y, YZ] as an optimum choice.

I will show below the remarkable difference in the resolution that is made by merely touching up the [X, Y] shims after I complete the Topshim 1D along the Z-axis (i.e. the magnet’s axis) from the model compound Strychnine in CDCl3.

In the following comparative spectrum :

GREEN : corresponds to the spectrum recorded after completion of Topshim 1D only.

RED :  When we spin the sample we see the dramatic improvement in resolution clearly bringing out the fine structure of the peaks.  Spinning effectively removes the magnetic field inhomegeneity in the XY plane.

BLUE : This is a spectrum that is recorded without spinning. But I have merely adjusted the X and Y shims alone after Topshim 1D.   The resolution we get now is pretty close to what we achive by spinning the sample.

You might wonder, why not simply spin the sample to obtain optimum resolution every time. Indeed, sample spinning is recommended to get the best overall resolution from the spectrometer. But there are caveats to keep in mind :

• If the XY plane shims are way off i.e. not just [X,Y] but the higher order shims such as [ XZ, YZ, etc.]  then you will see artifacts called spinning sidebands, confusing your data interpretation.
• For some experiments such as those that rely on relaxation mechanisms such as NOESY or ROESY,  we do not spin the sample and this advantage by spinning cannot be realized.
• For the so-called J-Young tubes, spinning is not recommended due to their tendency to wobble inside the magnet and get stuck in the process.

So, the recommended protocol for you to get the best resolution from the system is as follows:

• Before you start Topshim,  load the optimal default shim file we give you by entering : rsh stdbbi (I am taking the AV500 example here; please look at the cheat sheet on the console to know the name of the correct shim file to load)
•  If the sample can be safely spun,   do so by entering the command :  ro on
• Perform Topshim :  topshim
• Stop spinning by entering the command :  ro off
• Start the BSMS control suite by entering : bsmsdisp and bring up the following GUI window
• Perform the simplest X,Y shim adjustment as below :
  • Using the virtual keyboard above, select X.  Either increase OR decrease the X shim values by clicking on either Step+ OR Step- button so that the Lock Level steadily increases.  Do this until the Lock Level maxes out.
  • Repeat the above operation for Y ;  Select Y by clicking on it and do the same as before.
  • You will figure out in which direction you need to change the shims by couple of clicks. If the lock level goes down, then simply try the opposite sense of change.
• Believe it or not, this simple procedure itself will bring you pretty close to the optimal lineshape and resolution you are seeking.  The above manual process won’t take more than 30 seconds at the outermost.  But it is worth the investment in time.

Last, but not least, do not hesitate to contact us if you need any help.

Hope this will help you obtain optimal spectra from all our spectrometers.  The above recipe can be applied in AV300 and AV301 as well.

NMR Facility Manager

6/23/2021