Optimising the field homogeneity shimming

To provide an indication of progress when shimming, one requires a suitable indicator of field homogeneity. Essentially, there are three schemes that are in widespread use, all of which have their various advantages and disadvantages (1) the lock level, (2) the shape of the FID and (3) the shape of the NMR resonance. The ultimate measure of homogeneity is the NMR resonance itself, since defects apparent in the spectrum can often be related 

Shim gradient Gradient order Principle interacting shim gradients Shim gradient Gradient order 

When shimming, it is not always sufficient to take the simplest possible approach and maximise the lock level by adjusting each shim in turn, as this is may lead to a false maximum , in which the lock level appears optimum yet lineshape distortions remain. Instead, shims must be adjusted interactively. As an example of the procedure that should be adopted, the process for adjusting the Z and shims (as is most often required) should be  

Adjust the Z shim to maximise the lock level, and note the new level. 

Alter Z so that there is a noticeable change in the lock level, which may be up or down, and remember the direction in which Z has been altered. 

Shim gradient Gradient order Interacting shim gradients 

Not all these may be present on lower-field instruments, whilst on very high-field spectrometers additional high-order shims may be found. Those shown in brackets interact less strongly, and thus may not require subsequent readjustment. Those that interact with Z° (the main field) may cause momentary disruption of the lock signal when adjusted. 

Modem shim sets are capable of delivering non-spinning lineshapes that almost match those when spinning, and it is becoming increasing common not to spin samples. For multidimensional studies this is certainly the case, since sample spinning can introduce modulation effeets to the acquired data, leading to unwanted artefacts particularly in the form of so-called ti-noise (see Chapter 5). 

If the magnetic field happens to be close to the optimum for the sample when it is initially placed in the magnet, then simply maximising the lock level with each shim directly will achieve the optimum since you will be close to this already. Here again a reproducible sample depth makes life very much easier. 

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