Shimming gradient

This makes the straightforward shim procedure on the lock-level impossible. As most NMR probes are nowadays equipped with -gradients, shimming is usually accomplished by gradient-shimming on the acetonitrile signal. 

The introduction of gradient shimming (that makes this procedure very easy to automate) together with the possibility of performing automatic tuning and matching (via the use of motors mounted on the probe body) has further improved the whole procedure. 

Figure 3.47. Errors in the local static field along the length of a sample are encoded as signal phases when magnetisation is allowed to precess in a inhomogeneous static field for a time x. The resulting spatially dependent phase differences are used as the basis of gradient shimming.

The primary experimental requirement for gradient shimming is a sample containing a dominant strong, singlet resonance. A good candidate for proton observation is 90% HiO, but although this is ideal for biomolecular studies, it... 

Figure 3.50. Automatic deuterium gradient shimming. Spectrum (a) was acquired with all z-axis shims set to zero. After less than two minutes of gradient shimming of the z-z shims (3 iterations) spectrum (b) was obtained. The solvent was DMSO and one scan was acquired for each deuterium gradient echo collected via the probe lock coil. Automatic switching for deuterium observation was achieved with a home-built switching device.

The remarkable power of gradient shimming is illustrated in Fig. 3.50. The lower proton spectrum was recorded with the z-z shims all set to zero whilst the upper trace was the result of only 3 iterations of deuterium gradient shimming using the DMSO solvent resonance. The whole process took less than 2 minutes without operator intervention. Although a rather extreme example, the capabilities of this approach are clearly evident and it is likely to play a valuable role in automated spectroscopy, where irreproducible sample depths can lead to rather poor results with conventional simplex optimisation shim routines. The individual mapping of field errors within each and every sample overcomes these problems. 

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

Figure 3.55. A gradient edio sequence suitable for z axis field gradient shimming. Hie pulsed field gradients [nxivide die atial encoding lilst the delay r encodes the static field inhomogeneily as signal phase.

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