Lock level

First optimize the z-gradient to maximum lock level. Note the maximum value obtained. 

Again adjust the z-gradient for maximum lock level. 

Check if the strength of the lock level obtained is greater than that obtained in step 1. If not, then readjust z, changing the setting in a direction opposite to that in step 2. 

Readjust the z-gradient for maximum lock level, and check if the lock level obtained is greater then that in steps 1 and 3. 

Repeat the preceding adjustments till an optimum setting of z/z -gradients is achieved, adjusting the z -gradient in small steps in the direction so that maximum lock level is obtained after subsequent adjustment of the z-gradient. 

However, there is one major drawback implemented by the use of solvent mixtures. The commonly used solvent system of acetonitrile/water displays a strange behaviour. Typically, the line shape of the water and the acetonitrile is different. As normally the water is used in its deuterated form, the spectrometer is also locked on to the D2O component. Trying to shim such a sample by using the lock level, and therefore the line shape of the D2O, provides a good line shape for the water. However, for the acetonitrile component, severe distortions can be observed, and the sample signals show a similar behaviour like the signals of the acetonitrile. 

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. 

Another way to shim is to use the FID as a criterion for homogeneity instead of the lock level. The goal is to get a smooth exponential decay curve with the longest time constant (slowest decay) possible for the FID. Bruker uses the command GS to enter an interactive mode where the FID is acquired over and over again, displaying it each time... 

It is important to avoid lock instability due to either saturation or suspended particles, because instability interferes with magnet field regulation by the lock channel and, in severe cases, can result in loss of the lock signal altogether. When a stable lock level is achieved somewhere around midrange, the lock phase (see next subsection) should be maximized. Only an approximately maximum lock phase, however, is desired at this point, because the lock phase is dependent on the homogeneity of the magnetic field. 

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 Z 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... 

Check whether the lock level is greater than the starting level. If it is, repeat the whole procedure, adjusting in the same sense, until no further gain can be made. If the resulting level is lower, the procedure should be repeated but Z altered in the opposite sense. 

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. 

Optimise Z and Z interactively, as described above. If this is the first pass through Z and Z, then adjust the lock phase for maximum lock level. 

Although the lock level is used as the primary indicator of field homogeneity, it is not always the most accurate one. The lock level is dependent upon only one parameter the height of the deuterium resonance. This, whilst being ratber sensitive to the width of the main part of the resonance, is less sensitive to changes in the broad base of the peak. The presence of such low level lumps can be readily observed in the spectrum (particularly in the case of protons) but for this to be of use when shimming, the spectrometer must be able to... 

Initial thermal equilibrium, often based on the lock level stability, can be reached quickly taking as little as a few minutes (e.g. a room temperature sample inserted into a regulated magnet at 25 °C). Increasing the airflow to the variable temperature controller can speed this equilibrium process but unfortunately can easily introduce microphonics in the resulting spectra due to minute vibrations (i.e. wiggling of the sample in the probe). This is of course deleterious, and a compromise between temperature stability and positional stability must be reached for each spectrometer. For cryogenically cooled probes this becomes... 

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