The probe must be tuned to the observe frequency with the particular sample of interest. There can be a big difference depending on the solvents and concentration, such as water or organic solvents. When the probe is tuned the power is efficiently transferred from the transmitter to the probe, rather than reflect back to the transmitter, and pulse width is minimized. On the other hand, the detected signal power is efficiently transferred to the preamplifier and the S/N ratio is maximized. Probe tuning is essential for obtaining a good spectrum, and for some advanced experiments to get any meaningful results at all. For most of solid state NMR experiments, the probe must be properly tuned each time, otherwise high power could not efficiently delivered to the probe. It may cause arching or damage the probe.
lthough probe designs are different according to their functions, one has in general two adjustable capacitors. One is called tune and the other called match. The tune capacitor is used to adjust probe circuit to the desired frequency, most broad band probe have additional fixed external capacitors to extend the tune range. The match capacitor is used to adjust probe circuit to meet impedance requirement (50 Ohm). In most case, these capacitors are mutually interactive and therefore we should adjust them in turn.
In early sixties when larger electromagnets were used for NMR instrument, the field homogeneity was adjusted by mechanical alignment of the magnet pole faces. By place thin pieces of brass between the magnet and the pole pieces to make the poles more perfectly parallel. The metal piece was called shim and the adjustment was called shimming. A well-shimmed electromagnet (1.4T) could yield line widths of 0.2 Hz. As the magnetic field is increased it is necessary to add electronic shimming which is a small coil placed around the probe. As the field become higher and higher, more and more shimming coils are added into the room temperature shimming coil, the shimming process become much more difficulty.