Optimizing the field homogeneity: shimming
NMR experiments require a uniform magnetic field over the whole of the NMR sample volume that sits within the detection coil. Deviation from this ideal introduces various line shape distortions, compromising both sensitivity and resolution. Thus, each time a sample is introduced into the magnet it is necessary to ‘fine-tune’ the magnetic field and a few minutes spent achieving good resolution and lineshape is time well spent. For anyone actually using an NMR spectrometer, competence in the basic level of field optimization is essential, but even if you only need to interpret NMR spectra, perhaps because someone else has acquired the data or if the whole process is performed through automation, then some understanding of the most common defects arising from remaining field inhomogeneity’s can be invaluable.
The shim system
Maintaining a stable magnetic field that is uniform to 1 part in 109 over the active volume within modern NMR probes (typically 0.1–1.5 cm3) is extremely demanding. This amazing feat is achieved through three levels of field optimization. The first lies in the careful construction of the superconducting solenoid magnet itself, although the field homogeneity produced by these is rather crude when judged by NMR criteria. This basic field is then modified at two levels by sets of ‘shim’ coils. These coils carry electrical currents that generate small magnetic fields of their own, which are employed to cancel remaining field gradients within the sample. The superconducting shim coils sit within the magnet cryostat and remove gross impurities in the magnet’s field. The currents are set when the magnet is first installed and do not usually require altering beyond this. The room temperature shims are set in a former that houses the NMR probe itself, the whole assembly being placed within the bore of the magnet such that the probe coil sits at the exact center of the static field. These shims (of which there are typically around 20 to 40 on a modern instrument) remove any remaining field gradients by adjusting the currents through them, although in practice only a small fraction of the total number need be altered on a regular basis. The static field in vertical bore superconducting magnets also sits vertically and this defines, by convention, the z axis. Shimming In order to achieve optimum field homogeneity, high quality samples are essential. The depth of a sample also has a considerable bearing on the amount of Z shimming required, which can be kept to a minimum by using solutions of similar depth each time.
The most recent approach to field optimization comes from the world of magnetic resonance imaging and makes use of field gradients to map B0 inhomogeneity within a sample. This can then be cancelled by calculated changes to the shim settings. The results that can be attained by this approach are little short of astonishing when seen for the first time, especially for anyone who has had to endure the tedium of extensive manual shimming of a magnet, and this method now enjoys widespread popularity.