Shimming
In the context of NMR, the shims are small magnetic fields used to cancel out errors in the static magnetic field. These minor spatial inhomogeneities in the magnetic field could be caused by the magnet design, materials in the probe, variations in the thickness of the sample tube, sample permeability, and ferromagnetic materials around the magnet. A shim coil is thus designed to create a small magnetic field which will oppose and cancel out an inhomogeneity in the B o magnetic field. The most fundamental criterion of the homogeneity is the observed line shape, and in modern spectrometers, the two measures of homogeneity is the deuterium lock signal and the free induction decay (FID).
The deuterium lock is the mean by which long term stability of the magnetic field is achieved. The field strength might vary over time due to aging of the magnet, movement of metal objects near the magnet, and temperature fluctuations. The field lock can compensate for these variations. The field lock is a separate NMR spectrometer within the spectrometer. This spectrometer is typically tuned to the deuterium NMR resonance frequency. It constantly monitors the resonance frequency of the deuterium signal and makes minor changes in the Bo magnetic field to keep the resonance frequency constant.
The lock level is a convenient indicator of the homogeneity: the lock signal results from observing a single line, usually of the deuterated solvent. The area under this line is constant, but the width of the line depends on the homogeneity: as the line becomes narrower it must also become taller, thus the lock signal, displayed as the lock level, is higher. Thus the object is to maximize it
Adjusting the shim
There are different shim coils that can create a variety of opposing field. By passing the appropriate amount of current through each coil a homogeneous magnetic field can be achieved. The optimum shim current settings are found by either maximizing the signal from the field lock or maximizing the size of the FID. While shimming, our task is to find the best shim value by maximizing the lock signal.
One should take into consideration that high order shim fields (e.g. Z4) are 'contaminated' with shim fields of lower orders with the same parity (i.e. Z3 contains contributions from Z; Z 4 from Z2). So when adjusting high order components (Z3 , Z4 etc) it is essential to re-optimize the lower order components (Z, Z2 here).
A shim protocol starts with adjusting
Z, Z2
and then the low order horizontal gradients
X, XZ
Y, YZ
XY, X2 -Y2
and again
Z, Z2 .
Than go to higher orders if necessary
Z3 , Z
Z4 , Z2
Z4 , Z3, Z.
Try to improve the horizontals, and the low Z, again.
The shape of an NMR line is a good indication of which shim is misadjusted:
Shimming using the FID
If the FID can be displayed in real time (using the Bruker command 'gs'), the magnetic field can be shimmed by observing the shape of the decay envelope. Two features of the appearance of the FID are informative: its duration and its shape. The duration of the decay gives information about the eventual linewidth, and the appearance of the decay envelope gives information about its line shape:
(A) correct FID shape (B) Incorrect FID shape
The simple exponential shown in (A) above, is only observed with samples whose signal is dominated by a strong single resonance line (e.g. by that of the water solvent).
Gradient Shimming
The old gradshim feature, automatic shimming using gradients, is available on probes that are equipped with gradients. It requires the existance of reference mapping (both 1D as well as 3D maps) for each probe.
TOPSHIM
The topshim feature, a completely automatic shimming using gradients, is available on the AVIII800, NEO600, AVIII400 systems. For its graphic interface use the command topshim gui.