What is Shimming?
Shimming is adjusting the resolution of the signal by optimizing the homogeneity of the magnetic field. The lines in NMR spectra are very narrow-linewidths of 1 Hz or less are not uncommon-so the magnetic field has to be very homogeneous. What we have to do is to surround the sample with a set of shim coils, each of which produces a tiny magnetic field with a particular spatial profile which can be used to cancel out the inhomogeneities in the main magnetic field. The current through each of these coils is adjusted until the magnetic filed has the required homogeneity, something we can easily assess by recording the spectrum of a sample which has a sharp line.
The shims are labeled according to the filed profiles they generate. So, for example, there are usually shims labeled x, y and z, which generate magnetic fields varying in the corresponding directions.
The axial coils, z1-z4 modify the field profile along the axis of the main field, which is vertical. The radial coils are x and y, and other combinations containing one of the two. The radial inhomogeneities can be averaged out to some extent by spinning. Adjustment of the axial shims should be done with the sample spinning. The radial shims are done without spinning. There are several ways of shimming to optimize the homogeneity of the magnetic field (shimming on the FID, shimming on the spectrum, autoshim, gradient shimming and shimming on the lock). The shimming on the lock is more preferable for a standard user and you will open the lock window, which allows shimming to be performed on the lock. When shimming on the lock, you monitor the intensity of the lock signal. You need to adjust only z1 and z2 shims. Each shim setting (z1-z4) controls the current through shim coils that control magnetic field gradients in different directions. It is important to know that the z direction is parallel to the vertical direction of the probe and it is for this reason that the height (5 cm) of the sample in the NMR tube affects the z shim settings rather dramatically.
Note: I am periodically updating the latest shims. You should retrieve the ?stdshims? before establishing the lock.
Address:
C. Eugene Bennett
Department of Chemistry
406 Clark Hall
Prospect Street
P.O. Box 6045
West Virginia University
Morgantown, WV 26506
Phone:
(304) 293-3435 X6439
Fax:
(304) 293-4904
Email:
Chemistry@mail.wvu.edu
