Shimming is a process that is carried out to correct any inhomogeneities in the applied magnetic field during a nuclear magnetic resonance (NMR) experiment. The inhomogeneities can come from two main sources: Why Shim? In NMR spectroscopy, the analyst tries to achieve the best possible resolution, with the narrowest lines in the spectra.

In NMR and MRI, shimming is used prior to the operation of the magnet to eliminate inhomogeneities in its field. Initially, the magnetic field inside an NMR spectrometer or MRI scanner will be far from homogeneous compared with an "ideal" field of the device.

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.

Modern shims are coils that produce extra fields that can cancel the unwanted harmonics in B 0. The shim coils are designed to produce, when current is passed through them, a single harmonic. Then the current in that shim can be adjusted to exactly cancel that portion of the inhomogeneity described by that particular spherical harmonic.

Aug 23, 2007 · Bad Shimming Wide, asymmetrical lines • How do we know when the shimming is good? – Amplitude of the lock • Have you adjusted the shims so that you have maximized your lock level? – Decay of the FID • Short, stubby FID = bad shimming – Examine actual peaks in finished spectrum • Are they all symmetrical? No tails or bumps?

In order to obtain satisfactory spectra, any operator of an NMR spectrometer must be able to shim out the relatively small gradients caused by changing the sample and the sample temperature. This must be done fairly accurately, or lousy spectra will be produced.

One of the most important factors to get a good quality NMR spectrum is shimming. NMR spectroscopy demands a homogeneous magnetic field over the whole of the sample volume located within the probe's detection coil. If the field is not homogeneous the resulting spectra will suffer from distorted lineshape

Adjusting the magnetic field homogeneity is called "shimming." There are shim coils surrounding the area where the sample sits inside of the magnet. Currents run through the coils, producing local magnetic field gradients that minimize the magnetic …

Gradient shimming provides rapid, automatic adjustment of room temperature shims (mostly z shims). Typically it takes couple of minutes to obtain quality shims without much user intervention and all steps can be followed by the simple procedure written below.

In Part I of this series [1] a number of general concepts and recommendations were presented regarding NMR sample preparation and the physics and strategies involved in the optimization of magnetic field homogeneity (shimming) for high-resolution NMR spectroscopy.