Shimming

Shimming is another one of those things you have to do to record an NMR spectrum but what exactly is it, and why does it have to be done? Put simply, shimming is the process of optimising the magnetic field around your sample. Shimming your sample will produce spectra with sharper peaks and better resolution.

NMR spectrometers measure very small differences in frequency. To resolve a coupling constant of 3 Hz in a 600 MHz spectrometer the magnetic field needs to vary no more than 5 parts per billion throughout the entire sample. The field produced by superconducting magnets is not this homogenous and the environment in which the magnet is housed distorts the field as well. Furthermore, the sample itself distorts the field around it. Shimming is used to correct these distortions and needs to be done every time a sample is placed in the magnet.

The term shimming comes from the thin metal or wooden wedges, called shims, that were pushed into gaps to move and align mechanical equipment. Early NMRs were built with room temperature ferromagnets and shims were used to improve the field around the sample. As superconducting cryomagnets were introduced the physical shims were replaced by electrical coils which can induce small magnetic fields.

The shim coils are oriented along three orthogonal axes to give complete spatial control of the magnetic field. The z-axis is vertical, through the center of the sample, while the x and y axes are horizontal. The z shim induces a linear change in the magnetic field along the z-axis, like a gradient pulse. The z² shim creates a quadratic gradient along the z-axis, and z³ creates a cubic gradient. Similar functions are available for the x- and y- axes. Different combinations of the three axes and simple mathematical functions of them means that on our spectrometers there are 28 different shims that can be adjusted.

Optimising 28 shims requires searching for a maximum in 28 dimensional space, which is not a trivial task. Fortunately, automated shimming procedures that are very fast and reliable are available. Gradient shimming uses the probe's gradients to map the strength of the magnetic field over the entire sample. Then, with reference to a previously recorded shim map, the optimum shims can be calculated.

In most cases gradient shimming produces acceptable results on our cryoprobes, but sometimes slight adjustment of the lower order shims can improve the spectra. This is likely due to a temperature difference between the top and bottom of the sample that leads to convection in the solution. This is why I recommend using the "Tune after Z-X-Y-XZ-YZ-Z" option in the TopShim GUI on our spectrometers.

With older and lower field spectrometers the sample is often spun about the z-axis. This averages out the shims in the x-y plane and makes it easier to get sharp peaks. Unfortunately it also creates "spinning side bands", small artifact peaks that appear on either side of real peaks at the frequency at which the sample is spinning. With our 600 MHz spectrometers the gradient shimming easily optimises the x- and y-axis shims so that spinning is of no benefit and so it is not recommended.