To Spin Or Not To Spin?

For liquids NMR one can average out the transverse inhomogeneity of the magnetic field by spinning the sample tube about the direction of the magnetic field. This will lead to sharper lines and a higher signal-to-noise ratio however artifacts called spinning sidebands will appear at multiples of the spinning frequency on either side of an NMR line. The intensity of these artifacts depends on the degree transverse inhomogeneity of the magnet, the quality of the NMR tube and the NMR probe. The decision to spin depends on how well the transverse shims of the magnet (i.e. x, y, xy, (x2-y2).... etc..) are set, and your tolerance for spinning sidebands. In a very well shimmed magnet it may not be worth spinning at all. In a poorly shimmed magnet spinning will provide a substantial line narrowing as well as a huge boost in the signal-to-noise ratio. For 2D experiments, one should never spin as sidebands will introduce undesirable artifacts in the spectra. Below are two comparisons of spinning vs non spinning in a well shimmed and a not-so-well shimmed magnet. What do you think? Should you spin?

Why is the Tuning Display Unstable When I Tune the Probe?

On the Varian INOVA 500 you may notice that the tuning display is unstable while you are tuning the probe. On the Bruker spectrometers, this may show up as ripples or waves on the "wobble" tuning curve. This phenomenon is due to Q modulation in the probe as a result of spinning the NMR tube. The tuning of the probe is dependant on the orientation of the NMR tube within the probe. This problem is much worse for cheap NMR tubes or NMR tubes which have been damaged or lack perfect radial symmetry. The effect can also be seen for solids samples spinning in MAS probes.

To avoid the problem, simply stop spinning the sample.