Thursday, June 12, 2008
The Importance of Setting the Depth of Your Sample
Automated deuterium gradient shimming does a remarkable job at shimming NMR magnets and has really improved the quality of NMR data produced by novice NMR users. Despite the widespread success of this shimming method, I still hear complaints from students that "the shimming really sucks". A quick glance at the sample often explains the unsatisfactory results. There are a number of reasons why deuterium gradient shimming may not work well. One reason is that the sample tube height may not be set properly in the depth gauge such that the sample does not sit in the center of the probe coil. Deuterium gradient shimming uses a previously stored spatial map of the well shimmed magnetic field. The "shim map" is produced using a standard sample filled to the correct height and set to the correct depth with respect to the coil in the NMR probe. Any sample that does not sit in the same position as the standard sample used to produce the shim map will distort the magnetic field such that the stored shim map will not work very well. This is illustrated in the figure below. The left hand spectrum shows the result of gradient shimming for a properly positioned sample. For the spectrum on the right hand side, the NMR tube was lifted up 5 mm from a proper depth and the same gradient shimming routine was used.Take home message: Set the depth of your NMR tube properly.