Monday, January 7, 2008
Shimming Without a Lock Signal
Many people who do liquids NMR are at a loss when asked to shim a magnet without the benefit of a deuterium lock signal. Solids NMR people must do this routinely. To shim a magnet without a lock signal, you must set up the spectrometer to observe a strong FID (usually the proton signal is used but a strong narrow signal from any nucleus will do). The instrument must be setup to acquire scans without adding up the signals ("gs" on a Bruker instrument and "fid" on a Varian instrument). While observing the FID adjust the shims until the FID is as long lived as possible while retaining an exponentially decaying envelope. If multiple signals are present, the carrier frequency may have to be changed to get an FID with the "best appearance".