Gradient Shimming

Gradient shimming usually works well on the Merc400, 500, and 600 (there is a signal-to-noise problem on the 500, but if you set parameters well, it is good enough). It is moderately easy to learn. There is a basic protocol below, for individual instruction, you can contact me and setup a time when you have NMR time on the 500 or Merc 400, and I will explain how to gradient shim, what is happening and what problems to watch out for. There are instances when the gradient shimming does not work appropriately, you will need to reload your starting shims in this case.

I have setup standard shimmaps (essentially a map of the probe) so that is no longer required for standard tubes. The maps are stored in the /vnmr/gshimlib/shimmaps directory. The current standard shimmap will be in that directory, with the name always starting with asw (autoswitchable) for the standard probe on the Merc400 and always starting with PFG_SW (pulsed field gradient switchable) for the standard 500 probe.

To accomplish gradient shimming for the first time:
Turn spinner off. (the macro should turn spin off anyways). Note the new lock level after turning off the spinner. If the lock drops alot, shim quickly X1, Y1, XZ, YZ (how much is a standard drop spin to non-spin is solvent dependent, and instrument dependent; on the 500, expect ~5-10% drop in CDCl3, on the 400, expect 10-15% in CDCl3).
Type in vnmr window: gmapsys
Lots of things will happen, parameters will change, messages will appear... After the computer seems to have settled, click on Shimmaps button. Click on Shimmap files button. Click on CD to System directory button, should be on the far left side. If you do not see that button click on CD to User Directory. Then, you should see CD to System directory button, click on it. Highlight file that starts with asw on Merc400 or PFG_SW on the 500, click on Load Shimmap and Parameters. Click on Return button.

If you are on the 400: Set nt = 8 for CDCl3, or nt = 2 for any solvent with alot of deuterium (benzene, D2O, acetone, DMSO, methanol). For methanol or acetone, set d1 = 5. Then, click on Autoshim on Z button. The gradient shimming should finish in 2-4 iterations; the lock should have increased after the gradient shimming is done. If for some reason the gradient shimming does not succeed, abort acquisition (this rarely happens now); if this does occur check the lock level, if it is lower than when you started, click on Set Shims button, then click on Starting Shims button as gradient shimming failed and probably made the shims worse.

If you are on the 500: Set nt = 16 for CDCl3, or nt = 4 for any solvent with alot of deuterium (benzene, D2O, acetone, DMSO, methanol). For methanol or acetone, set d1 = 5. Set pw = 800 for any solvent. Set gzsize = 4 (shims Z1-Z4). Then, click on Autoshim on Z button. The gradient shimming should finish in 2-4 iterations; the lock should have increased after the gradient shimming is done. If for some reason the gradient shimming does not succeed, abort acquisition (this rarely happens now); if this does occur check the lock level, if it is lower than when you started, click on Set Shims button, then click on Starting Shims button. After that finishes, you could go back and set gzsize = 5 (shims Z1-Z5) and redo the Autoshim on Z (most of the time this helps, but sometimes, the gradient shimming fails for gzsize = 5).

After finishing the gradient shimming on any instrument, you might need to change back to your home directory to save any data that you acquire. To change to home directory, type in vnmr window: cd