NMR manager
Johns Hopkins School of Medicine
312 WBSB
Department of Pharmacology
725 N Wolfe St
Baltimore, MD 21205
410-502-4803(Office)
Shim is always a problem for most users. If you see your reference peak is bad, this means that shim is bad. This page tells you how to use autoshim to optimize shim parameters.
----------------------------load shim map from vnmr directory
You have the shim map which might not work for you. So you need to select one which has been updated by the manager. To do this, please follow the following procedure:
1. click Main Menu, then click File, then click Set Directory, then click Parent.
Now you can see all the users and the vnmr and vnmr1 directories.
use your mouse to select vnmr directory, then click Change;
use your mouse to select gshimlib, then click Change;
use your mouse to select shimaps, then click Change.
Now you see a window like this.
Now, type gmapsys, enter. The window you saw became dark. You need tot click Main Menu, then click File to bring it back.
Now, type gmapsys again,
Click Shim Maps, then Shimmap Files, then use your mouse to select dryprobe_lk_31Oct2006.fid. ( You can ask manager to see which is better).
Click Load Shimmap & Params.
You have successfully loaded the new shim map.
Note, you need to click Main Menu, File, Set Directory, Home to go back. You can only save something in your own directory.
-------------------------------shim procedure
1. type gmapsys , enter¿;
2. type gzwin? enter¿, you will see that the default number which is about 50;
3. type gzwin=38, enter¿; I found that it works well for our machine. This is after water disaster.
4. use the mouse to click at auto shim on Z. The machine will do shim for you for up to 10 times. Once the RMS is less than 1, the shim will be complete;
5. click quit. This will bring you back to the experiment at which you are going to run with the default parameters. This is supposed to be s2pul sequence. You should see it on top. If you do not see it or you can not see the quit button, type gmapsys again, and then use the mouse to click quit button.
6. You can manually shim to see if the lock level increases. Usually, it does not, because the machine has optimized the parameters.
7. type svs, enter¿
8. give it a name such as, cdcl3, dmso. The shim parameters are saved at your home directory. You can go there and take a look. ~home/vnmrsys/shims/
Next time, you can retrieve shim parameters by typing rts('cdcl3') su; Remember to type su so that the parameters are sent to the shim coil.
The shim parameters are retrieved and you should see the change of lock level at the log window.
9. How long does gradient shimming take?
Usually one cycle takes 20 seconds when you use the default nt of 4(nt= number of transients). If you think it is too long, you can change nt to 2.
Gradient shimming takes about 3 to 4 times and will stop when RMS (root mean square error) is less than 1. It will try up to 10 times and then stop.
See the picture.
