The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
SCHOOL of PHARMACY NMR FACILITY
Gradient Mapping/Shimming
In the VNMR input window type: gmapsys
The menu buttons will change in response to this. With the left mouse button click on the one labeled: Set Params
The menu buttons will again change; click on the one labeled: Gradient,Nucleus
The buttons will change again. Depending upon whether you will make a map using 1H (which is preferred, since it has a much larger signal) or 2H, click on either: Pfg H1 or Pfg H2.
In the VNMR Status Window the following message will appear: Parameters set for pfg on H1 (or H2, depending upon which you selected above), check gradtype
When you type: gradtype? in the VNMR status window, it should respond with: gradtype='ttt' which corresponds to our hardware.
You might also make sure that the gradients are turned on by typing: pfgon? (normally it should respond with pfgon='yyy', indicating that the x,y, and z gradients are on.)
To check how many gradients will be used for mapping/shimming, type: gzsize? The default for our Inova 500 is 6; if the sample is short you may want to set gzsize=5
If you selected 2H the default settings and nt=4 can be used for mapping. If you are using 1H you should set the gain=0 and tpwr=52 (or less) so the signal is not so great that the images are distorted; nt=1 is sufficient.
Click on the button labeled: Return
The buttons will change; click on the button labeled: Go, dssh
This will result in a pair of images being produced and displayed. Type: ds(1) and then type: f full to display the first of the pair. (You may also need to type: vp=0 and then vsadj).
Click on the button labeled: Th
This will display a horizontal yellow line; with the left mouse button, position this at the top of the displayed image, and then in the VNMR input window type: th=th*0.2 The line will then be positioned at the level corresponding to 20% of the height.
With the left mouse button, position one red vertical line at the left intersection of the image and the threshold you've just set, and with the right mouse button position the other red vertical line at the right intersection. (Be careful not to move the left line while positioning the right.)
In the VNMR input window type: gmapsys again. The buttons will change, and again click on the one labeled: Set Params
The buttons will change; click on the one labeled: Calculate gzwin
In the VNMR Status Window a message will appear, similar to: Cursors used to set gzxwin to 29.3, tof to -769.6
Click on the button labeled: Return
The buttons will change; click on the one labeled: Shim maps
The buttons will change; click on the one labeled: Make Shimmap
In the VNMR Status Window a message similar to this will appear: Eneter mapname:[last map name]
Type in the name you wish to call this shimmap. When you hit "Enter" on the keyboard, a shimmap will be made. (When done you will see pairs of images, the number equal to gzsize + 2.)
Click on the button labeled: Return
The buttons will change; to see the shimmap, click on the menu button labeled: Display
The buttons will change; click on the one labeled: Display shimmap which will then be displayed (a set of curves equal to gzsize; the power of the polynomial corresponds to the Z shim; i.e., Z1 is a straight line, whereas Z2 is a parabola, and so forth.)
Click on the button labeled: Return
The buttons will change. Then click on the button labeled: Autoshim on Z
Gradient shimming will commence, using the map that you just made. Iterations will be performed using only gradients Z1, Z2, Z3, and Z4 until the rms error is less than one; it will then begin again using shims Z1-Z6 (or Z1-Z5, if you set gzsize=5). This is done so that Z5 and Z6, which are "weaker", will not be "swamped" by Z1-Z4. In earlier versions of the software, one had to first type in the VNMR Input Window: gmap_z1z4='y' before starting Autoshimming.
When finished, clik on the button labeled: Quit
This will return you to the original experiment display from which you began.