Having a probe to reshim from scratch, I thought I'd have a go at
gradient shimming. I have tried this on a number of occasions, without
much success, but I did know there had been a revision of the GUI. Also,
Tim Horne from Bruker UK had been in and optimised some parameters for me
a little while ago - a procedure not well described in the 'FOCUS'
instruction leaflet.
Well, I'm afraid I couldn't make much of it, yet again.
The revised GUI is pretty much as bad as the old one, with a few new
things put in to make it worse. Almost all the text is now in a font
that's so small that it's illegible. The graph that's supposed to give
feedback about shim quality has both axes unlabelled, and the x-axis
scale changes arbitrarily so that you can't measure or compare anything.
The scale cannot be changed by the user.
The actual procedure for improving the shims is a complete mystery. All I
know is you have to improve the straightness of the displayed graph.
Allegedly you do this by choosing different groups of shims to adjust,
and altering their range of adjustment by typing a number in a box.
Unfortunately, I have not discovered what combination of shim groups and
numbers you have to use to generate improved resolution. The feedback
graph certainly changes, sometimes straighter, sometimes more curved.
Usually it's pretty much invisible because each iteration writes another
thin line in a pale pastel colour onto a pale cream background.
There are recognised good sequences for manual or simplex shimming when
you are maximising lock level or FID area, but what do you do with
gradient shimming? Nobody has been able to tell me. The results I've got
so far have been dreadful. I might just as well have spent the time
twiddling knobs by hand. After several iteration cycles, using gradually
more Z-shims, over various ranges, the result was a water line that had
visible large errors from all the shims I was adjusting. I quickly got a
better result from shimming by hand.
Clearly, I'm doing something wrong here. So, how _do_ you get good
resolution from gradient shimming? Does anyone know?
Before you ask, I have Z-gradients only, so I was adjusting Z-shims only,
using 1H; the probe was a 5mm 1H{13C}, and was being shimmed from
scratch; the X and Y shims had already been adjusted; the shim map
included all the available Z-shims (Z1 to Z6), and was remeasured at any
time there seemd to be an improvement to the shims; the sample was 50:50
H2O:D2O, 5cm deep, and I wasn't spinning it.
Any help on this would be useful,
Best wishes,
Alan
----------------------------------------------------
Dr A.S.F.Boyd Chemistry Department
NMR Spectroscopist Heriot-Watt University
phone: +44-131-451 3214 Riccarton
fax: +44-131-451 3180 Edinburgh
email: a.s.f.boyd@hw.ac.uk EH14 4AS
http://dava.che.hw.ac.uk/ Scotland
----------------------------------------------------