Despite 50 years of improvements in experimental techniques and
spectrometer hardware, the vital step of obtaining a homogeneous
static magnetic field in which to carry out experiments has remained
stubbornly resistant to automation until very recently. The skill
of shimming, manually adjusting the currents through a set of
10 to 40 small correction coils positioned around the sample in
order to flatten out errors in the magnetic field, is one which
all high resolution NMR spectroscopists have to learn. Thousands
of hours of instrument time are wasted in shimming each year,
despite the availability of effective automated techniques for
the admittedly simpler problem of shimming magnetic resonance
imaging systems. The difficulty lies in translating methods which
use the strong, fast pulsed field gradient systems and strong
proton signals found in MR imaging to the much more restricted
hardware used in high resolution spectrometers, and in devising
methods which can shim to much higher quality without the need
for very strong signals.
We have shown recently that the problem of shimming can be solved efficiently and generally for the restricted case of z shimming, on almost any modern high resolution spectrometer. The technique relies on measuring the phase change as a function of position in a sample when an extra delay is added to a deuterium spin echo experiment. The only field gradient required is that provided by the normal Z1 shim coil; 'homospoil' facilities for pulsing this coil are standard on most spectrometers. This method is faster, more accurate and more robust than manual shimming, typically taking less than a minute for routine shimming.
The problem of shimming in three dimensions is more difficult;
the sample spinning used in most routine experiments sweeps most
field errors under the carpet, but shimming transverse as well
as z gradients is crucial both in the initial setting-up of a
spectrometer and in experiments which require a non-spinning sample.
The first method which has been shown to work for high resolution
NMR without the need for fast 3D pulsed field gradients is edge
frequency profile shimming. This is uncomfortably slow for routine
use but is already cost-effective for initial shimming of new
magnets and probes; our more recent techniques are both faster
and more accurate, and have been adopted by a major manufacturer. We have also produced a simple solution to
the problem of automated shimming in the presence of bulk sample
motion such as that caused by convection.
EPSRC final report summary on grant GR/L17443
Morris research group home page
Please send any comments or questions about these pages to:
g.a.morris@manchester.ac.uk
Most recent revision 15th April 2007