NMR spectroscopy is one of the most useful analytical techniques available to the chemist. It is also extremely expensive, making efficient use of instrument time a priority. One of the most irritating tasks in maintaining an NMR instrument is the need continually to readjust the correction ('shim') coil currents used to ensure a uniform magnetic field. This process of 'shimming' has until recently proved surprisingly resistant to automation, largely because the traditional experimental indicator of field quality, the amplitude of the steady-state deuterium lock signal, offers no information on the nature of the field homogeneity. The time spent on shimming varies greatly according to the purposes for which a spectrometer is being used, but a figure of an hour a day spent either in manual shimming or on more or less unsatisfactory automatic methods is not unusual. Considerably longer times are needed for the initial or 'cold' shimming of a new magnet or probe, where no approximate shim settings are available; cold shimming adds significantly to the installation costs of new instruments.
We set out to develop 'gradient shimming' methods, based on magnetic
resonance imaging techniques, which would allow a conventional
high resolution NMR spectrometer to map its own magnetic field,
allowing automatic shimming to be carried out rapidly and accurately
without the need for expensive extra hardware. To be of general
use in high resolution NMR, an automated shimming method should
fulfil a number of criteria. It should be (i) robust; (ii) accurate;
(iii) fast; (iv) applicable to all samples; (v) require little
or no extra hardware; and (vi) be usable with all types of probe.
Our first development was a deuterium spin echo method for 1D
(single axis) shimming. This method is rapid, accurate, highly
convergent, and gives results for spinning samples which are better
than those obtained manually by the most experienced and patient
operators. The shim mapping process (which need only be carried
out once) takes 5 - 10 minutes, and iterative correction rarely
requires more than 2 -3 minutes even for the most severe field
inhomogeneity. For the special case of 1D shimming, the method
meets all the six criteria outlined above. This was the first
gradient shimming method to be published that did not require
a fast pulsed field gradient facility, and hence could be used
with almost any spectrometer; it has very rapidly proved its worth
in laboratories around the world, and is now a standard feature
of most new commercial spectrometers.
The problem of full 3D shimming is considerably more difficult,
because of the need to map the field using the shim coils, which
take tens of milliseconds to settle. Our first approach was to
use a profile edge frequency method for field mapping, because
this makes only very modest demands on the spectrometer hardware.
This worked surprisingly well given its limitations, and represented
a considerable advance on manual shimming methods. It remains
the first and only published 3D gradient shimming technique applicable
to conventional instruments, but unlike the 1D method outlined
above it fails to meet criterion (iv) above, being restricted
to samples with a single strong, dominant proton resonance.
To circumvent the limitations of edge frequency mapping we turned
to a 3D shimming method based on the same principles of phase
mapping as 1D gradient shimming, using stimulated echoes to compensate
for the very slow switching speeds of the shim coils. This method
has been implemented on 400 and 500 MHz spectrometers, and satisfies
all six of our desiderata. It is more than competitive in accuracy
and speed with manual shimming, being capable of complete 3D mapping
and shimming of a 400 MHz spectrometer in one hour, and requires
only minor and inexpensive hardware changes for implementation
on any modern high resolution NMR spectrometer.
The 1D and 3D phase mapping techniques essentially solve the problem
of both routine and cold automated shimming on high resolution
spectrometers. The 1D technique has already been widely accepted
and implemented, and we anticipate equal enthusiasm for the 3D
technique which should be published shortly.
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Please send any comments or questions about these pages to:
g.a.morris@man.ac.uk