Page 107 - Tandem Techniques

Page 88

Today, however, modern high-resolution NMR spectrometers operate at a frequencies of 250 or even
750 MHz and, as seen from Table 2.1, for proton spectroscopy, this requires magnetic fields of about 60
and 180 kilogauss respectively. Such fields cannot be obtained from conventional electromagnets and
consequently, superconducting magnets must be used.

Superconducting magnets require a continuous supply of current and, unfortunately, consume large
quantities of cryoscopic fluids. The magnet consists of a main field superconducting coil with a number
of other smaller coils that can control field gradients in different directions with respect to the main
field. These smaller coils are called shim coils and are used to improve the homogeneity of the field.

A modern NMR machine fitted with a superconducting magnet is depicted in Figure 2.27. The
superconducting coils must remain submerged in liquid helium during use, with the current in each,
established during installation. Outside the liquid helium bath is a liquid nitrogen bath, which reduces
the heat transfer to the helium bath, and thus conserves helium. There are liquid level sensors that
actuate warning devices in both the helium and nitrogen baths to ensure they do not become exhausted.
There is also a number of shim coils associated with the probe inside the magnet that operate at room
temperature. These shim coils provide the final adjustments to field homogeneity which, in modern
instruments, are usually under computer control. An air supply, provided through appropriate conduits
to the probe, actuates the turbine that spins the sample and provide energy for any automatic sample
handling devices.

A diagram of an NMR probe is shown in Figure 2.28. Inside the probe is a Dewar vessel which holds
the sample tube, the various sensor coils and the conduits to the system. The Dewar is also fitted with a
heater to control the probe and sample at a prescribed temperature. The Dewar contains two coils; the rf
lock-coil that is usually tuned to deuterium as the reference nucleus which, in effect, provides the
calibrating scale for the spectrum, and the rf coil for the nucleus under examination. The total rf circuit
is not included in the diagram to avoid confusion. There are two