Figure 9.27
Schematic diagram of a continuous wave NMR spectrometer.frequency of the transmitter. By employing a superconducting magnet, field strengths approaching 10
tesla (~ 10^5 Gauss) and operating frequencies up to 500 MHz can be utilized. The magnetic field and
the axes of the transmitter and receiver coils are all mutually perpendicular. This arrangement ensures
that radiation from the transmitter can be detected in the receiver coil only when the sample absorbs
energy. Some instruments use a single coil, the absorption of energy being detected by an out-of-
balance signal in a bridge circuit which incorporates the coil and a capacitor. The spectrum may be
recorded on a chart or viewed on a VDU screen by repetitive scanning.
The effect of long-term field instability or drift is counteracted by means of an internal or external field-
frequency locking system. This consists of a secondary RF circuit which generates a signal at the
resonance frequency of a selected reference nucleus (usually^1 H in tetramethylsilane (TMS) or^2 D in a
deuterated compound). The reference nucleus is continuously irradiated and the corresponding
absorbance signal monitored. If this diminishes due to field drift, a voltage is generated in a servo-loop
circuit which in turn adjusts the current flowing in coils of the electromagnet in such a way as to re-
establish the original field. An external locking system employs a second probe, located close to the
sample probe, and which houses the reference compound tube. With an internal locking system, the
reference compound is added directly to the sample or sealed in a thin tube which is put into the sample
tube. Adequate sensitivity can usually be obtained only from solutions of 0.1 M or more, although by
computer processing of the data accumulated by multiple scans spectra from 0.01 M solutions or less
can be recorded.
A considerable improvement in speed and sensitivity can be achieved with a pulsed Fourier transform
(FT) spectrometer. Here the sample is subjected to a series of short duration high intensity RF pulses
(1– 100 μs)