Instant Notes: Analytical Chemistry

spectrum mathematically in less than a second by a computer using a fast

Fourier transform(FFT) algorithm. Multiple interferograms can be rapidly

accumulated and averaged to increase sensitivity by as much as three orders of

magnitude.

A block diagram of a typical NMR spectrometer is shown in Figure 9 and

comprises five main components:

● a superconducting solenoid or electromagnet providing a powerful magnetic

field of up to about 17 Tesla;

● a highly stable RF generator and transmitter coil operating at up to about

750 MHz;

● a receiver coil with amplifying and recording circuitry to detect and record

sample resonances;

● a sample probe positioned between the poles of the magnet;

● a dedicated microcomputer for instrument control, data processing (FFT of

interferograms) and data storage.

The homogeneity and stability of the magnetic field should be at least 1 in 10^9

to ensure narrow absorption bands and good resolution. Sample tubes are made

to spin in the sample probe at about 50 Hz by an air turbine so as to increase the

apparent field homogeneity further. The direction of the magnetic field and the

orientations of the transmitter and receiver coils must be mutually perpendic-

ular to detect sample resonances and eliminate spurious signals in the detector

cicuit as shown in Figure 9.

E12 – Nuclear magnetic resonance spectrometry: principles and instrumentation 259

Transmitter

coil

Receiver coil

Sample tube

Magnet

Sweep coils

z, B 0

RF amplifier

AF amplifier Detector RF amplifier

Recorder

ADC

Switch

Pulse

generator

RF generator

x

y

AF AF RF

Computer RECEIVER

TRANSMITTER

Fig. 9. Diagrammatic representation of an NMR spectrometer. Reproduced from R. Kellner et al., Analytical
Chemistry, 1998, with permission from Wiley-VCH, and from Nuclear Magnetic Resonanceby P.J. Hore (1995) by
permission of Oxford University Press © P.J. Hore, 1995.