Nuclear Magnetic Resonance Spectroscopy: Instrumentation

INSTRUMENTATION

The NMR-spectrum can be scanned either by changing the frequency of the radio-frequency oscillator or by changing the spacing of the energy levels while making a small change in the applied magnetic field.

The sample is introduced in a test-tube between the pole faces of a DC-electromagnet whose gap field can be varied from zero upto 14,092 gauss and even scaled upto 23,000 gauss in sophisticated versions of the instrument. The pole pieces are nearly 12 inches in diameter and are spaced approximately 1.75 inches apart. In order to flip the rotating nuclear axis with regard to the magnetic field an oscillating radio-frequency field, supplied by low power, crystal-controlled oscillator is strategically placed at right angles that would be perpendicular to the plane of the paper. The coil that transmits the radio-frequency field is made into two-halves to allow insertion of the sample holder, and the two halves are placed in the gap of the magnetic poles. Coils located within the pole gap allow a sweep to be made through the applied magnetic field that produces resonance in the range of precession frequencies.

A few turns of wire wound tightly around the sample tube forms a separate radio-frequency coil which picks up the resonant signals emitted from the sample. The receiver coil is perpendicular to both the stationary field and the radio-frequency transmitter coil so as to minimise pick-up from these fields. Thus, energy is absorbed from these receiver coils when nuclear transitions are induced. Absorption of energy causes the radio-frequency voltage across the receiver coil to drop. This voltage change is amplified and detected by a high-gain-radio frequency amplifier and a diode-detector which is tuned to the same frequency as the ratio frequency transmitter.

The resulting DC-voltage is placed on the vertical plates of an oscilloscope to produce an intensity as a function of frequency which is nothing but the desired NMR-spectrum.