Sensitivity

Abstract

Abstract It is an unfortunate fact that magnetic resonance is not a sensitive technique, that is to say, we need a relatively large amount of material to produce a detectable signal. We have already had a clue to the reason (§ 2.4 ); most of the nuclear spins in a sample are equally divided between the upper and lower energy levels, with only a tiny excess in the ground state. Because upward and downward transitions are equally probable, one upward jump (absorption of energy) is cancelled by one downward jump (emission), generating no signal in the receiver. Only the small excess population in the lower level (of the order of only 5 nuclei in 100,000) is actually utilized, because this permits net absorption of radiofrequency energy, not matched by equal emission from the upper level. We often loosely refer to this effect as the ‘Boltzmann factor’ since it is the Boltzmann distribution function that determines the spin populations at equilibrium. Almost all of the tiny nuclear magnets aligned along the magnetic field direction are matched by an equal number that are opposed to the field.