FEBRUARY. 1955
NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY
INDUSTRIALhboratories are already applyiug one of the mare subtle techniques of modern nuclear physics. Nuclear magnetic resonance spectroscopy, disoovered only eight years ago, is used to detect and distinguish between the nuclear particles present in s. samole. In this i t differs from conventional snectroseoov. the two analytical techniques, n-m-r spectroscopy can draw the finer distinctions. Normally, the atomic nucleus is almost impervious to any o u b side influence, because i t is shielded by its surrounding electrons. Some nuclei, however, actually resemble small magnets and therefore can be acted upon by an external magnetic field. I n addition, most nuclesr species spin like a top. But, if a spinning magnet is placed in e. strong magnetic field, i t tends to flip over, and resists this tendency in exactly the same way that a spinning top resists the effect of gravity-by precessing. This is what happens when a top slaws down; i t tilts and in addition to spinning about its axis, i t rotates slowly around the verticrtl. The stronger the disturbing force in either case, the more rapid this precession. The precession rate of magnetized, spinning nucleus depends on the strength of the nuclear magnet, on the amount of its spin, and also on the strength of the external magnetic field. For example, if a sample of hydrogen is placed in a magnetic field of 10,WO gauss-readily attainable in the labaratory-the nuclei (protons) will precess 42 million times per second. This rapid
a t ~ a h a r d warkihg , independently, found that by applying a second magnetic field, alternating a t precisely the frequency of precession, the effect could be observed. One way to explain i t is to say that. the stationary magnetic field gets the spinning tops all lined up and precessing a t the same rate. Then the alternating field causes them all to wohble in rhythm, or resonance as the physicist would have it. But it takes energy to do this, and Bloch and Purcell noted this absorption of energy from the alternating field whenever it was exactly in resonance with the precession. For any given strength of the stationary magnetic field, each type of nucleue has its own characteristic precession frequency. Thus, i t should be possible to identify the elements in a sample by first applying a steady field, then scanning over the suitable frequency range with the alternating'field, and noting when energy is absorbed.
EQUIPMENT AVAIWLBLE
Instuments are now on the market for rapid, non-destructive n-m-r analysis of small liquid samples, including melted solids, solids in solution, and liquefied gases. The sample in a test tube is surrounded by a coil of wire connected to a source of radio-frequency current, and hoth are placed between the pole pieces of a strong magnet. When nuclear resonance occurs, energy is absorbed, and an external electric circuit detects this absorption-as a trace on a cat.hode-ray tube, for example. These instruments are used to identify components of a complex mixture and to determine the structure of a molecule. Since the n-m-r effect depends on the strength of the nuclear magnet, and since there are differences between nuclei, n-m-r spectroscopy can also differentiate between isotopes, or different forms of the same element. Protans-hydrogen nuclei-happen to produce one of the strongest effects observed; n-m-r was first discovered with protans in water or paraffin. The fluorine nucleus is also easy to detect by nuclear resonance; n-m-r spectroscopy is often a sensitive way of detecting protons or fluorine nuclei in chemical oompaunds. Some nuclei, however, do not act like magnets and cannot be detected by n-m-r; such is the case with the most abundant isotope of oxygen, but most nuclear species can be analyzed. The sample to be tested may be gaseous, liquid, or solid, though liquids are preferred. I n hoth gases and liquids, random motion of the particles tends to average out any influence that may be exerted on a nucleus by its neighbors, but in gases the nuclei are relatively so few in number that the total effect is weak. In solids, where the positions of the neighbors are rigidly fixed, the influence of these neighbors shows up. As it depends in a predictable way on the arrangement of the neighbors, one can learn something ahout the local crystal structure by studying such disturbances of the normal effect; n-m-r spectroscopy of solids may thus be a help in crystal analysis. Hydrogen atoms, for example, are difficult to locate by other means, such as x-ray diffraction. When the sensitivity of the u-m-r spectrograph is pushed as far as possible, one can observe minute differences caused by the chemical environment of the atom. Thus, for instance, it makes a perceptible difference whether the proton is surrounded only by other protons, or whether it is a part of a distinct chemical group. It is then possible to determine the position of the orotons within s. comnlex molecule and arrive a t a better undersearoh tool for struoturd and organic chemistry.-Reprinted f ~ o mthe Industrial Bulletin sf -4rthur D. Little, Ine.
