TECHNOLOGY
Superconducting Magnet Has Uniform Field Large, uniform field could lead to nuclear magnetic resonance as biggest potential application for magnet A large, uniform magnetic field has been demonstrated in a superconducting magnet that reaches field strengths roughly twice those of iron-core magnets now commonly used. The field within a 2-cm. sphere inside a superconducting solenoid at 47 kilogauss is homogeneous to better than one part in 10 6 , according to tests made by the magnet's developer, Varian Associates, of Palo Alto, Calif. The magnet, Varian feels, should find its greatest potential application in nuclear magnetic resonance. Varian is reluctant to talk of equipment applications of the magnet now. It is so new the company hasn't had time to assess the problems of incorporating it into an instrument that could be run routinely by a research laboratory. Varian does believe, however, that equipment made using the new magnet would find ready application to problems ranging from use of carbon-13 in structural analyses to characterization of minute amounts of biologically important substances. Operating at 47 kilogauss, the new magnet yields a flux density at the sample some two times greater than present iron-core magnets in commercial instruments. This flux density has been reached before by superconducting magnets, but the 2-cm. spherical uniform field has not, Varian claims. It is the combination of the two properties—high field and uniformity—that gives the magnet its potential for NMR. Increases in the flux density in magnets used in NMR have been achieved with regularity since the technique began to find wide application in 1958 with fields of 14 kilogauss. Now, with iron magnets, commercial equipment is available at 23.5 kilogauss. NMR. Magnetic field strength is important in getting a detectable signal in NMR studies. NMR works because each isotope with nonzero nuclear spin has a characteristic gyromagnetic ratio. This is the ratio of the magnetic moment of a nucleus to
Increasing Field Leads to Greater Resolution 19.3 megacycles/14.1 kilogauss
32.1 megacycles/23.5 kilogauss
Magnetic field
60.0 megacycles/43.9 kilogauss
As the field intensity, measured in kilogauss increases, the NMR spectrum of decaborane has more pronounced peaks
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its spin angular momentum. Place the nucleus in a strong magnetic field and it will precess with an angular frequency that is the product of the strength of the strong magnetic field and the gyromagnetic ratio of the nucleus. This is called the Larmor precession frequency. To get information out of a molecule in a magnetic field, a small radio-frequency field is induced around a sample. When this r.f. field equals the Larmor frequency for a particular nucleus, the nucleus will resonate, absorbing energy from the r.f. field. Detecting this absorption or responding signal along with those of other nuclei is a way of fingerprinting the molecule. Nuclei with higher magnetic moments yield stronger signals at their characteristic Larmor frequency. Tritium, the proton, and fluorine are the top three in magnetic moment. The NMR signal at a fixed frequency shifts slightly with changes in field strength depending on the chemical environment of the nucleus. Thus an important use of NMR is answering questions about molecular structure. To be useful in NMR generally, a given nucleus should be present in many compounds. A high magnetic moment is desirable so that the signal can be detected above thermal noise of the radio-frequency pickup coil. The nucleus should have little or no electric quadrupole moment because this tends to blur the sharpness of the NMR signal. Local magnetic shielding caused by interactions of the surrounding electrons with the applied magnetic field adds to or subtracts from the value of external magnetic field at which nuclei resonate. The higher the magnetic field, the greater is this effect, which is known as the chemical shift. Ethyl alcohol yields a well-known example of the chemical shift. It contains six protons in three chemical environments: the CH 3 group, the CH 2 group, and the OH group. The proton in the OH group resonates at the lowest applied field. The CH 2 group protons resonate next, because of the influence of the oxygen. The CH 3 protons are affected less strongly by the oxygen because of the additional intervening chemical bond. The field spacing or chemical shift between the three resonances increases linearly with increasing magnetic field. Splitting of the peaks into subpeaks 56
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is due to coupling between the spins of the protons in the structural groups. This is independent of the field strength, but can be observed only in a homogeneous, stable field. Sensitivity is greater with increasing Larmor frequency. Thus whether or not an isotope yields a detectable signal depends on both its gyromagnetic ratio and the strength of the magnetic field. Thus with increasing field strength and uniformity come three gains: an increased signal strength, bigger chemical shift, and greater resolution of the fine structure or subpeaks. Solenoids. Ten years ago, Varian points out, it couldn't have built its superconducting solenoid. The idea behind setting up a uniform magnetic field in a solenoid dates back to Helmholz and Maxwell. Basically a solenoid is a cylinder, with a hole in the center, wound with a conductor. Passing electricity through the coil sets up a field in the bore. With very careful attention to the way the solenoid is wound, it is possible to set up a uniform magnetic field in the bore. Determining how to wind the superconducting solenoid is very complex. Here Varian adopted a mathematical procedure published by Dr. M. W. Garrett at Swarthmore in 1951. Dr. Garrett devised a unified mathematical framework that expresses a magnetic field due to a given current distribution. This involved several hundred numerical calculations and had to be solved by computer to design the solenoid winding. A second problem was choice of materials. Up to about 23 kilogauss, iron-core magnets work well. Above this, iron pole pieces saturate, and increasing the magnetic field is a matter of just pouring in more power, which is expensive. The solution was to use materials known as Type II defect-saturated superconductors. These are alloys of niobium with either tin, titanium, or zirconium. These alloys have been investigated extensively by Bell Laboratories, General Electric, and Atomics International in recent years. At liquid helium temperature they all have high tolerances for magnetic fields. For practical reasons (ductility, availability, cost) Varian has used an Nb-Zr alloy in its solenoid. A superconducting solenoid can be first energized from an external source of current. Once the desired field is obtained, a superconducting shunt is
introduced across the solenoid terminals. The current induced in the solenoid from the external source then circulates undiminished. This persistent current gives rise to an extremely stable magnetic field in the solenoid. After adjustment, no further electrical power is required to maintain the field. The solenoid is kept cold inside a cylindrical Dewar. The bore of the Dewar makes it possible to introduce samples into the uniform magnet field at room temperature. The inner access bore is separated from the liquid helium (4° K.) by a vacuum which contains an intermediate copper liner kept at liquid nitrogen temperature (77° K.). Problems. Since the magnet is so new, Varian hasn't had a chance to test it with some of the persistent problems of NMR. It has, however, used the standard NMR trace for ethyl alcohol to determine field uniformity. One possibility with the stronger field is use in NMR of nuclei with small gyromagnetic moments. Carbon-13 is one isotope that fits most requirements. It is present in many chemicals, has a nuclear magnetic moment and no quadrupole moment, and shows a large chemical shift. But its resonance signals are weak, and its natural abundance is low—about 1.1%. Techniques are available now for observing C 13 . These either do not show finer features of the NMR spectra or take extremely long times to average out random noise. Doubling the field for this isotope would increase the availability of direct skeletal information for many organic compounds, since the attainable sensitivity increases roughly at the rate of increase of the magnetic field. Proton studies of natural products where quantities of material are sometimes low and spectra are often complex would benefit from the increased field, as would studies of metabolites containing phosphorus-31. Doubling the field in some cases could increase the chemical shift enough to resolve puzzling overlapping multiplets that occur in many boron-11 compounds. Similarly, the higher field and consequent increased sensitivity should permit work with nitrogen-14, lithium-7, and others. Right now, Varian intends to tackle several such specific problems in its research and applications laboratories.
