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Due to the limited techniques available in the early days of NMR, conventional iron cored electromagnets or ceramic permanent magnets had to be used to produce very strong magnetic fields and so field strengths were limited to about 14 kilogauss. As a result, the early NMR spectrometers could only operate at about 60 MHz. Today, very strong magnetic fields can be obtained by employing Superconducting Magnets. It is seen from Table 1, that for proton spectroscopy, magnetic fields of about 60 and 180 kilogauss respectively will allow the use of frequencies of 259 or even 750 Mhz and so, modern high-resolution NMR spectrometers operate at these frequencies. Although hardly used today, in order to illustrate the relative simplicity of the NMR instrument, the original low resolution 60 MHz permanent magnet NMR spectrometer will be briefly described.
The Perkin Elmer Corporation pioneered the production of the permanent magnet, 60MHz, NMR machine in the early 1960s and a simple diagrammatic form of the instrument is shown in figure 5. This instrument includes the basic parts of all NMR spectrometers and although modern instruments have more sophisticated adjustments and electronics and employ superconducting magnets they all have the basic parts of the original 60 MHz instrument of Perkin Elmer. The first requirement is a magnet having the dimensions and strength required for the radiation frequency to be used. In the above instrument this consisted of a ceramic permanent magnet having a field-strength of about 1400 gauss. The second requirement is a coil to provide a small magnetic field that is adjusted to scan the range appropriate for the protons to be examined. This coil can vary in complexity and in the original instrument includes a series of coils (called the Golay coils (after Marcel Golay the inventor of the capillary column in gas chromatography)) that could also be used adjust the field to a maximum degree of homogeneity. The thirds requirement is an RF coil that both supplies the radiation to the sample and also senses the energy absorption at the positions of resonance. Finally there is a tube to contain the sample that is attached to an air turbine to rotate the sample at high speeds. The rotation of the sample is essential to ensure that the sample to be exposed to a magnetic field that is as homogeneous as possible. As the sample rotates, the net field experienced is the average of that swept out during a single rotation. The effect of spinning the sample on resolution is shown in figure 6.
It is seen that the effect of spinning the sample is quite dramatic and is essential as it would be impossible to obtain high-resolution spectra without using the sample spinning technique. Field homogeneity of better than one part in 109 is necessary to obtain a resolution of 1 Hz in a field of 176 500Mhz.
About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.