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Electron impact ionization is generally a fairly harsh method of ionization but the energy used to produce the ions is controllable, which is practically an advantage. The procedure usually produces a range of molecular fragments that in most cases, helps to elucidate the structure of the molecule. However, although molecular ions are often produced, which is important for structure elucidation, sometimes only small fragments of the molecule are observed, with no molecular ion. Under such circumstances, alternative ionizing procedures may need to be used. A diagram of a simple electron impact ionization source is shown in figure 2.
Electrons are formed by thermal emission from a heated tungsten or rhenium filament and accelerated by an appropriate potential to the anode trap. The magnitude of the accelerating potential may range from 5 to 100 V depending on the electrode geometry and the ionization potential of the substances to be ionized. The filament current can be automatically controlled to provide a constant trap anode current and, thus, maintain steady ionizing conditions. The sample is introduced into the gas stream at the center of the electron beam. The ions formed are repelled by a suitable potential, through a hole in the wall of the ion source enclosure and, thus, pass into the accelerating field of the mass spectrometer. A more detailed and practical layout of an electron impact source is shown in figure 3. The central stainless steel block is made from stainless steel and the sample is introduced through a small hole in the center. Molecules move into the ionization chamber where they meet a stream of energetic electrons and ions are produced. These ions are repelled into the focus area by suitable potentials being applied to repeller plates 1 and 2. The positive ions are accelerated by the repeller plates and the negatively charged accelerator electrode and pass into the analyzer unit. A magnetic field of a few hundred gauss is often maintained along the axis of the electron beam, to confine the electrons to a narrow helical path. In general only about 0.1% of the molecules entering the ion source are ionized.
The optimum ionization energy of the electron varies with different compounds, but an average value appears to lie between 50 and 100 eV. The approximate relationship between ion current and electron energy takes the form shown in figure 4. The ionizing energy of the electrons is controlled by the accelerating potential applied to the anode in the ion source. This facility is important, as it allows the energy of the electrons to be adjusted so that optimum fragmentation will take place.
Optimum fragmentation will provide the maximum information to allow the structure of the compound to be elucidated. An example of the effect of electron energy on the fragmentation pattern of a compound is shown in figure 5. It is seen from the pattern obtained at low energies (ca 14 electron volts) that a large parent ion is produced but relatively few fragments. This means that the molecular weight of the material could be fairly easily identified but its structure would not be easily identifiable as there were very few fragments to work with.
In contrast, at high electron energies (ca 60 electron volts) there are a large number of fragments, particularly at low molecular weight, but the parent ion is hardly discernible. This means that although some of the secondary structure of the molecule may be revealed, the lack of a definite parent ion would again make the total molecular structure difficult to identify. However, at mid–electron energies (ca 20 electron volts) numerous fragments together with an unambiguous parent ion are produced, providing ample information for structural identification.
It is seen that the electron energy in electron impact ionization is an important parameter on which to optimize, to ensure that the best possible data is generated for structure elucidation. If a more gentle form of ionization is required, however, Chemical Ionization should be used.
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.