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A diagram of a Flame Atomic Absorption Spectrometer is shown in figure 11. The light source is a cold cathode lamp that produces (almost exclusively) the light that would be naturally emitted by the element to be measured at a high temperature. A large range of such lamps are available that includes the vast majority of the elements of general analytical interest. Consequently, the light will contain specifically those wavelengths that the element in the flame will selectively absorb. The light passes through the flame, which is usually rectangular in shape so as to provide an adequate path length of flame for the light to be absorbed, and then into the optical system of the spectrometer.
The flame is fed with a combustible gas, customarily air/acetylene, nitrous oxide/acetylene or air/propane or butane. The sample, dissolved in a suitable solvent, is nebulized and fed into the gas stream at the base of the burner. The light, having passed through the flame, can be focused directly onto a photo-cell or onto a Diffraction Grating by means of a spherical mirror. The Diffraction Grating can be made movable, and so it can be set to monitor a particular wavelength that is characteristic of the element being measured, or it can be scanned to produce a complete absorption Spectrum of the sample. After leaving the grating, light of a selected wavelength, or range of wavelengths, is focused onto the photocell. The position of the Diffraction Grating determines the wavelength of the light that is to be monitored.
The flame absorption spectrometer is fairly sensitive, and can be readily used as a tandem instrument combined with a chromatograph, providing that an appropriate interface is employed. A photograph of a Perkin Elmer Flame Atomic Absorption Spectrometer is shown in figure 12.
This instrument is normally fitted with 4 different cold cathode lamps and, thus, can examine 4 different elements, automatically, from the same analysis; 8 lamp units are also available. A diagram of a hollow cathode lamp is shown in figure 13.
The cylindrical hollow cathode of the lamp contains one or more of the elements of interest in the analysis. The cylindrical cathode is screened from the anode connections by means of a ceramic cylinder The anode is situated above the cathode and is made of tungsten or nickel and the whole electrode system is enclosed in a glass envelope. The glass envelope is filled with neon or argon to a pressure of 1kPa and on applying a potential of 100 to 200 volts across the electrodes a glow discharge is formed.
A simple explanation of the process is as follows, Accelerated electrons from the cathode collide with the gas atoms and produce ions. The ions are accelerated to the cathode by the electric field and when they strike the cathode surface the elements of interest are ejected from the surface and are excited to provide radiation in the discharge environment. These lamps can be combined in groups so that a given instrument can determine a number of different elements by merely switching the lamps.
A photograph of four unit lamp mount is shown in figure 14. A single lamp can be made to generate characteristic radiation for up to two or three elements without interference problems. Modern atomic adsorption spectrometers are deigned to provide extremely simple operation, easy maintenance and, when required, fast and inexpensive servicing. Many ancillary and alternative devices are available to suit specific applications.
A number of different sampling devices are available including very sophisticated automatic samplers that allow instruments to be operated 24 hours a day. An example of an automatic sampler that can be used for atomic absorption spectrometers is shown in figure 15,
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.