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The atomic emission spectrometer is versatile, highly sensitive and very selective. The procedure is to aspirate the sample, which will be a solution of the element (usually an aqueous solution) into the high temperature volume of the emission system and then monitor the light emitted with a suitable optical system that usually terminates in a Diode Array sensor. The nebulizer is normally a very simple device a diagram of which is shown in figure 2.
The sample, often in an aqueous
solution, is pumped by means of a peristaltic pump through a jet into
a concentric tubular cavity through which a suitable gas is flowing
at relative high velocity. The system tends to act as a miniature
‘spray drier’ the liquid breaking up into fine droplets
that evaporate and leave the solid sample as minute particles
entrained in the moving gas. The evaporation process, however, is not
completely efficient and water is carried forward with the particles
and must be removed. Another type of nebulizer is the ‘cross-flow
nebulizer’ a diagram of which is shown in figure 3.
The advantage claimed for this type of nebulizer is more efficient spray production due to the sample liquid being fed in, normal and close to the nebulizing gas stream. However, this is also not one hundred percent efficient and a device still has to be incorporated into the sampling system that removes excess, liquid sample. The device used for this purpose is also very simple and is called the spray ‘chamber’. A diagram of a spray chamber is shown in figure 4.
It is seen that the spray chamber consists of two concentric tubes sealed at either end. The outer tube has an exit at the top that allows the gas containing the particles to pass to the ‘torch’. At the bottom of the tube there is a ‘drain’ exit where the liquid caught on the walls of the inner and outer tubes collect and pass to waste. The gas carrying the sample particles then passes into the torch where the sample atoms are heated to a very high temperature A diagram of a ‘torch’ is shown in figure 5. The torch consists of three concentric tubes made of quartz or some other appropriate material. A copper coil, though which water circulates for cooling purposes, surrounds the top portion of the torch and is connected to a radio frequency source. The plasma gas, e.g. argon, is used as the nebulizing gas and a second flow of argon enters the torch at the base to act both as a coolant and as part of the plasma forming agent. In many torches the base of the torch is also cooled by a water jacket. A spark produced by a Tesla coil initiates the plasma formation by generating some electrons. The RF power (0.2 to 2.0 kW and 27 to 40 Mhz)) produces electric and magnetic fields that accelerates the electrons by inductive coupling (inductively coupled plasma ICP) The resulting high energy electrons cause further argon ionization by collision which continues as a chain reaction producing the plasma. The temperature of the plasma ranges from 6000 C to 10,000 C and appears as an intense brilliant white ‘tear-drop’ shaped fireball.
The distribution of the different zones in the plasma is depicted in figure 6. The lower part of the plasma is the induction zone where the effect of the RF radiation starts to become affective. Above this is the initial radiation zone where the plasma temperature may reach as high as 10,000 C. Above the initial zone is a larger plasma volume that is called the analytical zone and it is the light from this area that is used for analytical purposes. The analytical zone changes into a plasma tail before it eventually becomes extinguished. The nebulizer spray chamber, torch and other essential parts of the instrument are connected together if the manner shown in figure 7.
Figure 7 shows a general layout of ICP the atomic Emission Spectrometer system. Individual instruments may differ in detail but the layout given in figure 7 will be a reliable guideline for most atomic Emission Spectrometers
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.