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The Inductively Coupled Plasma Ionization source is very similar to the ICP torch previously described. The sample is first nebulized (possibly in a solvent) and the atomized sample passes through a spray chamber and then enters the ICP torch. The gas and ions leaving the jet first enter an expansion chamber through a cooled sampling cone with an orifice, about 1 mm in diameter. In this chamber the gas expands and is pumped away. A portion of the jet from the torch then passes through a second orifice called the skimmer cone.
The ICP produces singly charged positive ions for most elements, which makes it a very efficient source for coupling to a mass spectrometer. A series of electrostatic lenses focus the ions into the quadrupole mass spectrometer analyzer. After resolution the ions are detected in the normal way with an electron multiplier tube. A diagram of the ICP interface and the Quadrupole Mass Spectrometer is shown in figure 12.
The ICP ionizer is not a very efficient excitation/ionization source for nonmetals (e.g. halogens) and for elements such as arsenic and selenium . helium plasma, however, has an ionization potential of 24.5 eV (compared with that of argon, 5.75 eV) and is consequently a more efficient excitation/ionization source. Thus, the microwave induced helium plasma is likely to be more efficient. The microwave induced helium plasma apparatus is very similar to that of the ICP torch as depicted in figure 12, except that the plasma is induced by a resonant microwave cavity surrounding the body of the torch, and not by a cooled coil. With the ICP, the isotopes of argon, oxygen , nitrogen and hydrogen can combine with themselves, or with other elements, to produce isobaric interferences. The use of helium , which is essentially mono-isotopic, significantly reduces the number of interferences compared with the argon plasmas.
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.