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Plasma desorption is achieved by the use of a radioactive source, the fission particles being used to ionize the sample. Employing 252Cf as the source, this ionization technique can be used with the time of flight (TOF) mass spectrometer in a rather clever manner. 252Cf has a half-life of 2.7 years and decays giving an alpha particle and two charged fission fragments simultaneously emitted in opposite directions. Typically a pair of fission fragments might be 106Te and 142Ba with energies of 104 and 79 MeV respectively. A diagram of the plasma desorption apparatus is shown in figure 11.
The sample is placed on a thin aluminum sheet (or a sheet of aluminized polyester film) that is connected to a high positive potential (assuming the sample ions will be positively charged). When fission occurs, one particle strikes the sample and produces ions, while the other, emitted in the opposite direction, is sensed by the trigger detector and starts the time of flight measurement.
The ionized sample molecule or fragment is accelerated to its characteristic velocity, passes through the drift region of the spectrometer and is finally sensed by the stop sensor, which arrests the time measurement. This procedure will be better understood when the TOF mass spectrometer is discussed in detail,
The yield of sample ions is very low and so a large number of spectra need to be collected, which may take several minutes.
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.