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Matrix assisted laser desorption/ionization can provide charged ions of substances having extremely high molecular weights. The Nd–Yg–Laser, having a wavelength of 286 nm, and a pulse width of about 10 ns is normally employed for this purpose. The sample is dispersed in an involatile liquid to prevent decomposition and to allow the surface to be continually renewed. A number of different liquids have been explored, including glycerol and nicotinic acid. nicotinic acid is particularly appropriate as it absorbs very strongly at 286 nm, the wavelength of the laser light. A diagram showing the basic layout of the matrix assisted laser desorption/ionization source and mass spectrometer is shown in figure 15.
The laser light is arranged to strike the surface at a 45˚ angle, and the emitted ions are collimated, by a three element Einzel lens and then passed through an ion deflector to the time of flight mass spectrometer. The ions are accelerated by an appropriate voltage, and then allowed to drift to an ion reflector and then back to the electron multiplier. This system was employed by Karas and Hillenkamp  to obtain the mass Spectrum of a number of proteins including bovine albumin. An example of a Spectrum they obtained for bovine albumin is shown in figure 16.
The Spectrum shows the single, double and triple charged parent ions, the molecular weight of the parent ion being about 66750 daltons. The standard deviation of the parent ion peak is about 300 daltons. The technique can provide mass spectra of very large molecules, apparently with little or no decomposition.
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.