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The laser can serve two purposes, one to desorb the sample as vapor into the ionizing system, and two, to actually produce the sample ions. However, ions created by pulsed lasers produce bursts of ions and are obviously unsuitable for use with a scanning spectrometer. Intermittent ion production, however, is compatible with the time of flight (TOF) mass spectrometer, which can record all the ions produced by each laser pulse. Another advantage of this combination is the nearly unlimited mass range of this particular type of mass spectrometer. A diagram of the laser desorption/time of flight mass spectrometer combination is shown in figure 13.
The sample is placed on a probe that receives high–energy laser pulses from a carbon dioxide laser. An expanding argon stream propels the bursts of vaporized sample from a 100 μm orifice, through a skimmer, into the accelerating section of the mass spectrometer. In this section, the molecules are again subjected to a high–energy laser light beam, this time from a dye laser, which produces ions by photo–ionization. The ions that are formed are immediately accelerated through the region into a drift section of the mass spectrometer, where they are deflected by an ion reflector, to an electron multiplier.
Laser desorption can also be used with conventional electron impact and high–pressure chemical ionization sources, and also in conjunction with other ionization modes. A diagram of a sample probe that can be used with an electron impact ion source is shown in figure 14.
The laser light is focused by means of suitable lenses onto an optical fiber pipe, which passes though the probe and onto a glass window at the end of the probe. The sample is placed next to the glass window and the desorbed molecules pass along the hollow tube to be ionized by electron impact. This type of desorption causes very little thermal degradation, and has been used satisfactorily for measuring the spectra of labile materials, such as certain antibiotics.
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.