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Direct inlet interfaces allow a liquid to be injected straight into ion source via a suitable restriction without any prior concentration procedure. A diagram of a simple direct inlet interface is shown in figure 18. The restriction consists of an orifice about 2-5 μm in diameter through which the liquid sample or solution is forced. The liquid jet that is first formed rapidly breaks up into droplets. The droplets then pass into a heated chamber, where they are vaporized and the sample and solvent vapor then enter the ionization chamber. The tip of the jet can be suitably cooled to prevent premature evaporation of the sample solution drops. The spectrometer pumping system cannot usually cope with much more that a few micro-liters of liquid solvent as vapor, so the liquid sample is usually split (this is accomplished using a down-stream needle valve. This type of interface is usually employed with a Chemical Ionization source and the solvent vapor is used as the reagent gas.
The use of the solvent as the reagent gas can place restrictions on the choice of solvent that will control the nature of the ions that are produced in the Chemical Ionization source. This interface has the disadvantage that a very small orifice is necessary for rapid vaporization, which can easily become blocked. This type of interface has been most successfully used for solutes that have a reasonable vapor pressure at the source temperature
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.