Focus
Jamin plate
Collimator Photodetectors
Objective
Figure 3 . The helium abundance interferometer
mosphere, the reference gas is expanded into its interferometer chambers by means of a membrane valve that keeps the differential pressure between the Jovian gas and the reference gas chambers very low. The interferometer produces a welldefined interference pattern of consecutive, equidistant bright and dark fringes at the detector array when the
chambers are filled with gas mixtures that have the same refractive index. However, any difference between the Jovian and reference gas refractive indexes causes a continuous shifting of the pattern with the increasing pressure caused by the probe penetrating the Jovian atmosphere. An array of nine photodetectors should allow the scientific team to measure the instan-
taneous position of the fringe pattern to within one-sixteenth of the fringe separation. Project Galileo involves the combined efforts of many research centers and aerospace companies, including the Ames Research Center, Hughes Aircraft, General Electric, and the German version of NASA, Deutsche Forschungs-und Versuchsanstalt Fur Luft-und Raumfahrt. Overall project management is being handled for NASA by the California Institute of Technology's Jet Propulsion Laboratory in Pasadena, Calif. Next month's FOCUS will describe the two analytical instruments to be carried on Galileo's orbiter, the near infrared mapping spectrometer and the ultraviolet spectrometer. M.D.W. Suggested reading Yeates, C. M; Johnson, T. V.; Colin, L.; Fanale, F. P.; Frank, L.; Hunten, D. M. Galileo: Exploration of Jupiter's System; National Aeronautics and Space Administration: Washington, D.C., 1985; NASA SP-479, available from the National Technical Information Service, Springfield, Va. 22161, for $16.95. Johnson, T. V.; Yeates, C. M. Sky & Telescope 1983, 66, 99-106.
Supercritical Fluid Chromatography at thePittsburghConference SFC commercialization moves ahead amidst some claims that its advantages are limited The supercritical state refers to a physicochemical phenomenon in which a pure substance is raised to a temperature above its critical temperature—the point beyond which it can no longer condense into a liquid or solid, no matter how much pressure is applied. For some time now, researchers have worked to apply this phenomenon to supercritical fluid chromatography (SFC), a technique with higher chromatographic efficiencies per unit time than liquid chromatography (LC) and the ability to handle higher molecular weight molecules than gas chromatography (GC). Advantages such as these have engendered enough interest in the SFC technique to encourage two companies, Lee Scientific and Suprex, to develop and introduce commercial supercritical fluid chromatographs.
At last month's Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy in Atlantic City, Lee Scientific introduced the 501 SFC ($34,900 plus computer), a syringe pump-based capillary SFC system with a flame ionization detector. The 501 SFC is compatible with the International Business Machines PC-AT and other computers for control of pressure programming and instrument operation. Suprex's SFC/200A ($34,000) is a microprocessor-controlled syringe pump-based system with a flame ionization detector. The Suprex instrument accommodates capillary and packed microbore columns. These two instruments do not represent the first commercial supercritical fluid chromatograph. Credit for that achievement goes to Hewlett -
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Packard, which introduced a prototype LC add-on device on a tentative basis at the 1983 Pittsburgh Conference but has since withdrawn it from the market. This device was fundamentally different from the two instruments introduced this year. It was a conventional packed-column system designed to be used with a carbon dioxide mobile phase, whereas the Lee Scientific and Suprex instruments accommodate capillary columns. The relatively high flow rate of conventional packed columns tends to restrict mobile phase selection to safe, nontoxic substances (such as carbon dioxide) that attain the supercritical state under very mild temperature and pressure conditions. The much lower flow rate of capillary columns minimizes any potential safety hazard from mobile phase leaks, permitting
