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ANALYTICAL CURRENTS
Using a light-scattering detector for SFC In packed-column SFC, modifiers and additives are often necessary to efficiently elute polar compoundsfromthe column. Unfortunately, these modifiers can interfere with the use offlameionization and UV detectors. Alternatively, Larry T. Taylor and J. Thompson B. Strode, III, of the Virginia Polytechnic Institute and State University modified an evaporative lightscattering detector designed for LC and interfaced it with a supercritical fluid chromatograph. They evaluated the performance of the detector by monitoring the response of progesterone, testosterone, and 17-ahydroxyprogesterone with varying nitrogen makeup gas flow rate, C0 2 modifier type, modifier concentration, detector
Friction yields nanolatch After three centuries of studying friction, adhesion, and wear at the macroscopic level, researchers can now explore tribological properties at the nanometer scale by using atomic force microscopy (AFM). Theoretically, results should be interpretable in terms of fundamental principles.
Computer model of the atomic structure of the Mo03/MoS2 interface. (Adapted with permission from the American Association for the Advancement of Science.)
detector. A total gas flow rate of 1000 mL/ min gave acceptable peak widths and signal response. Detection limits of < 10 ng were obtained for all three steroids using 2%, 10%, and 20% (v/v) methanol-modified C0 2 on a Deltabond cyano column. The authors were also able to separate Detection of ginkgolides in a Ginkgo biloba leaf using UV and evaporative light-scattering detection. (Adapted with permission and detect the five ginkgolides in an exfrom Preston Publications.) tract from a Ginkgo biloba leaf and suggest that SFC with evaporative lightorifice size, and detector temperature. scattering detection is a promising techThe researchers found that increased nique for assaying food stuffs, polymers, nitrogen gasflowresults in decreased and pharmaceuticals. (J. Chromatogr. analyte response but in improved peak shape similar to that obtained with a UV Sci. 1996, 34, 261-71.)
However, AFM investigations into tribology are hampered by the lack of a welldefined description of the probe tipsample sliding interface. Paul E. Sheehan and Charles M. Lieber of Harvard University have circumvented this problem by investigating the friction of Mo03 nanocrystals sliding across single-crystal MoS2, a widely used solid lubricant. The interface structure and contact area are atomically defined, and the Mo03 nanocrystals can be moved controllably with the AFM tip. When they ran the experiments, the authors found that the friction was extremely anisotropic, with Mo03 sliding along only one of the three equivalent MoS2 surface directions. An atomic model of the interface was developed to explain these observations. The researchers exploited this friction anisotropy to fabricate a nanometer-scale mechanical latch and groove lock using two Mo03 crystals that have different preferred sliding directions. The latch and groove slide in and out reversibly, whereas breaking the latch required a large lateral force of 41 nN. Such reversible latches could be the basis for mechanical logic gates. (Science, 1996,272, 1158-61.)
Studying the catalytic properties of an enzyme The mechanism by which complex redox metalloenzymes catalyze enzyme transport and coupled reactions is important for understanding biological energy transduction. The various kinetic and spectroscopic techniques used to study electron transport tend to produce disparate pieces of information rather than a unified picture of the process. Fraser A. Armstrong and colleagues at the Oxford Inorganic Chemistry Laboratory (U.K.) and the University of CaliforniaSan Francisco have used electrocatalytic voltammetry to study the catalytic behavior of succinate dehydrogenase (SDH), the membrane-extrinsic component of Complex II. SDH adsorbs onto a pyrolytic graphite edge electrode and catalyzes interconversion of succinate and fumarate depending on the applied electrochemical potential. Under reversible conditions (near the reduction potential of the fumarate/succinate couple) and at the physiological temperature of 38 °C, SDH
Analytical Chemistry News & Features, August 1, 1996 461 A
