Supercritical fluid separations concepts

For the elucidation of chemical reaction mechanisms, in-situ NMR spectroscopy is an established technique. For investigations at high pressure either sample tubes from sapphire [3] or metallic reactors [4] permitting high pressures and elevated temperatures are used. The latter represent autoclaves, typically machined from copper-beryllium or titanium-aluminum alloys. An earlier version thereof employs separate torus-shaped coils that are imbedded into these reactors permitting in-situ probing of the reactions within their interior. However, in this case certain drawbacks of this concept limit the filling factor of such NMR probes consequently, their sensitivity is relatively low, and so is their resolution. As a superior alternative, the metallic reactor itself may function as the resonator of the NMR probe, in which case no additional coils are required. In this way gas/liquid reactions or reactions within supercritical fluids can be studied... 

In HPLC, the direct serial coupling of achiral and chiral columns for the simultaneous achiral/chiral separation of a range of components in a complex mixture was not feasible due to either the increased backpressure or the different mobile-phase requirement. However, this concept is not restricted for SFC because the supercritical fluid not only has a significantly lower viscosity than a liquid but is also an effective eluent for both chiral and achiral stationary phases [52], As an example, an SFC-MS approach using the coupled achiral/chiral column combination was reported to significantly enhance the separation efficiency for the reaction mixtures of cinna-monitrile and hydrocinnamonitrile intermediates to determine the diastereometric/ enantiometric composition of the final product in one step [52], Characterization of... 

The gradient concept can be applied also in SMB chromatography. This approach was first suggested for separations using supercritical fluids (Clavier, Nicoud, and Perrut, 1996). It is capable of increasing productivity and product concentrations, as well as of reducing eluent consumption. Theoretical and experimental studies... 

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