The Contribution of Supercritical Fluids to Laboratory Automation

Extreme cases of non-selectivity are encountered at the maximum and minimum densities. At maximum densities, the supercritical fluid has maximum solvent power so usually everything that is soluble at the various discrete lower densities is soluble at the maximum density - i.e., there is no selectivity if just the highest density is used in the extraction scheme. At that point, a different selectivity can be superimposed on the highest-density supercritical fluid by adding additional components, called modifiers, to the bulk fluid to form solvent mixtures. Typical modifiers are methanol, ethanol, methoxy ethanol, and methylene chloride. With carbon dioxide, the onset of noticeable solvent power occurs at about 0.1 g/mL this is the point at which the carbon dioxide makes a transition from ideal-gas behaviour (PVT equations) to critical-region behaviour where the density is an even more sensitive function of pressure (compared to ideal-gas behaviour). The result is that liquid-like, but selective, solvation occurs for carbon dioxide over the density range of about 0.1 

In analytical-scale SFE, the usual approach has been to expand the compressed mixture to nearly ambient pressure the supercritical fluid expands to ideal-gas conditions and the solutes precipitate in a collection region. This makes an SFE instrument conceptually quite simple. Contact a sample with a supercritical fluid at the designated temperature, density (controlled by pressure), and chemical composition to dissolve the analytes of choice impose a flow through the sample to remove the dissolved analytes from the sample region and expand the resultant mixtiu-e of extraction solvent plus extracted components to ambient pressure to get rid of the solvent and collect the concentrated extracts. At that point, what is left is a reconstitution step to get the extract into a solvent compatible with the target analytical instrument. 

Not only are four important sample preparation processes automated by an SFE instrument, but furthermore, with a modular design approach, the SFE instrument can become a component in an automated system. One example is the direct coupling of SFE and analytical instruments. One such system is referred to as a bridge configuration between the SFE and GC (or GC/MS, HPLC, SFC) [10]. In such a system the operator inputs a queue of samples to the SFE and receives the analytical report - with no manual intervention. Operation of the SFE and the analytical instrument can be overlapped to maximise through-put. Sample tracking capabilities are strengthened. This 

The capacity for lab automation that SFE brings to sample preparation is analogous to the analytical instrument automation that has been developing for the last twenty years. Such tools as automatic injectors and integrators for gas and liquid chromatographs are examples. The shift from manual technique towards instrument control helps to improve the robustness and repeatability of laboratory methods. 

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