Collection and Reconstitution of the Extracted Components

Reality is often quite different. When a supercritical fluid mixture expands into pressures as high as ambient conditions, the resultant expansion plume can be a complex mixture it is a high velocity gas stream that entrains precipitated particles of extracted materials and often frozen carbon dioxide. Much adjustment needs to take place in the collection zone in order to achieve something close to 100 % recoveries of solutes with concentration ranges from parts per billion (PCBs) up to 50 % (total fat in a chocolate candy). Besides the flow dynamics of the expansion, several physicochemical parameters cause the deviation from the initial simple model. They include, but are not limited to, volatility of the solute, degree of co-precipitation of solid carbon dioxide (followed almost immediately with uncontrolled subhmation of the solid), aerosol formation, surface tension, occlusion in solid carbon dioxide, rebound from impinging surface, and many other interacting phenomena. 

Beyond simple empty vessels, a time-honoured approach has been to bubble the expanding carbon dioxide into another liquid. Although an improvement, this approach still suffers from many of the other above complications and a few new ones such as solute volatility and aerosol formation between the solutes and the chosen trapping liquid. 

Most recently, some of the commercial SFEs have used a thermally controlled solid trap as a collection zone such a trap is comprised of a porous packing material that can have some chemical affinity for analytes. In effect, the trap acts as a mechanical, thermal, and chemical filter to separate the extracted components from the expanding gas. Since the extracted components are 

The preceding sections have outlined the theoretical basis of supercritical fluids as solvents and some engineering considerations for designing an analytical-scale instrument. In the next section we will present details of specific examples of applying SFE to samples of interest in the food industry and a summary of other examples. 

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