Minimal-Length Fixed Restrictors

The answers to aU of these three questions are difficult, if not impossible, for the average practitioner to obtain. The first question brings up the observation that a fixed restrictor couples the flow rate and pressiu-e pressure is a variable dependent upon flow rate and vice versa. As flow rate increases, the system pressure increases similarly, a decrease in flow yields a decrease in pressure. If this is combined in an environment where the resistance changes as a solute flux passes, then the flow rate-pressure domain is uncontrolled, unpredictable and very difficult to reproduce between any two fixed restrictors. Flow rate, pressure (density), temperature, and time must be controlled precisely in order to deliver the same quantity of supercritical fluid needed to partition a given amount of analyte from a unit amount of matrix. Quoting extraction efficiency or percent recovery under such uncontrolled parameters is thus an empirical matter where coincidence is random or indiscriminate. 

There are many variations on fixed restrictors including a carefully defined fritted zone at the end of a capillary tube. Most of these have led to a un-ending number of comments at supercritical fluid extraction conferences such as the biggest hurdle in SFE is limited restrictor availability and technology. Greibrokk [15, 16] has summed up general experience and feeling about fixed restrictors, especially for SFC. 

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