Dynamic and geometric variables

Dynamic factors are among the key variables to be optimized in an SFE process. In addition to extracting the analytes, the primary function of the supercritical fluid is to transport the solutes to the collecting vessel or to an on-line coupled chromatograph or detector. Ensuring efficient transportation of the analytes following separation from the matrix entails optimizing three mutually related variables, namely the flow-rate of the supercritical fluid, the characteristics of the extraction cell and the extraction time. These factors must be carefully combined in order to allow the flow-cell to be vented as many times as required. 

In some cases, the flow-rate is the single factor most strongly influencing the extraction efficiency the higher it is, the greater the amount of analyte that is extracted over a given interval [38]. In other cases such as the extraction of fluoranthene from soil, the flow-rate scarcely affects the results above a 0.3 ml/min level [41]. The optimum conditions for a particular extraction cannot be established on the basis of the number of times the extraction cell is vented because the extraction of most analytes is kinetically limited. Occasionally (e.g. in the extraction of cocaine from coca leaves), the process is governed primarily by the solubility of the analyte rather than by its desorption kinetics (see Fig. 7.10) [22]. 

The size, geometry and void volume of the extraction cell, in addition to its amenability to decreasing compaction by stirring or ultrasonication, all influence the SFE process. 

As a rule, natural samples require longer times for quantitative extraction of analytes than do spiked samples. Thus, complete extraction of cholesterol from egg yolk takes 

60 min compared to only 30 min for the quantitative extraction of this substance added to glass wool [18]. Even with natural samples, the time required to extract a given analyte varies markedly depending on the type of matrix that contains it. One case in point is the extraction of PAHs from certified reference materials (CRMs) with a matrix consisting exclusively of soil [46], 

---