Supercritical fluid extraction (SFE) and Supercritical fluid chromatography (SFC) are very closely linked. SFE/SFC utilizes extreme conditions of temperature and pressure in such a way that the mobile phase remains as a supercritical fluid, as can be seen in Figure 1. Supercritical fluids possess unique properties, intermediate between those of gas and liquids. These depend on the pressure, temperature and composition of the fluid. In particular, their viscosity is lower than that of liquids, and the diffusion coefficients are higher, allowing more efficient extractions. In addition, the density (and therefore the solvent power of the fluid) may be adjusted by varying both the pressure and the temperature, affording the opportunity of theoretically performing highly selective extractions. This is where SFC is used in large scale applications. Table 1 shows a range of large scale industrial applications
Table 1 Large scale industrial applications Super critical fluid extraction
The basic equipment is very similar to a standard HPLC system, with the following exceptions/additions:
- A source of CO2 (commonly a tank)
- The ability to regenerate the CO2 – i.e. remove the organic modifier prior to re-circulating or re-depositing in the tank
- A backpressure restrictor placed after the analytical column/extraction cell
An example SFC/SFE apparatus is shown in Figure 2.
Figure 2 Example of SFC/SFE apparatus
The SFE may be carried out in either static or dynamic mode. The pressure in the system is maintained by means of a restrictor (either fixed or variable, the latter making the pressure independent of the flow rate). At the end of the restrictor, the fluid is depressurized and the extracted analytes are trapped in an organic solvent or on a solid phase filled cartridge (from which the analytes are later eluted with a small volume of organic solvent).
Due to the numerous parameters affecting the extraction efficiencies, SFE affords a high degree of selectivity and it is this reason that supercritical fluid extraction has a wide range of industrial-scale uses. However, on the other hand, this makes the optimisation quite tedious and difficult in practice and maybe prevent all potential species being extracted.
The parameters to consider are linked to the extraction parameters inside the cell, to the nature of the solutes or to the nature of the matrix. The important parameters in SFE are both the pressure and temperature inside the cell. A pressure increase leads to a higher fluid density, thus increasing the solubility. The inverse is observed with the temperature; however, increasing the temperature may enhance the solubility of volatile analytes. In addition, higher temperatures may be required to overcome solute–matrix interactions, as observed for the extraction of polycyclic aromatic hydrocarbons (PAHs) and polychlorinated dibenzo-p-dioxins from environmental matrices.
The polarity of compounds is the most significant factor to be considered when working with SFE. Pure CO2 efficiently extracts non-polar to low polarity compounds. For polar solutes, a modifier is added to enhance the extraction. For very polar and ionic compounds, the modifier may be a complexing agent, an ion-pair reagent or a derivatisation reagent. As an example, the addition of tetrabutylammonium enabled the extraction of anionic surfactants from sewage sludge to be performed. The addition of the modifier directly to the matrix (prior to the extraction) may help in disrupting the analyte–matrix interactions; however, it requires that a static extraction be performed first, to avoid sweeping the modifier out of the cell. In cases where the analytes do not readily derivatise, the addition of a derivatisation reagent may still be useful as it can react with the active sites of the matrix, thus enhancing the extraction, as has already been observed during the extraction of PAHs from urban dust. 
The users of SFE must be aware of the fact that the addition of a modifier to CO2 presents severe drawbacks, due to the technical factors, and so it should be avoided or minimized whenever possible. The presence of the modifier changes the values of the critical pressure and temperature, so that too high a modifier content may result in a temperature lower than the critical value, resulting in a subcritical state, with higher viscosity and lower diffusion coefficients than the supercritical state; in this case, the technique is commonly called enhanced-fluidity liquid extraction (EFLE). In addition, as the modifier enhances the solvating power of the fluid, it reduces the extraction selectivity as more matrix materials or non-target analytes are co-extracted. Finally, the modifier condenses upon depressurization, which may result in elution of the retained compounds when a solid trap is used as the collection device, since then it may act somewhat like a chromatographic device. The nature of the matrix (water content, percentage of organic carbon, humic/fulvic materials, etc.) and its physical characteristics (such as porosity or particle size) are of prime importance for the success of an extraction,9 as with other extraction techniques. Milling the matrix is recommended, to limit the diffusion step inside the matrix and to increase the surface area, which increases the rate of extraction when it is limited by matrix effects. Also, the addition of a drying agent (such as sodium sulfate) may prevent the plugging of the restrictor by ice in the presence of humid matrices. Caution must also be taken when filling the vessel to ensure a homogeneous bed of material (to prevent channelling) and to take into account possible swelling of the matrix (such as polymers) upon introduction of supercritical CO2. In addition, very fine particles may be swept out of the cell by the fluid and result in plugging and mechanical transfer problems. Finally, a sorbent may be added in the cell to retain matrix material and increase the selectivity of the extraction.
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