Using an alternative supercritical fluid

One way of improving SFE efficiency is by using a more suitable SF to extract the target analyte. Unfortunately, the choice of fluids other than CO, is restricted by the desire to have reasonable critical parameter values and costs, chemical inertness, low toxicity and little environmental impact. The use of supercritical N2O has proved to increase the extraction efficiency for high-molecular weight PAHs and chlorinated dibenzo-p-dioxins from fly ash and sediment [52]. This extractant, however, does not always improve the extraction efficiency [53] also, it can be explosive in the presence of reactive organics. Other polar fluids such as CHCIF, (Freon-22) have exhibited increased efficiency in the extraction of nitrated and non-nitrated PAHs from particulate matter in diesel exhaust [54]. Freon-22 has also been found to allow significantly fast and effective extraction of 

One of the most promising alternatives to CO, is supercritical water. Like CO, water is an environmentally acceptable solvent, but has not yet received attention as an analytical extractant for environmental solids because, in contrast to CO, water is too polar to efficiently leach most non-ionic organics associated to environmental solids. As can be seen in Table 7.2, water has a dielectric constant of ca. 80 at ambient temperature and pressure fortunately, however, the constant can be substantially lowered to accommodate much milder and potentially useful conditions simply by raising the temperature at a moderate pressure (see Fig. 7.13, which shows that water can be made even less polar than CCI4). Thus, simply by changing its temperature, the solvent polarity of water can be expanded over a very broad range. By contrast, the dielectric constant of supercritical CO, ranges from 1 to 1.6 only. As shown by Hawthorne et al. [59], water can be 

The behaviour of water under supercritical conditions has been used to separate ionic species by precipitation as salts combined with the oxidation of organic species [60]. 

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Palenzuela, B. et al.. Bioguided extraction of polyphenols from grape marc by using an alternative supercritical-fluid extraction method based on a liquid solvent trap. Anal. Bioanal. Chem., 378, 2021, 2004. 

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