Chemistry Reference
In-Depth Information
Table 7.2
Critical properties of solvents used in SFE.
Solvent
Critical property
Temperature [°C]
Pressure [atm]
Ethene
10
51
Water
374
221
Carbon dioxide
32
72
Ethane
32
48
Nitrous oxide
37
72
Sulfur hexafluoride
46
38
Pentane
-77
33
Fluoroform
26
48
Ammonia
133
113
7.3.2
Supercritical fluid extraction
Supercritical fluid extraction was introduced into laboratory practice by K. Zosel in 1978, since when there has
been a steady growth of interest in this analytical technique. Environmentally-benign and efficient, SFE has
been extensively with respect to the extraction of organic compounds from various matrices (e.g. medicinal
plants [70], food [71], seeds, fruits, leaves, flowers, rhizomes [72], soil [73], drugs [74], sediments [75],
honey [76] and water [77]). Unfortunately, some organic pollutants are polar, so they cannot be extracted
quantitatively from environmental solids with SFCO
2
[78]. But by modifying SFCO
2
with methanol, the
solvent becomes more polar and is then suitable for the extraction of more polar target compounds.
SFE has been adopted by the EPA as a reference method for extracting Petroleum Hydrocarbons (Method
3560, in 1996), PAHs (Method 3561, in 1996) and PCBs (Method 3562, in 2007) from solid environmental
matrices. SFE has recently been used for extracting POPs from different plant materials and several analytical
applications dealing with the extraction of POPs from different animal tissues have also been reported [79].
A wide variety of solvents is available for use as SFs, including carbon dioxide, nitrous oxide, ethane,
propane, n-pentane, ammonia, fluoroform, sulfur hexafluoride and water. Table 7.2 lists the most common
compounds that can be used as supercritical fluids. Carbon dioxide is widely used in SFE because it is
safe (low critical parameters: Tc
72 atm) (see Figure 7.8), readily available, inexpensive and
non-toxic [78]. Carbon dioxide is a gas at room temperature, so once the extraction is completed, and the
system decompressed, substantial elimination of CO
2
is achieved without residues, yielding a solvent-free
extract [80].
The only serious drawback of SFE is that the investment costs are higher compared to those of the traditional
process (extraction plus separation), which is relatively cheap and very easily scaled up for industrial purposes
[72, 74]. Because of their properties, supercritical fluids are uniquely versatile for controlling extraction and
chromatographic selectivity. Furthermore, the solvating power of supercritical fluids can be adjusted by
altering pressure and temperature.
As an extraction medium, supercritical fluids have many advantages, the most important of which are:
=
32°C, Pc
=
the minimization or complete exclusion of organic solvents;
●
the possibility of selective extraction if minor changes are made to the pressure and temperature in the
extraction system;
●
a much higher degree of extracting agent purity than is achievable with organic solvents, because carbon
dioxide and nitrous oxide, commonly used as supercritical fluids, are gases under ambient conditions;
●
