the physico-chemical properties of the solvent, such as its density, diffusivity, or
solubility characteristics (Padrela et al. 2009; Naresh et al. 2013).
Nowadays, more than 90% of all analytic SFE are carried out with supercritical
carbon dioxide (Saha et al. 2017; Pannu et al. 2018) which has been available since
the 1980s (Buckle 2015). It is an attractive alternative to organic solvents such as
halogenated hydrocarbons (e.g. chloroform, dichloromethane) since it has a relatively low critical temperature (31
C) and pressure (74 bar), it is non-flammable,
accessible in high purity and very low cost (it can be captured for use with no need
for synthesis and processing), it can be readily isolated from the extract and it leaves
no toxic residues behind, thus posing no threat to the environment or human health
(Rozzi et al. 2002; Naresh et al. 2013). In the supercritical state, CO 2 has a polarity
comparable to liquid pentane what makes it suitable for the solubilization of
lipophilic compounds (Saha et al. 2017). The major drawback of CO 2 as a
non-polar solvent medium and a linear molecule with no net dipole moment is a
significant difficulty in dissolving polar and ionic species (Peach and Eastoe 2014).
The addition of a small amount of polar co-solvent (modifier) enhances the
solubilizing capacity of the supercritical CO 2 , making it possible to extract more
polar molecules (Wrona et al. 2017). Most popular co-solvents are ethanol, water
and methanol. Other fluids used in SFE include methane, ethane, ethylene, propane,
propylene, ammonia, pentane, n-butane, n-hexane, acetone, nitrous oxide (N 2 O),
dichlorodifluoromethane, trifluoromethane, argon, xenon, etc. Other supercritical
fluids that have been getting more attention in environmental SFE are freons and
sulfur hexafluoride (SF 6 ). The latter is a non-polar but easily polarizable molecule
and as a supercritical fluid it has been reported to selectively extract aliphatic
hydrocarbons with a chain length of up to C-24 from mixtures containing both
aliphatic and aromatic hydrocarbons (Saha et al. 2017). Due to the legal limitations
of solvents and solvent residues (Vági et al. 2005) the interest in supercritical fluid
technology has grown in the past two decades, being increasingly proposed as an
alternative to many conventional solvent-based processes (Padrela et al. 2009). It is
widely used in petroleum industry for the determination of aromatics as well as for
other hydrocarbon separations.
Ultrasound-assisted extraction (UAE), also known as ultrasonic solvent extraction, is also considered a good option for the extraction of organic compounds from
different matrices. The use of ultrasound enhances contact between solvent and solid
due to an increase of pressure which aids penetration of the solvent into material and
mass transfer, and increase of temperature which improves solubility and diffusivity
(Saha et al. 2017). Even if originally developed for solid samples, the ultrasound
radiation has recently been coupled to microextraction techniques to enable the
analysis of liquid samples (Albero et al. 2015). Sonication can be used with other
extraction techniques to enhance their efficiency. For instance, it can be combined
with a stir bar sorptive extraction technique (SBSE) for an extraction and
preconcentration of halogenated phenols from sediments (Tölgyessy et al. 2009).
Besides ultrasonic waves, microwaves are another form of radiation that is
recognized as a simple and valuable tool in applied chemistry. Microwave-assisted
extraction (MAE) is a procedure that uses microwaves that easily penetrate into the
5 The Recent Strategies Employed in Chemical Analysis of Contaminated Waters,. . .
145
solubility characteristics (Padrela et al. 2009; Naresh et al. 2013).
Nowadays, more than 90% of all analytic SFE are carried out with supercritical
carbon dioxide (Saha et al. 2017; Pannu et al. 2018) which has been available since
the 1980s (Buckle 2015). It is an attractive alternative to organic solvents such as
halogenated hydrocarbons (e.g. chloroform, dichloromethane) since it has a relatively low critical temperature (31
C) and pressure (74 bar), it is non-flammable,
accessible in high purity and very low cost (it can be captured for use with no need
for synthesis and processing), it can be readily isolated from the extract and it leaves
no toxic residues behind, thus posing no threat to the environment or human health
(Rozzi et al. 2002; Naresh et al. 2013). In the supercritical state, CO 2 has a polarity
comparable to liquid pentane what makes it suitable for the solubilization of
lipophilic compounds (Saha et al. 2017). The major drawback of CO 2 as a
non-polar solvent medium and a linear molecule with no net dipole moment is a
significant difficulty in dissolving polar and ionic species (Peach and Eastoe 2014).
The addition of a small amount of polar co-solvent (modifier) enhances the
solubilizing capacity of the supercritical CO 2 , making it possible to extract more
polar molecules (Wrona et al. 2017). Most popular co-solvents are ethanol, water
and methanol. Other fluids used in SFE include methane, ethane, ethylene, propane,
propylene, ammonia, pentane, n-butane, n-hexane, acetone, nitrous oxide (N 2 O),
dichlorodifluoromethane, trifluoromethane, argon, xenon, etc. Other supercritical
fluids that have been getting more attention in environmental SFE are freons and
sulfur hexafluoride (SF 6 ). The latter is a non-polar but easily polarizable molecule
and as a supercritical fluid it has been reported to selectively extract aliphatic
hydrocarbons with a chain length of up to C-24 from mixtures containing both
aliphatic and aromatic hydrocarbons (Saha et al. 2017). Due to the legal limitations
of solvents and solvent residues (Vági et al. 2005) the interest in supercritical fluid
technology has grown in the past two decades, being increasingly proposed as an
alternative to many conventional solvent-based processes (Padrela et al. 2009). It is
widely used in petroleum industry for the determination of aromatics as well as for
other hydrocarbon separations.
Ultrasound-assisted extraction (UAE), also known as ultrasonic solvent extraction, is also considered a good option for the extraction of organic compounds from
different matrices. The use of ultrasound enhances contact between solvent and solid
due to an increase of pressure which aids penetration of the solvent into material and
mass transfer, and increase of temperature which improves solubility and diffusivity
(Saha et al. 2017). Even if originally developed for solid samples, the ultrasound
radiation has recently been coupled to microextraction techniques to enable the
analysis of liquid samples (Albero et al. 2015). Sonication can be used with other
extraction techniques to enhance their efficiency. For instance, it can be combined
with a stir bar sorptive extraction technique (SBSE) for an extraction and
preconcentration of halogenated phenols from sediments (Tölgyessy et al. 2009).
Besides ultrasonic waves, microwaves are another form of radiation that is
recognized as a simple and valuable tool in applied chemistry. Microwave-assisted
extraction (MAE) is a procedure that uses microwaves that easily penetrate into the
5 The Recent Strategies Employed in Chemical Analysis of Contaminated Waters,. . .
145