186
Deep eutectic solvents and natural deep eutectic solvents are known for their
various properties: low vapor pressure, nonflammability, high thermal stability, and
low thermal conductivity (Nam et al. 2015; Radošević et al. 2016; Ruesgas-Ramón
et al. 2017; Moura et al. 2017; Khataei et al. 2018). Deep eutectic solvents are generally hydrophilic; the first hydrophobic one, synthesized from decanoic acid and
quaternary ammonium salts, was used for the extraction of volatile fatty acids from
aqueous solutions (van Osch et al. 2015). These solvents are considered “green” and
excellent alternatives to conventional and nonconventional organic solvents, being
effective to extract hydrophilic and hydrophobic compounds (Tang et al. 2014).
New techniques miniaturizing solid-phase extraction or liquid-liquid extraction
have arisen such as solid-phase microextraction and liquid-phase microextraction,
respectively (Hawthorne et al. 1992; Pedersen-Bjergaard and Rasmussen 1999).
These techniques are characterized by using low amounts of sample matrices and
small volumes of organic solvents. They are recently recommended because they
offer many advantages such as high degree of concentration and minimized extraction time and energy consumption (Aydin et al. 2018). The application of these
solvents in these techniques will be discussed later on.
This chapter presents an emphasis on the extraction techniques that used deep
eutectic solvents as extraction solvents. Also, the effects of their properties and of
the method parameters on the extraction efficiency are discussed. This review examines additionally the advantages and drawbacks of each extraction method.
Moreover, the recent combinations of different extraction techniques using deep
eutectic solvents in extraction processes are also reviewed.
6.2 Microextraction Techniques: Description
and Application with Deep Eutectic Solvents
Liquid-liquid extraction and solid-phase extraction techniques were first introduced
in the early 1970s. Liquid-liquid extraction involves adding a solvent to the sample
that is immiscible, followed by a selective partitioning of analytes between the two
phases. Solid-phase extraction technique consists on passing aqueous samples
through a solid sorbent where the analytes will be retained (Okenicová et al. 2016).
Therefore, the selection of an appropriate sorbent is very important. This technique
is considered better than liquid-liquid extraction because it reduces and even eliminates the use of toxic and inflammable solvents, thus becoming more environmentally friendly (Picó et al. 2007; Ince et al. 2010).
Since then, extraction techniques trends in analytical chemistry have been focusing toward less organic solvent consumption, faster extraction time, automation,
and improved quantification, which includes higher recoveries, better reproducibility, and lower method detection limits. That led to the invention of small-scale miniaturizing versions of liquid-liquid extraction and solid-phase extraction techniques:
the liquid-phase microextraction and solid-phase microextraction techniques,
respectively (Raynie 2004). In this part of the chapter, different types of
L. Nakhle et al.
Deep eutectic solvents and natural deep eutectic solvents are known for their
various properties: low vapor pressure, nonflammability, high thermal stability, and
low thermal conductivity (Nam et al. 2015; Radošević et al. 2016; Ruesgas-Ramón
et al. 2017; Moura et al. 2017; Khataei et al. 2018). Deep eutectic solvents are generally hydrophilic; the first hydrophobic one, synthesized from decanoic acid and
quaternary ammonium salts, was used for the extraction of volatile fatty acids from
aqueous solutions (van Osch et al. 2015). These solvents are considered “green” and
excellent alternatives to conventional and nonconventional organic solvents, being
effective to extract hydrophilic and hydrophobic compounds (Tang et al. 2014).
New techniques miniaturizing solid-phase extraction or liquid-liquid extraction
have arisen such as solid-phase microextraction and liquid-phase microextraction,
respectively (Hawthorne et al. 1992; Pedersen-Bjergaard and Rasmussen 1999).
These techniques are characterized by using low amounts of sample matrices and
small volumes of organic solvents. They are recently recommended because they
offer many advantages such as high degree of concentration and minimized extraction time and energy consumption (Aydin et al. 2018). The application of these
solvents in these techniques will be discussed later on.
This chapter presents an emphasis on the extraction techniques that used deep
eutectic solvents as extraction solvents. Also, the effects of their properties and of
the method parameters on the extraction efficiency are discussed. This review examines additionally the advantages and drawbacks of each extraction method.
Moreover, the recent combinations of different extraction techniques using deep
eutectic solvents in extraction processes are also reviewed.
6.2 Microextraction Techniques: Description
and Application with Deep Eutectic Solvents
Liquid-liquid extraction and solid-phase extraction techniques were first introduced
in the early 1970s. Liquid-liquid extraction involves adding a solvent to the sample
that is immiscible, followed by a selective partitioning of analytes between the two
phases. Solid-phase extraction technique consists on passing aqueous samples
through a solid sorbent where the analytes will be retained (Okenicová et al. 2016).
Therefore, the selection of an appropriate sorbent is very important. This technique
is considered better than liquid-liquid extraction because it reduces and even eliminates the use of toxic and inflammable solvents, thus becoming more environmentally friendly (Picó et al. 2007; Ince et al. 2010).
Since then, extraction techniques trends in analytical chemistry have been focusing toward less organic solvent consumption, faster extraction time, automation,
and improved quantification, which includes higher recoveries, better reproducibility, and lower method detection limits. That led to the invention of small-scale miniaturizing versions of liquid-liquid extraction and solid-phase extraction techniques:
the liquid-phase microextraction and solid-phase microextraction techniques,
respectively (Raynie 2004). In this part of the chapter, different types of
L. Nakhle et al.