sludges, wastes and biological structures (Wells 1978; Domini et al. 2005; Kassim
et al. 2005). It is very useful when it comes to separation of poorly soluble
compounds. Soxhlet extraction normally uses polar organic solvents or mixtures;
non-polar solvents such as n-hexane are used to extract non-polar contaminants,
such as OCPs and PCBs (Wells 1978). The long Soxhlet extraction times have
prompted the development of modified extractors, such as revolutionary RaFaTec
solvent extraction unit (Foss Tecator AB, Höganäs, Sweden). The apparatus offered
considerable advantages in terms of time and solvent use, it also allowed the
evaporation and collection of solvent further improving efficiency and the results
were comparable to the traditional method (Bicking 2000). First introduced in 1975,
it had quickly gained widespread acceptance and become an inspiration for an
improved system Soxtec™ launched by FOSS 6 years later.
The more recent improvement of the method allows the application of
microwaves to Soxhlet apparatus. This significantly accelerates the process of
extraction of PCBs from contaminated soils (from 24 h to 70 min) and reduces the
organic solvent disposal since 75–80% of the extractant can be recycled. The
extractor also enables a fully automated extraction of two samples simultaneously
(Luque-Garcia and Luque de Castro 2003). Other innovative methods include
ultrasound-assisted Soxhlet (UASE) developed in 2004 (Luque-Garcia and Luque
de Castro 2004) with an ultrasonic probe immersed in a water bath that circulates
around the Soxhlet chamber or Sono-Soxhlet system with an ultrasonic probe
inserted directly into the extraction chamber so an ultrasound can be applied within
an extraction reactor (Djenni et al. 2013). Nevertheless, Soxhlet extraction is frequently referred to as the benchmark technique so the results obtained with newer
extraction techniques are compared to those obtained by Soxhlet extraction (Dean
2000).
Traditional extraction techniques, such as liquid-liquid extraction and Soxhlet
extraction, require large volumes of toxic organic solvents, thus creating new
environmental hazards, increasing the risk of cancer and contributing to the ozone
layer depletion. The newer extraction techniques such as SFE, MAE, ASE, SPE,
SPME and hot water extraction are very attractive because they use much smaller or
no amounts of solvents (O’Reilly et al. 2005; Risticevic et al. 2010), they are rapid
and environmentally friendly.
Unlike Soxhlet, supercritical fluid extraction (SFE) represents a green technology
that offers several advantages over conventional extraction methods. It is a process
of separating the extractant from the matrix using supercritical fluids as an extracting
solvent. A solute is usually separated from a solid matrix, but liquid matrices can be
used as well. Supercritical fluid is any substance at a temperature and pressure above
its critical point, where distinct liquid and gas phases do not exist. It can diffuse
through solids like a gas, which facilitates the extraction of analytes located in not
easily accessible pores, and dissolve materials into their component parts like a
liquid (Sapkale et al. 2010; Chirila and Drăghici 2013). The power of supercritical
fluid as a solvent depends largely on its density. Near the critical point of a fluid,
even modest change in pressure or temperature may result in significant changes in
144
L. Nemček and I. Hagarová
et al. 2005). It is very useful when it comes to separation of poorly soluble
compounds. Soxhlet extraction normally uses polar organic solvents or mixtures;
non-polar solvents such as n-hexane are used to extract non-polar contaminants,
such as OCPs and PCBs (Wells 1978). The long Soxhlet extraction times have
prompted the development of modified extractors, such as revolutionary RaFaTec
solvent extraction unit (Foss Tecator AB, Höganäs, Sweden). The apparatus offered
considerable advantages in terms of time and solvent use, it also allowed the
evaporation and collection of solvent further improving efficiency and the results
were comparable to the traditional method (Bicking 2000). First introduced in 1975,
it had quickly gained widespread acceptance and become an inspiration for an
improved system Soxtec™ launched by FOSS 6 years later.
The more recent improvement of the method allows the application of
microwaves to Soxhlet apparatus. This significantly accelerates the process of
extraction of PCBs from contaminated soils (from 24 h to 70 min) and reduces the
organic solvent disposal since 75–80% of the extractant can be recycled. The
extractor also enables a fully automated extraction of two samples simultaneously
(Luque-Garcia and Luque de Castro 2003). Other innovative methods include
ultrasound-assisted Soxhlet (UASE) developed in 2004 (Luque-Garcia and Luque
de Castro 2004) with an ultrasonic probe immersed in a water bath that circulates
around the Soxhlet chamber or Sono-Soxhlet system with an ultrasonic probe
inserted directly into the extraction chamber so an ultrasound can be applied within
an extraction reactor (Djenni et al. 2013). Nevertheless, Soxhlet extraction is frequently referred to as the benchmark technique so the results obtained with newer
extraction techniques are compared to those obtained by Soxhlet extraction (Dean
2000).
Traditional extraction techniques, such as liquid-liquid extraction and Soxhlet
extraction, require large volumes of toxic organic solvents, thus creating new
environmental hazards, increasing the risk of cancer and contributing to the ozone
layer depletion. The newer extraction techniques such as SFE, MAE, ASE, SPE,
SPME and hot water extraction are very attractive because they use much smaller or
no amounts of solvents (O’Reilly et al. 2005; Risticevic et al. 2010), they are rapid
and environmentally friendly.
Unlike Soxhlet, supercritical fluid extraction (SFE) represents a green technology
that offers several advantages over conventional extraction methods. It is a process
of separating the extractant from the matrix using supercritical fluids as an extracting
solvent. A solute is usually separated from a solid matrix, but liquid matrices can be
used as well. Supercritical fluid is any substance at a temperature and pressure above
its critical point, where distinct liquid and gas phases do not exist. It can diffuse
through solids like a gas, which facilitates the extraction of analytes located in not
easily accessible pores, and dissolve materials into their component parts like a
liquid (Sapkale et al. 2010; Chirila and Drăghici 2013). The power of supercritical
fluid as a solvent depends largely on its density. Near the critical point of a fluid,
even modest change in pressure or temperature may result in significant changes in
144
L. Nemček and I. Hagarová