The DLLME technique uses somewhat bigger volumes of organic solvents than
SDME, but it still only involves microliters. DLLME, much like SDME, is also often
being combined with GC, HPLC, ET-AAS and FAAS methods. DLLME can also be
coupled to other extraction techniques, such as SPE, SFE, SBSE and nano
techniques. The use of ultrasonic waves with DLLME has gained increasing popularity in recent years because UA-DLLME provides an increased rate of mass
transfer of the analyte from aqueous phase into the fine extracting droplets (Albero
et al. 2015). For instance, UA-DLLME was successfully applied for the simultaneous determination of alkylphenols, parabens and bisphenols (Jain et al. 2014),
polybrominated diphenyl ethers (Zhang et al. 2013) or dechlorane along with other
chlorinated flame retardants (Hsieh et al. 2013) in water samples. A sample preparation technique related to DLLME (Przyjazny 2019) which uses the property of
non-ionic surfactants to form micelles in aqueous solutions and to become turbid
when heated to particular temperature (so-called cloud point temperature) is termed
cloud point extraction (CPE). Above this temperature, the micellar solution separates
into two phases: a surfactant-rich phase of a small volume and a diluted aqueous
phase (Hagarová and Urík 2016). CPE is a simple, rapid, versatile, inexpensive and
environmentally friendly method utilized for separation and preconcentration of
trace metals from aqueous solutions which has recently become an attractive area
of research and an alternative to LLE (Saha et al. 2017). Regarding HF-LPME, the
extraction yields obtained by this method are higher than those gained by direct
SDME, since hydrophobic hollow fibres permit the use of vigorous stirring rates to
increase the speed of the extraction kinetics (Saha et al. 2017).
Supported liquid membrane extraction (SLME) and microporous membrane
liquid-liquid extraction (MMLLE) are techniques that use hydrophobic porous
membranes impregnated by organic solvent, which separates two immiscible phases.
Liquid membranes have been successfully used in an extraction of metals and
organic pollutants, mainly pesticides, such as phenoxy acid, dinitrophenol, sulfonylurea and triazine herbicides from waters and wastewaters (Bartolome et al. 2007;
Msagati et al. 2008; Chirila and Drăghici 2013). A competitive advantage of
membrane-based techniques is that they can be coupled to a variety of other
analytical methods such as ion chromatography (IC), LC, GC, capillary electrophoresis and atomic absorption spectrometry.
5.3.2 Solid-Liquid Extraction
When the sample matrix is solid and the phase required to be analysed is of liquid
nature, the process is called solid-liquid extraction. It begins with wetting of the
matrix surface with a solvent or immersing the whole matrix in a solvent and letting
the solvent penetrate into the pores of the sample particulates. This is followed by
the dissolution of extractable material, transport of the solute(s) from inside to the
surface of the solid particles and the dispersion of the solute within the bulk of the
solvent surrounding solid particles by diffusion and agitation (Berk 2013). The solid
phase is then removed by filtration. The key factor in a procedure is the proper
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L. Nemček and I. Hagarová
SDME, but it still only involves microliters. DLLME, much like SDME, is also often
being combined with GC, HPLC, ET-AAS and FAAS methods. DLLME can also be
coupled to other extraction techniques, such as SPE, SFE, SBSE and nano
techniques. The use of ultrasonic waves with DLLME has gained increasing popularity in recent years because UA-DLLME provides an increased rate of mass
transfer of the analyte from aqueous phase into the fine extracting droplets (Albero
et al. 2015). For instance, UA-DLLME was successfully applied for the simultaneous determination of alkylphenols, parabens and bisphenols (Jain et al. 2014),
polybrominated diphenyl ethers (Zhang et al. 2013) or dechlorane along with other
chlorinated flame retardants (Hsieh et al. 2013) in water samples. A sample preparation technique related to DLLME (Przyjazny 2019) which uses the property of
non-ionic surfactants to form micelles in aqueous solutions and to become turbid
when heated to particular temperature (so-called cloud point temperature) is termed
cloud point extraction (CPE). Above this temperature, the micellar solution separates
into two phases: a surfactant-rich phase of a small volume and a diluted aqueous
phase (Hagarová and Urík 2016). CPE is a simple, rapid, versatile, inexpensive and
environmentally friendly method utilized for separation and preconcentration of
trace metals from aqueous solutions which has recently become an attractive area
of research and an alternative to LLE (Saha et al. 2017). Regarding HF-LPME, the
extraction yields obtained by this method are higher than those gained by direct
SDME, since hydrophobic hollow fibres permit the use of vigorous stirring rates to
increase the speed of the extraction kinetics (Saha et al. 2017).
Supported liquid membrane extraction (SLME) and microporous membrane
liquid-liquid extraction (MMLLE) are techniques that use hydrophobic porous
membranes impregnated by organic solvent, which separates two immiscible phases.
Liquid membranes have been successfully used in an extraction of metals and
organic pollutants, mainly pesticides, such as phenoxy acid, dinitrophenol, sulfonylurea and triazine herbicides from waters and wastewaters (Bartolome et al. 2007;
Msagati et al. 2008; Chirila and Drăghici 2013). A competitive advantage of
membrane-based techniques is that they can be coupled to a variety of other
analytical methods such as ion chromatography (IC), LC, GC, capillary electrophoresis and atomic absorption spectrometry.
5.3.2 Solid-Liquid Extraction
When the sample matrix is solid and the phase required to be analysed is of liquid
nature, the process is called solid-liquid extraction. It begins with wetting of the
matrix surface with a solvent or immersing the whole matrix in a solvent and letting
the solvent penetrate into the pores of the sample particulates. This is followed by
the dissolution of extractable material, transport of the solute(s) from inside to the
surface of the solid particles and the dispersion of the solute within the bulk of the
solvent surrounding solid particles by diffusion and agitation (Berk 2013). The solid
phase is then removed by filtration. The key factor in a procedure is the proper
142
L. Nemček and I. Hagarová