752
N. A. Manaf et al.
step in obtaining significant pre-concentration. Thus, this technique is both expensive
and environmentally unfriendly (Kozlowska et al. 2003). In this context, SPE seems
to be better, as smaller amounts of organic solvents are usually used. However, SPE
cartridges are used only once; not only are these expensive but they also generate
a great deal of waste. After use, SPE cartridges are sent to dumping grounds or,
in certain cases, waste incineration plants. These practices are not environmentallyfriendly (Agnieska and Tomasz 2011). In this case, microextraction techniques may
offer a better option.
Microextraction techniques are generally defined as non-exhaustive sample preparation methods that utilise a very small volume of the extracting phase (μL) relative to
the sample volume (Torres Padrón et al. 2014). Of these, the solid-phase microextraction (SPME), introduced by Arthur and Pawliszyn in 1990, have found their place in
modern analytical laboratories while others such as the single-drop microextraction
(SDME) (Jeannot and Cantwell 1996) and hollow-fibre liquid-phase microextraction
(HF-LPME) (Pedersen and Ramussen 2008) have yet to become mainstream techniques. In 2006, the dispersive liquid-liquid microextraction (DLLME) was introduced. DLLME is a modified solvent extraction method and its acceptor-to-donor
phase ratio is greatly reduced compared with other methods. In DLLME, the appropriate mixture of the extraction and disperser solvents is rapidly injected by syringe
into aqueous samples containing analytes. Thereby, a cloudy solution forms. In fact,
the cloudy state results from the formation of fine droplets of the extraction solvent which disperse in the sample solution. Then, this cloudy solution is centrifuged
and the fine droplets become sediment at the bottom of the conical test tube. The
determination of analytes in the sedimented phase can be performed by instrumental
analysis (Rezaee et al. 2010). The DLLME technique, apart from its green features,
is appealing as it uses common glassware and chemicals. However, the use of dispersive solvents may decrease the partitioning and the mass transfer of the analytes
into the extraction solvent, thereby reducing the enrichment efficiency (Xue et al.
2014). The use of high density and toxic organic solvents (e.g., carbon tetrachloride,
chloroform, dichloromethane, etc.) are other disadvantages of this technique (Leng
et al. 2012). Further refinement of the DLLME technique gives rise to vortex-assisted
liquid liquid microextraction (VALLME) and manages to overcome the drawbacks
of the DLLME technique (Yiantzi et al. 2010). Yiantzi et al. (2010) introduced a
new microextraction method termed VALLME, whereby dispersion of low density
extraction solvent into water is obtained through vortex mixing, a mild emulsification
procedure. The fine droplets can rapidly extract target analytes from water because
of the shorter diffusion distance and larger interfacial area. After centrifugation,
the floating extractant phase restores its initial single-drop shape for the following instrumental analysis. The VALLME technique, in which the dispersion of the
extraction solvent is enhanced by vortex mixing instead of ultrasound irradiation,
was devised (Yiantzi et al. 2010). VALLME overcomes the complexity of DLLME,
where by the necessity of using a disperser solvent and the problem of possible analyte degradation resulting from the high temperatures and pressures, as well as the
free radicals that are generated when using ultrasound; using a vortex for mixing
is more cost-effective than an ultrasonic bath and a great deal less expensive than
N. A. Manaf et al.
step in obtaining significant pre-concentration. Thus, this technique is both expensive
and environmentally unfriendly (Kozlowska et al. 2003). In this context, SPE seems
to be better, as smaller amounts of organic solvents are usually used. However, SPE
cartridges are used only once; not only are these expensive but they also generate
a great deal of waste. After use, SPE cartridges are sent to dumping grounds or,
in certain cases, waste incineration plants. These practices are not environmentallyfriendly (Agnieska and Tomasz 2011). In this case, microextraction techniques may
offer a better option.
Microextraction techniques are generally defined as non-exhaustive sample preparation methods that utilise a very small volume of the extracting phase (μL) relative to
the sample volume (Torres Padrón et al. 2014). Of these, the solid-phase microextraction (SPME), introduced by Arthur and Pawliszyn in 1990, have found their place in
modern analytical laboratories while others such as the single-drop microextraction
(SDME) (Jeannot and Cantwell 1996) and hollow-fibre liquid-phase microextraction
(HF-LPME) (Pedersen and Ramussen 2008) have yet to become mainstream techniques. In 2006, the dispersive liquid-liquid microextraction (DLLME) was introduced. DLLME is a modified solvent extraction method and its acceptor-to-donor
phase ratio is greatly reduced compared with other methods. In DLLME, the appropriate mixture of the extraction and disperser solvents is rapidly injected by syringe
into aqueous samples containing analytes. Thereby, a cloudy solution forms. In fact,
the cloudy state results from the formation of fine droplets of the extraction solvent which disperse in the sample solution. Then, this cloudy solution is centrifuged
and the fine droplets become sediment at the bottom of the conical test tube. The
determination of analytes in the sedimented phase can be performed by instrumental
analysis (Rezaee et al. 2010). The DLLME technique, apart from its green features,
is appealing as it uses common glassware and chemicals. However, the use of dispersive solvents may decrease the partitioning and the mass transfer of the analytes
into the extraction solvent, thereby reducing the enrichment efficiency (Xue et al.
2014). The use of high density and toxic organic solvents (e.g., carbon tetrachloride,
chloroform, dichloromethane, etc.) are other disadvantages of this technique (Leng
et al. 2012). Further refinement of the DLLME technique gives rise to vortex-assisted
liquid liquid microextraction (VALLME) and manages to overcome the drawbacks
of the DLLME technique (Yiantzi et al. 2010). Yiantzi et al. (2010) introduced a
new microextraction method termed VALLME, whereby dispersion of low density
extraction solvent into water is obtained through vortex mixing, a mild emulsification
procedure. The fine droplets can rapidly extract target analytes from water because
of the shorter diffusion distance and larger interfacial area. After centrifugation,
the floating extractant phase restores its initial single-drop shape for the following instrumental analysis. The VALLME technique, in which the dispersion of the
extraction solvent is enhanced by vortex mixing instead of ultrasound irradiation,
was devised (Yiantzi et al. 2010). VALLME overcomes the complexity of DLLME,
where by the necessity of using a disperser solvent and the problem of possible analyte degradation resulting from the high temperatures and pressures, as well as the
free radicals that are generated when using ultrasound; using a vortex for mixing
is more cost-effective than an ultrasonic bath and a great deal less expensive than