15.2.2 Liquid–Liquid Microextraction
Liquid–liquid extraction uses a large volume of organic solvents and samples,
i.e. dozens of milliliters to hundred milliliters, which is not friendly to the environment. Liquid–liquid microextraction, proposed by Liu (Liu and Dasgupta 1996) and
Jeannot et al. (Jeannot and Cantwell 1996, 1997) in the mid-1990s, provides a
miniaturized liquid–liquid extraction mode in which the organic solvent used is in
the range of tens to hundreds of microliters, and the amount of sample is in the range
of several to tens of milliliters. This greatly reduces the cost of analysis, and very
small volume of organic solvents can bring very high enrichment factor, so no
subsequent concentration steps are needed. Liquid–liquid microextraction integrates
sampling, separation, purification, and concentration into one step. It is especially
suitable for the analysis of trace phthalates in water samples. In addition, fewer
operational steps and less solvent consumption could reduce the risk of phthalates
contamination. The technique has been utilized for the extraction of analytes in
complex matrices, including water, food, and biological materials. But its extraction
efficiency and precision need further improvement.
Liquid–liquid microextraction can be classified into single drop microextraction,
dispersive liquid–liquid microextraction, solidified floating organic drop
microextraction, and hollow fiber liquid–liquid microextraction (Yamini et al. 2019).
Except single drop microextraction, hollow fiber liquid–liquid microextraction, dispersive liquid–liquid microextraction, and solidified floating organic drop
microextraction have been extensively applied in the extraction of phthalates in
water samples.
Hollow Fiber Liquid–Liquid Microextraction
Hollow fiber liquid–liquid microextraction is membrane-based liquid–liquid
microextraction which has become popular for their advantages over the other
microextraction methods, such as more effective sample cleanup, more stable
extraction procedures, greater enrichment factor, and negligible consumption of
organic solvents. It has a three-phase mode and two-phase mode. The three-phase
mode is an aqueous–organic–aqueous system in which the immobilized organic
solvent or supported liquid membrane is exposed to two aqueous phases of the sample solution and the aqueous acceptor phase located inside the hollow fiber. In
contrast, in the two-phase mode, the acceptor phase solution is also the same as an
organic solvent of supported liquid membrane, and both are immiscible in water
(Esrafili et al. 2018). In hollow fiber liquid–liquid microextraction, assisted by
magnetic stirring, the extraction time is commonly more than tens of minutes
(Sarafraz-Yazdi and Amiri 2010). The method needs a special extraction device,
and the operational requirements are relatively high. Toluene and ionic liquid
[BMIm]PF6 have been utilized as extractants by many researchers to extract
phthalates in water samples. For example, Psillakis et al. (Psillakis and Kalogerakis
548
C. Sun et al.
Liquid–liquid extraction uses a large volume of organic solvents and samples,
i.e. dozens of milliliters to hundred milliliters, which is not friendly to the environment. Liquid–liquid microextraction, proposed by Liu (Liu and Dasgupta 1996) and
Jeannot et al. (Jeannot and Cantwell 1996, 1997) in the mid-1990s, provides a
miniaturized liquid–liquid extraction mode in which the organic solvent used is in
the range of tens to hundreds of microliters, and the amount of sample is in the range
of several to tens of milliliters. This greatly reduces the cost of analysis, and very
small volume of organic solvents can bring very high enrichment factor, so no
subsequent concentration steps are needed. Liquid–liquid microextraction integrates
sampling, separation, purification, and concentration into one step. It is especially
suitable for the analysis of trace phthalates in water samples. In addition, fewer
operational steps and less solvent consumption could reduce the risk of phthalates
contamination. The technique has been utilized for the extraction of analytes in
complex matrices, including water, food, and biological materials. But its extraction
efficiency and precision need further improvement.
Liquid–liquid microextraction can be classified into single drop microextraction,
dispersive liquid–liquid microextraction, solidified floating organic drop
microextraction, and hollow fiber liquid–liquid microextraction (Yamini et al. 2019).
Except single drop microextraction, hollow fiber liquid–liquid microextraction, dispersive liquid–liquid microextraction, and solidified floating organic drop
microextraction have been extensively applied in the extraction of phthalates in
water samples.
Hollow Fiber Liquid–Liquid Microextraction
Hollow fiber liquid–liquid microextraction is membrane-based liquid–liquid
microextraction which has become popular for their advantages over the other
microextraction methods, such as more effective sample cleanup, more stable
extraction procedures, greater enrichment factor, and negligible consumption of
organic solvents. It has a three-phase mode and two-phase mode. The three-phase
mode is an aqueous–organic–aqueous system in which the immobilized organic
solvent or supported liquid membrane is exposed to two aqueous phases of the sample solution and the aqueous acceptor phase located inside the hollow fiber. In
contrast, in the two-phase mode, the acceptor phase solution is also the same as an
organic solvent of supported liquid membrane, and both are immiscible in water
(Esrafili et al. 2018). In hollow fiber liquid–liquid microextraction, assisted by
magnetic stirring, the extraction time is commonly more than tens of minutes
(Sarafraz-Yazdi and Amiri 2010). The method needs a special extraction device,
and the operational requirements are relatively high. Toluene and ionic liquid
[BMIm]PF6 have been utilized as extractants by many researchers to extract
phthalates in water samples. For example, Psillakis et al. (Psillakis and Kalogerakis
548
C. Sun et al.