Liquid membranes extraction techniques

In all liquid membrane extraction techniques, the membrane is an organic liquid, which is in contact with the aqueous sample. Analytes are extracted either by simple partitioning of uncharged species into the organic phase or by the action of some extractant, a compound present in the membrane liquid which can form complexes with the analyte, thereby facilitating its transport into the membrane liquid. So far, this is in principle the same as classical LLE. The difference between the various liquid membrane extraction techniques refers mainly to the acceptor side of the membrane. 

In all liquid membrane extraction techniques, the membrane is an organic liquid, which is in contact with the aqneous sample. Analytes are extracted either by simple partitioning... 

Liu J-F, Chao J-B, Wen M-J, Jiang G-B. Automatic trace-enrichment of bisphenol A by a novel continuous flow liquid membrane extraction technique. J Sep Sci 2001 24 874-878. 

Supported liquid membrane extraction techniques employ either two or three phases, with simultaneous forward- and back-extraction in the latter configuration. The aqueous sample phase is separated from the bulk organic or an aqueous receiver phase by a porous polymer membrane, in the form of either a flat sheet or a hollow fiber that has been impregnated with the organic solvent phase. The sample phase is continuously pumped, the receiver phase may be stagnant or pumped, and the organic phase in the membrane pores is stagnant and reusable [8-10]. 

Classical LLEs have also been replaced by membrane extractions such as SLM (supported liquid membrane extraction), MMLLE (microporous membrane liquid-liquid extraction) and MESI (membrane extraction with a sorbent interface). All of these techniques use a nonporous membrane, involving partitioning of the analytes [499]. SLM is a sample handling technique which can be used for selective extraction of a particular class of compounds from complex (aqueous) matrices [500]. Membrane extraction with a sorbent interface (MESI) is suitable for VOC analysis (e.g. in a MESI- xGC-TCD configuration) [501,502]. 

LC techniques are widely diffused for the determination of hydrophilic but not volatile and thermally unstable pesticides. Since the European Community Directive [68] indicates 0.1 pg L" as the concentration threshold level for a single pesticide in waters destined for human consumption, to quantify these concentration levels, suitable pre-concentration and extraction procedures must be generally performed prior to the HPLC determination. The extraction methods are based on LLE, MAE, on-line continuous flow liquid membrane extraction (CFLME), and mainly on SPE and SPME. Many SPE procedures are used the packing materials are graphitized carbon, ODS, styrene-divinylbenzene co-polymers, or selective phases based on immunoafflnity. The extraction can be performed on- and off-line, manually, or in a semi-automated way. 

In membrane extraction, the treated solution and the extractant/solvent are separated from each other by means of a solid or liquid membrane. The technique is applied primarily in three areas wastewater treatment (e.g., removal of pollutants or recovery of trace components), biotechnology (e.g., removal of products from fermentation broths or separation of enantiomers), and analytical chemistry (e.g., online monitoring of pollutant concentrations in wastewater). Figure 18a shows schematically an industrial hollow fiber-based pertraction unit for water treatment, according to the TNO technology (263). The unit can be integrated with a him evaporator to enable the release of pollutants in pure form (Figure 18b). 

Sandahl, M., L. Mathiasson, and J.A. Jonsson. 2000. Determination of thiophanate-methyl and its metabolites at trace level in spiked natural water using the supported liquid membrane extraction and the microporous membrane liquid-liquid extraction techniques combined on-line with high-performance liquid chromatography. J. Chromatogr. A 893 123-131. 

Membrane Techniques The interest in membrane techniques for sample preparation arose in the 1980s. Extraction selectivity makes membrane techniques an alternative to the typical sample enrichment methods of the 1990s. Different membrane systems were designed and introduced into analytical practice some more prominent examples are polymeric membrane extraction (PME), microporous membrane liquid-liquid extraction (MMLLE), and supported liquid membrane extraction (SEME) [106, 107]. Membrane-assisted solvent extraction (MASE) coupled with GC-MS is another example of a system that allows analysis of organic pollutants in environmental samples [108-111] ... 

For two-phase liquid membrane extraction, (MMLLE), the basic principles are more simple than those for SLM, as there is only one phase boundary involved in the extraction, usually from an aqueous to an organic phase, which is chemically equivalent to LLE in a separatory funnel, etc. The extraction is driven by the difference in chemical potential of the analytes in organic solvent and in aqueous solution, which is described as a partition coefficient. In many cases, the octanol-water partition coefficient (log Kqw) is considered as estimates for the partition coefficient in MMLLE and LLE, even if the organic solvents used usually are other than octanol. The techniques work best for relatively nonpolar compounds, having values of log A ow > 3. 

Liu J-E, Chao J-B, and Jiang G-B. Continuous flow liquid membrane extraction A novel automatic trace-enrichment technique based on continuous flow liquid-liquid extraction combined with supported liquid membrane. Anal. Chim. Acta 2002 455 93-101. 

Dzygiel P and Wieczorek P. Extraction of glyphosate by supported liquid membrane technique. J. Chromatogr. A 2000 889 93-98. Dzygiel P and Wieczorek P. Supported liquid membrane extraction of glyphosate metabolites. J. Sep. Sci. 2001 24 561-566. 

Chimuka L, Cukrowska E, Soko L, and Naicker K. Supported-liquid membrane extraction as a selective sample preparation technique for monitoring uranium in complex matrix samples. J. Sep. Sci. 2003 26 601-608. 

Norberg J, Zander A, Jonsson JA. Microporous membrane liquid-hquid extraction technique combined with gas chromatography mass spectrometry for the determination of organotin compounds. Anal Chim Acta 2000 404 319-328. 

There are different variations of the MASX technique, including supported liquid membrane extraction (SLM), microporous membrane liquid-liquid extraction (MMLLE), polymeric membrane extraction (PME) and membrane extraction with a sorbent interface (MESI). These techniques will be briefly described below. 

The supported liquid membrane extraction is generally one of the most selective membrane based extraction techniques. Selectivity is tuned by adjusting the foctors described above. However, like other membrane based extraction techniques, so far it has been applied to mostly environmental and biological samples as seen in review articles (30,31,34). 

Sample preparation by means of liquid membrane extraction is a technique that in essence contains two LLE extractions in one step. The setup is easily automated, and sample preparation is performed in a closed system, thus minimizing the risk for contamination and losses during the process. Because the extraction is made from an aqueous phase (donor) to a second, also aqueous phase (acceptor), further enrichment on a precolumn is possible before injection into the LC apparatus. Liquid membranes were used for enrichment of metsulfuron-methyl and chlorsulfuron from clean aqueous samples and natural waters. A similar device was developed by a Chinese... 

FIGURE 13.10 Schematic diagram of the CFLME system. (From Anal. Chim. Acta, 455, Liu, J.-F., Chao, J.-B., and Jiang, G.-B., Continnons flow liquid membrane extraction A novel antomatic trace-enrichment technique based on continnous flow liqnid-hqnid extraction combined with snpported liqnid membrane, 93-101, Copyright 2002, with permission from Elsevier.) S, sample solntion R, 0.5 M snlfnric acid O, organic solvent A, acceptor W, waste PI, P2, peristaltic pnmps P3, piston pnmp MC, mixing coil EC, extraction coil VI, V2, 6-port valves SLM, SLM device PDA, detector, 240 nm. 

Separation science plays a pivotal role in many hydromet-allurgical processes, inclnding industrial wastewater treatment [1-7]. Ont of the various separation techniques, solvent extraction, ion exchange, and precipitation are the workhorse for various industrial applications. At the same time, there is a growing interest in membrane-based separation methods that are considered environmentally benign [7-10]. A combination of membrane separation and solvent extraction techniques, known as the liquid membrane (LM) technique, has drawn considerable attention for the separation scientists and technologists. LM-based separation methods are associated... 

The eflect of temperature deserves special mention since it tepresenb a major advantage for the liquid-membrane process. Figure 19.4-5 compares the influence of temperature on extraction of uranium by the SX and LM techniques. In the case of liquid membranes, extraction efficiency increases with temperature... 

Supported liquid membrane extraction (SLME) is emerging as a fast and efficient sample preparation technique. Aromatic aminophosphonate isolation from water samples based on SLME allowed the identification and study of the operational parameters (pEI and ionic strength of the aqueous phase, composition of the membrane phase, and concentration of analytes) as well as the structure-extraction efficiency relationship. 

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