Sample preparation supported liquid extraction

Sandahl, M., L. Mathiasson, and J.A. Jonsson. 2002. On-line automated sample preparation for liquid chromatography using parallel supported liquid membrane extraction and microporous membrane liquid-liquid extraction. J. Chromatogr. A 975 211-217. 

Maquille et al. [121], due to the physicochemical properties (i.e., polarity and ionization state) of the investigated drugs (opiates, amphetamine, cocaine and metabolites), concluded that LLE should be selected. To automate the sample preparation procedure, this team proposed urine extraction by supported liquid-liquid extraction (SLE), a promising technique that appeared in 1997 [122], which can be easily automated in a 96-well plate format. It has been demonstrated that matrix effect is significantly minimized. 

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] ... 

Classically, flat-sheet porous PTFE or polypropylene membranes are used as support for the membrane liquid and mounted in holders (cells, contactors) permitting one flow channel on each side of the membrane [1,3,6,8,25]. See Figure 12.1. Such membrane units are typically operated in flow systems and in principle apphcable to aU versions of membrane extraction for analytical sample preparation or sampling. Such a setup can be easily interfaced with different analytical instmments, such as HPLC and various spectrometric instmments, and thereby provides good possibdities for automated operation. Drawbacks of this type of devices are relatively large costs and limited availability, as well as some carryover and memory problems as the membrane units are utilized many times, necessitating cleaning between each extraction. 

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. 

Solid-phase extraction (SPE) is the method of sample preparation that concentrates and purifies analytes from solution by sorption onto a disposable solid-phase cartridge, followed by elution of the analyte with an appropriate solvent. The SPE technique was developed in the mid-1970s as an alternative means of liquid-liquid extraction but become particularly attractive for its automation, parallel purification, and pre-concentration. Since 1995, SPE has been applied in various fields, environmental, food sciences, biomedical analyses, pharmaceutical analyses, and organic synthesis. " There are a numbers of publications and reviews on the subjects of development of new solid-phase supporting materials, instrumentation and device, techniques, and theoretical aspect. ... 

Hgure 8 Supported liquid membrane and microporous membrane liquid-liquid extraction. (Adapted from Jonsson JA and Mathiasson L (1999) Liquid membrane extraction in analytical sample preparation. Trends in Analytical Chemistry 18 318-324.)... 

Figure 10 Enrichment and separation setup for SLM extraction. A - peristaitic pump, B - membrane device with hoiiow fiber instaiied into fused siiica capiiiary, C - iniet on the donor side, D - washing iine used after enrichment, E - microinjection pump to transport the acceptor into ioop F, G - Cis packed capillary column, H - detector window, I - splitter unit, J - high pressure pump. (Adapted from Thordarson E, Palmarsdottir S, Mathiasson L, and Jonsson JA (1996) Sample preparation using a miniaturized supported liquid membrane device connected on-line to packed capillary liquid chromatography. Analytical Chemistry 68 2559-2563.)...

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. 

The samples are extracted with hexane in such a way that the water, or water-sediment mixture, and the container itself are exposed to the solvent. Improved analytical columns are used to analyze the extracts by electron capture gas chromatography. These columns are prepared by coating the support with Carbowax 20M, prior to coating with the selected liquid substrate. Using two different types of columns will substantially increase confidence in the results through the different partitioning action of two liquid substrates. 

When the analytes are to be retained in a sorbent, the sample (which can be solid, semi-solid, liquid or gaseous) is inserted in the solid state into the extraction cell. Samples in the latter three forms are supported on an appropriate material in order to ensure effective attack by the supercritical fluid. Solid supports are not used for liquid, gaseous and semi-solid samples only, however. Some research work conducted so far on solid samples has involved not natural samples but synthetic ones prepared from a selected sorbent (a natural matrix where the presence of the analytes of interest was previously excluded or a synthetic support such as polyurethane foam or glass wool) with which a solution containing the analytes was homogenized. Quantitative evaporation of the analyte solvent is mandatory as any residual solvent may alter the polarity of the supercritical fluid and hence its action to an extent dependent on the particular fluid and solvent properties, and also on the amount of solvent retained. 

Audunsson [29] reported on a sandwich-type extraction module equipped with liquid membranes, prepared by immersing hydrophobic microporous membranes (e.g. PTFE membranes) in organic solvents for about 15 min. The inert men ranes then act as supports for the immobilized solvent. When an aqueous sample passes by the membrane, non-ionic components in the sample are extracted into the hydrophobic liquid film and transferred into an appropriate acceptor solution on the other side of the membrane. When the acceptor remains stagnant while the sample flows continuous ) for a defined period, a preconcentration is effected in the acceptor solution, which is subsequently transferred to the detector. The procedure is equivalent to extraction and back-extraction in a single step. More details on such a system used for sample cleanup in gas-liquid chromatography is presented in Sec. 3.7. 

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