Membrane extraction devices

In another type of membrane extraction devices, porous polypropylene hollow fibers are used, often in a disposable way, which minimizes carryover problems and reduces costs [26-33]. On the other hand, manual manipulations are needed, limiting the possibility for automation. With these devices, the extraction can be carried out in a static mode, either in large sample volumes, where the extraction is not intended to be complete, or in small volumes aiming for complete extraction. Usually, stirring is applied to increase the speed of mass transfer. Some typical practical arrangements are shown in Figure 12.2. This type of SLM extraction is often called hollow fiber liquid phase microextraction, or three-phase liquid phase microextraction or two-phase liquid phase microextraction but the terminology in this active field of research has not been settled. Also hollow fibers can be connected in flow systems [34,35]. 

Fig. 4. a Schematic of porous membrane-based separation of immiscible liquids with different wetting characteristics, b Liquid-liquid extraction device (Kralj et al. 2007)... 

R 18] A modular set of devices was developed within the pChemTec project introduced above. It consists of a base plate which is identical for all four devices. This base plate acts as the fluidic interface to the piping and is equipped with a micro structured mixer. The base plate can be combined with a heated tube to deliver a mixer-tube reactor. A combination with a porous tube delivers a degasser unit. A combination with a membrane unit (not shown) or a settler results in an extraction device (Figure 4.58). 

To simplify the above-mentioned MMLLE systems and, unlike the automated flowing MMLLE, the nonautomated, nonflowing design of MMLLE is simple to prepare manually and is an easy-to-use extraction procedure that is always done off-line prior to GC analysis. In this context, only a short piece of HF membrane is employed as an extraction device after the HF lumen and pores96 or only the pores97 have been filled with an appropriate organic solvent, the membrane is immediately immersed in the aqueous sample. The principle of this two-phase HF-MMLLE system is also called HF liquid-phase microextraction (HF-LPME) and will be briefly commented on in the next section. 

Luo, Y.Z. and J. Pawliszyn. 2000. Membrane extraction with a sorbent interface for headspace monitoring of aqueous samples using a cap sampling device. Anal. Chem. 72 1058-1063. 

Collection of analytes from a stream of gas on a sorbent bed and their release by thermal decomposition priori to the final determination stage, Membrane Extraction with Sorbent ) Interface - MESI), (Hollow Fiber Sampling Analysis -HFSA), (On-line Membrane Extraction Microtrap -OLME), (Membrane Purge and Trap - MPT), (Pulse Introduction Membrane Extraction - PIME), (Semi Permeable Membrane Devices - SPMD)... 

Membrane extraction with sorbent interface (MESI) is an interesting example of an extraction device, which is the most useful system for interfacing with GC. In this approach, the donor phase is a gas or a liquid sample, and the acceptor phase is a gas. The volatiles are continuously trapped on sorbent and then desorbed into GC [112]. Another solution is a combination of off-line GC-MESI through a cryogenic trap, which allows preparation of environmental samples in the field and performance of GC analysis after transportation to the laboratory [113,114]. MESI allows the extraction of volatile and relatively nonpolar analytes. 

Processes for production of ethanol and acetone-butanol-ethanol mixture from fermentation products in membrane contactor devices were presented in Refs. [88,89]. Recovery of butanol from fermentation was reported in Ref. [90]. Use of composite membrane in a membrane reactor to separate and recover valuable biotechnology products was discussed in Refs. [91,92]. A case study on using membrane contactor modules to extract small molecular weight compounds of interest to pharmaceutical industry was shown in Ref. [93]. Extraction of protein and separation of racemic protein mixtures were discussed in Refs. [94,95]. Extractions of ethanol and lactic acid by membrane solvent extraction are reported in Refs. [96,97]. A membrane-based solvent extraction and stripping process was discussed in Ref. [98] for recovery of Phenylalanine. Extraction of aroma compounds from aqueous feed solutions into sunflower oil was investigated in Ref. [99]. 

A process to separate naphthenes from paraffins is claimed in Ref. [103]. It involves the use of a polar solvent for separation in a microporous membrane device. Use of membrane extraction to remove p-nitrophenol in wastewater from dye and pesticide synthesis was investigated in Ref. [104]. Removal of nonvolatile pesticide components from water is presented in Ref. [105]. Removal of several important organic pollutants such as phenol, chlorophenol, nitrobenzene, toluene, and acrylonitrile from wastewater was investigated in Ref. [106]. 

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. 

FIGURE 12.2 Hollow-fiber devices for membrane extraction, (a) Hollow-fiber loops for equilibrium extraction redrawn after Liu et al. (From Liu, J.-F., Jbnsson, J.A., and Mayer, P., Anal. Chem., 77, 4800, 2005.) (b) Liquid-phase microextraction after Pedersen-Bjergaard and Rasmussen. (From Grpuhaug Halvorsen, T., Pedersen-Bjergaard, S., Reubsaet, J.L.E., and Rasmussen, K.E., J. Sep. ScL, 24, 615, 2001. With permission.) (c) Syringe-based hollow fiber LPME. (Erom Zhao, L. and Lee, H.K., Anal. Chem., 74, 2486, 2002. Copyright 2002 American Chemical Society. With permission.)... 

Simple and cheap membrane extraction flow systems for relatively large sample volumes can be built up around a peristaltic pump. An example of such a system is seen in Figure 12.6a. Here, the sample is pumped through the donor channel and the acceptor phase is manually removed by the use of a syringe after each extraction. Such systems have been used both for laboratory work [69,70] and for sampling in natural waters [71]. An tutorial for the operation of this type of devices has been published [8]. 

After this isolation step, analytes are usually concentrated in sorbent traps (membrane extraction sorbent interface [MESI]) and thermally desorbed in a cryofocusing device or directly in a gas chromatographic column. In other cases,no sorbent trap is used, analytes are desorbed directly from the membrane unit cell (thermal membrane desorption [TMD]) and carried by a gas stream onto the front of the chromatographic column of the analytical system. 

A passive sampling device was constructed using the commercially available Twister sorbent stir bar by enclosing it inside a dialysis membrane. This device has been called a membrane-enclosed sorptive coating sampler or MESCO. Extraction efficiencies were three orders of magnitude lower than for SPMD due a lower sampling rate, however, the sensitivity was comparable because all of the collected analyte is desorbed into the GC whereas in SPMD, only a small sample in injected for analysis. Twister stir bars are also much smaller and can be deployed less conspicuously. 

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... 

The extraction of metals based on a membrane contactor system with conventional solvents is a process widely studied using different configurations, extractants, and extraction solvents. One of the upcoming applications of membrane contactors is supercritical extraction. This process is called porocritical extraction. Porocritical process or porocritical extraction is a commercial supercritical fluid extraction (SFE) technique that utilizes an hollow fiber membrane contactor (HFMC) to contact two phases for the purpose of separation. As an improvement, the extraction of Cu + from aqueous solutions by means of dense gas extraction was achieved by using a hollow fiber membrane contactor device [7]. The authors... 

Nonsupported Liquid Membrane Extraction In addition to SLM, which is the most commonly used three-phase extraction principle, at least in analytical chemistry, also other ways of placing an organic phase between two aqueous phases are known. In the classical bulk liquid membrane (BLM) setups, U-tubes or similar devices are used to confine bulk volumes of organic liquids between two aqueous phases. This type of devices is very little used for sample preparation in analytical chemistry, as the extraction process becomes slow and the emichment factors possible are very limited. 

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. 

Gupta, S.K., Rathore, N.S., Sonawane, J.V., Pabby, A.K., Singh, R.R., Venugopalan, A.K., Dey, P.K., Venkatramani, B., Hollow fiber membrane contactor Novel extraction device for plutonium extraction, BARC Newsletter 181, 2002 (Founder s Day Special Issue). 

Extraction devices using parallel flow combined with a membrane have also been developed. The fluids of two immiscible phases are separated by a membrane. Cai et al. performed extraction of butyl Rhodamine B (BRB) in aqueous solution into isobutanol. Polytetrafluoroethylene (PTFE) membranes with different pore sizes of 0.2, 0.45 and 1.0 pm (60 pm thick, 70-80% porosity) was placed between the two phases [10]. Aqueous flow rates in the range 22-65 pL min were investigated with a solvent flow-rate of 8.2 zpL min. The extraction efficiency improves with increasing membrane pore size and aqueous flow rate. 

Wang et al. developed an extraction device with a Celgard 2400 microporous polypropylene membrane [11]. It has an average thickness of 25 pm, the pore size is 0.05 pm and 38% of the surface is porous. The channel is made of polycarbonate and... 

LLE, liquid-liquid extraction MAE, microwave-assisted extraction SEE, solid-phase extraction SPME, solid-phase microextraction LPME, liquid-phase microextraction SOME, single-drop microextraction D-LLLME, dynamic liquid-liquid-liquid microextraction SEE, supercritical fluid extraction MIP, molecularly imprinted polymers sorbent SPMD, device for semipermeable membrane extraction PDMS, polydimethylsiloxane coated fiber PA, polyacrylate coated fiber CW-DMS, Carbowax-divinylbenzene fiber PDMS-DVB, polydimethylsiloxane divinylbenzene fiber CAR-PDMS, Carboxen-polydimethylsiloxane coated fiber DVB-CAR-PDMS, divinylbenzene Carboxen-polydimethylsiloxane coated fiber CW-TPR, Carbowax-template resin HS-SPME, headspace solid-phase microextraction MA-HS-SPME, microwave-assisted headspace-solid-phase microextraction HEM, porous hollow fiber membrane PEl-PPP, polydydroxylated polyparaphenylene. 

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