MMLLE extraction

The impact of several factors on the MMLLE extraction yield of PAHs in water has been comprehensively studied using a flowing FS-MMLLE system and off-line analysis with GC-flame ionization detection (GC-FID).84 The flowing FS-MMLLE procedure combined with off-line GC-mass spectrometry (GC-MS) analysis has been utilized for the extraction of nonionic and derivatized ionic organotin compounds in river water.85... 

For gas chromatography, the most suitable membrane extraction technique is MMLLE. The organic acceptor is better compatible with GC than with HPLC, as are the analytes that are best extracted in such a system, i.e., relatively hydrophobic compounds. A new development is the ESy instrument (ESyTech AB, Lund, Sweden) [76-78] where an MMLLE extraction in microscale (1 mL extracted into a volume ca 1 p,L) is automatically performed and the organic extract is directly injected into the GC by an injection needle, directly connected to the extraction cell. See Figure 12.7. 

Classical LLEs have also been replaced by membrane extractions such as SLM (supported liquid membrane extraction), MMLLE (microporous membrane liquid-liquid extraction) and MESI (membrane extraction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]. 

MMLLE Microporous membrane liquid-liquid extraction... 

Microporous membrane liquid-liquid extraction (MMLLE) is a two-phase extraction setup. In MMLLE procedures, the membrane material and format (FS and HF), extraction units, and system configurations are identical to those described in SLM (Section 4.4.1.2).63 The two-phase HF-MMLLE system is identical to that used in Section 4.4.3, although sometimes with minor differences. In contrast to three-phase SLM extraction, MMLLE employs a microporous membrane as a miniaturized barrier between two different phases (aqueous and organic). One of the phases is organic, filling both the membrane pores (thus making the membrane nonporous) and the compartment on one side of the membrane (acceptor side). The other phase is the aqueous sample on the other side of the membrane (donor side). In this way, the two-phase MMLLE system is highly suited to the extraction of hydrophobic compounds (log Ko/w > 4) and can thus be considered a technique complimentary to SLM in which polar analytes (2 < log Ko/w < 4) can be extracted. 

On-Line Systems Flowing MMLLE systems have been established in different layouts with automation and on-line hyphenation to GC and HPLC analysis. An automated on-line FS-MMLLE-GC system with a loop-type interface compatible with LVI was used for the extraction of pesticides and PAHs in surface waters.86 In another study, pressurized hot water extraction (PH WE) was coupled on-line to a FS-MMLLE-GC-FID system and applied to the analysis of PAHs in soil, where MMLLE was used as a cleanup and concentration step of the PH WE extract prior to final GC analysis.87 In addition, an HF-MMLLE setup was incorporated in PHWE and GC, resulting in an online PHWE-HF-MMLLE-GC system, where the HF membrane module contained 10-100 HFs. The system served for the extraction and analysis of PAHs in soil and sediments ... 

Flowing FS-MMLLE with on-line hyphenation to FtPLC has also been investigated. Sandahl et al. were the first to interface FS-MMLLE with reversed-phase HPLC for the on-line extraction of methyl-thiophanate in natural water, obtaining an LOD of 0.5 pg L-1.89 Also, a parallel FS-SLM and FS-MMLLE design was coupled on-line to reverse-phase HPLC for the extraction of methyl-thiophanate (by MMLLE) and its metabolites (by SLM) in natural water.90 In addition, on-line coupling of FS-MMLLE and normal-phase HPLC has been successfully applied in the determination of vinclozolin (Ee =118 and LOD = 1 pg I. ) in surface water91 and of in-sample ion-paired cationic surfactants (Ee > 250 and LOD = 0.7-5 ug L-1) in river water and wastewater samples.92... 

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. 

A very simple HF-MMLLE configuration has been employed by flame-sealing the two ends of the HFs. The HFs were then soaked in n-undecane for a period of time so as to allow them to fill with solvent this makes simple HF-MMLLE devices. In this way, a single HF was utilized for the MMLLE of eight polybrominated diphenylethers (PBDEs) in 100 mL samples of tap, river, and leachate water. The analysis was done by manual injection of 2 pL of the HF lumen content into a splitless GC injector followed by GC-MS analysis in selected ion monitoring (SIM) mode. Under optimal HF-MMLLE conditions, the extraction was exhaustive (E = 57-104%), giving very good enrichment (Ee = 2800-5200-fold), very low LOD (<1.1 ng I. ), and relative recoveries of 85-110%. Two PBDEs were detected and quantified in leachate water at concentrations of 3.5 ng I. for BDE 153 and 23 ng L 1 for BDE 183.96... 

Membrane SLM MESI MMLLE MIMS PME HF-LPME MASE LGLME Headspace-solid phase dynamic extraction (HS-SPDE) Purge-and-membrane extraction Headspace/membrane extraction w. sorbent interface (HS-MESI) Headspace/membrane inlet mass spectrometry (HS-MIMS)... 

MMLLE Microporous membrane liquid-liquid extraction Nonporous (microporous) Aqueous/organic/organic Organic/organic/aqueous... 

Application of polymer membranes to separation of aqueous and organic phases in liquid-liquid extraction processes is called microporous membrane liquid-liquid extraction (MMLLE). An organic acceptor solvent, filling the pores of the hydro-phobic membrane, stays in direct contact with the aqueous phase near the membrane surface, where mass transfer takes place. This kind of extraction is similar to SEME, but takes place in a two-phase system and is slower and less selective because of the absence of carrier agent. Because the polymer membranes are insoluble, an arbitrary combination of aqueous and organic phase is possible and the extraction efficiency mainly depends on the partition coefficient. 

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] ... 

Two-phase liquid membrane Microporous membrane liquid—liquid extraction MMLLE [6,86]... 

As with three-phase membrane extraction, it is also possible here to work either with flat membranes, or with hollow-fiber membranes. In the first case, the technique is usually called microporous membrane liquid-liquid extraction (MMLLE), a name originating from Cussler [40]. With hollow fibers, it can be called two-phase liquid phase microextraction. 

A number of applications with polymeric membranes have been described. The most commonly used membrane material is silicon rubber or polyethylene. The possibility for both aq/polymer/aq extraction (including trapping in the acceptor, very similar to SLM extraction) and also, e.g., aq/polymer/org extraction (similar to MMLLE) has been demonstrated. 

In the aq/polymer/org situation, the organic solvent typically penetrates the polymer causing it to swell considerably, and the situation is very similar to that of MMLLE. With a fixed composition of the membrane, the possibilities for chemical tuning (such as application of carriers) of the separation process are greatly reduced compared to SLM extraction or MMLLE. Also, as diffusion coefficients in polymers are lower than in liquids, the mass transfer is slower, leading to slower extractions. On the other hand, as the membrane is virtually insoluble, any combination of aqueous and organic liquids can be used, and the entire system becomes very stable. 

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. 

Here, is the partition coefficient between the organic acceptor phase and the aqueous donor (sample) phase. For acidic or basic compounds ar> is given by Equation 12.3 or 12.4, while for non-chargeable compounds d = 1- Thus in MMLLE, the organic/aqueous partition coefficient directly governs the extraction. 

Membrane Extraction in Preconcentration, Sampling, and Trace Analysis 12.3.5 Differences between MMLLE and SLM... 

The MMLLE technique can be seen as a complement to the SLM extraction, permitting membrane-based extraction to be extended to further classes of compounds. Compared with SLM, MMLLE has the following characteristics ... 

Extract ends up in the organic solvent, not in water. Thus MMLLE is more easily interfaced to gas chromatography and NP-HPLC than to SLM, which is most compatible with reversed-phase HPLC. 

Note SLM, supported liquid membrane (aq/org/aq) MMLLE, microporous membrane liquid-liquid extraction (aq/org) PME, polymer membrane extraction (aq/polymer/org) MESI, membrane extraction with sorbent interface (aq (or gas)/polymer/gas/sorbent) CFLME, continuous flow liquid membrane extraction (aq/org (in flow)/aq) LPME2, two-phase liquid phase microextraction in hoUow fibers (aq/org) LPME3, three-phase liquid phase microextraction in hollow fibers (aq/org/aq). 

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. 

In MMLLE, the acceptor and membrane phases are both organic, while the donor phase is aqueous. This technique is more suitable than SLM for extracting non-polar compounds. The chemical principle of MMLLE is the same as that of LLE, but performed in a dynamic flow-system, which makes it easy to hyphenate MMLLE with other separation techniques such as GC (68). For example, HyOtylainen and colleagues coupled PHWE on-line with MMLLE and GC for... 

Microporous membrane liquid- liquid extraction MMLLE Aq/org/org lonizable and neutral e.g. triazines in oil (38). 

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