Three-Phase LPME

In three-phase LPME, the SLM separates the sample matrix from the acceptor phase, which is situated in the lumen of the frher. Extraction is based not only on the partition of the uncharged analyte between the donor phase and the SLM (f SLM/sampie) ut also Ott thc partition of the analytes between the SLM and the acceptor phase (fCacceptor/SLM)- As will he discussed later, adjustment ofthe pH in the acceptor phase to charge the analytes improves the extraction recoveries, since it influences the partition of the analyte between the SLM and the acceptor phase. 

Three-phase LPME can be fully compatible with HPLC when the acceptor phase has a pH that does not damage the column (for silica-based reversed-phase columns, between pH 2 and 8). Since it is an aqueous solution, it can directly be injected into the chromatographic system. 

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The table also shows that a three-phase LLE (organic extraction followed by back-extraction into aqueous phase) yields lower recoveries and enrichment compared to three-phase LPME, as reflected in peak heights from the two techniques as shown in Figure 1.29. Furthermore, three-phase LLE is sensitive to the magnitude of Ka/org and LPME is not. 

Calculated Recovery and Enrichment in Three-Phase LPME and Simple LLE at Different Ka/d Values... 

FIGURE 1.29 Three-phase LPME versus three-phase LLE of promethazine.154 (Reproduced with permission from Elsevier.)... 

FIGURE 1.30 Simple experimental set-up for three-phase LPME.155 (Reproduced with permission from the Royal Society of Chemistry.)... 

Depending on the nature of the analytes and the chromatographic system needed to analyze the extracts, LPME can be carried out in a two- or three-phase mode. In both two- and three-phase LPME, diffusion of the analyte in the sample matrix and in the SLM is of great importance. Good diffusion and agitation (of the LPM E device) lead to improved extraction. Ideally, the extraction comes to an end when an overall equilibrium is reached. 

Factors affecting the extraction recovery are pH of the donor phase, pH of the acceptor phase (in case of a three-phase LPME), composition of the SLM, and extraction time. 

To improve extraction recoveries in a three-phase LPME system, the partitioning of the analyte between the SLM and the acceptor phase can be influenced greatly by adjusting the pH such that basic and/or acidic analytes become charged. In this way the partition equilibrium changes, and most of the analytes move from the organic phase (SLM) to the aqueous acceptor phase. In practice, this means a pH in the acceptor phase is ideally 2 pH units below the value for a base and 2 pH units above the pf value for an add. 

At present, LPME is still a noncommercial microextraction technique and many different SLM compositions have been used. Generally, for nonionized compounds, 1-octanol and toluene have been mostly used in two-phase LPME, while 1-octanol and dihexyl ether have been mostly used in three-phase LPME. In some cases, carriers or ion pair reagents are added to the sample phase to extract charged analytes. An example of this is the addition of aliphatic sulfonic add to the donor phase at pH 7. At this pH, it will form ion pairs with positively charged basic compounds. The neutral ion pair is extracted into the SLM. At the interface between the SLM and the acceptor phase, which exhibits a low pH, the basic analytes are released through protonation of the aliphatic suFonic acid (Figure 9.12). 

LPME has been accomplished either by extraction into a small droplet of organic solvent hanging at the end of a microsyringe needle [181-191] (microdrop) or into small volumes of acceptor solution present inside the lumen of porous hollow fibers [192-194]. In both the microdrop concept and in the hollow fiber format, the analytes of interest are extracted and preconcentrated into a few microliters of appropriate solvents. Because of this, LPME may be very effective for analyte enrichment and may result in a major reduction in the use of organic solvents. A detailed review based on hanging droplets in two- and three-phase LPMEs has been presented by Psillakis and Kalogerakis [195]. 

Bardstu, K.F., Ho, T.S., Rasmussen, K.E., Pedersen-Bjergaard, S. and Jonsson, J.A. (2007) Supported liquid membranes in hollow fiber liquid-phase microextraction (LPME) — Practical considerations in the three-phase mode. Journal of Separation Science, 30, 1364. 

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