Two-Phase LPME

FIGURE 1.36 Experimental set up for two-phase LPME.156 (Reproduced with permission from Elsevier.)... 

FIG U RE 1.37 Effects of extraction time and fiber length on two-phase LPME of mirtazapine stereoisomers.156 Influence of extraction time (A) and acceptor-to-donor phases volume ratio (B) on the efficiency of LPME. Plots for the (+)-(5)-mirtazapine (white bars) and (-)-(R)-mirtazapine (black bars) enantiomers (response in area counts). Extraction conditions (A) 1 mL plasma sample 0.1 mL 10 M NaOH 3.0 ml deionized water 7.0 cm fiber length 22 /rL toluene (B) 30 min of extraction 1 mL plasma sample 0.1 mL 10 M NaOH 3.0 mL deionized water toluene. (Reproduced with permission from Elsevier.)... 

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). 

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. 

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. 

The chemical principle of two-phase HF-LPME is identical to that in the FS- and HF-MMLLE systems discussed in the previous section. Thus, it is merely a case of different names for similar technical approaches. The intention of this section is to give a brief synopsis describing the most recent and important developments in this technique, and its potential automation. 

The two-phase HF-MMLLE/HF-LPME procedure with potential automation is expected to achieve wide acceptance among researchers and experimentalists as it is direct and easy-to-use. 

For two-phase HF-LPME, the lumen, as well as the porous wall of the fiber, is fiUed with the organic solvent. Since the fibers are hydrophobic, organic solvents easily diffuse through the wall hence, the most convenient way to fill the fibers is to seal them in both ends by heat and place... 

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. 

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]. 

More recently, a miniaturised version of liquid-liquid extraction has been developed, known as liquid-phase microextraction (LPME). Two main forms of this... 

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