LPME

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. 

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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.)... 

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.)... 

FIGU RE 1.38 Chromatograms of LPME-treated drug-free plasma, mirtazapine enantiomers, and mefloquine.156 Chromatograms refer to drug-free plasma (A) plasma spiked with 62.5 ng mL-1 of (+)-(5)-mirtazapine (2) and (-)-(i )-mirtazapine (3) and 500 ng rnL-1 of (R, S)-mefloquine (1,4) (B) plasma sample from a patient treated with 15 mg/day of rac-mirtazapine (C). All samples were pre-treated by LPME. The analysis was performed on a Chiralpak AD column using hexane ethanol (98 2, v/v) plus 0.1% diethylamine at a flow rate of 1.5 mL min-1, A = 292 nm. (+)-(5)-mirtazapine (2), (-)-(f )-mirtazapine (3) and (1, 4) internal standard. (Reproduced with permission from Elsevier.)... 

So far, LSE is the most popular for extracting contaminants in food. However, over the last years LPME in its different application modes (single drop microextraction, dispersive liquid-liquid microextraction and hollow fiber-LPME) has been increasingly applied to food analysis because of its simplicity, effectiveness, rapidity, and low consumption of organic solvents. Different applications have been recently reviewed by Asensio-Ramos et al. [112]... 

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

The second variant of LPME uses a disposable, porous, hollow microfibre. In this case, the sample solution is placed in a vial equipped with a modified lid. A loop of microfibre passes in and out of the lid, such that the loop dips into the sample solution in the vial. The extraction solvent is injected into the capillary fibre from the outside. Again, stirring may be used. After an equilibration period, where solutes are able to traverse the fibre wall into the extraction solvent, the contents of the capillary are withdrawn and injected onto the chromatographic system. The two forms of this technique are illustrated in Figure 4.9. 

The majority of the applications of LPME have been analysis of drugs in biofluids [35 0] or of pollutants in water samples [41 5]. However, it has been successfully applied to the analysis of triphenylphosphine oxide (TPPO) as an impurity in pharmaceuticals [46]. 

It is now possible to purchase commercially available systems for achieving something akin to LPME. In this instance, instead of a capillary microflbre, the extraction solvent is contained in a porous insert that sits inside the sample vial. This can be sampled automatically using the autosampler syringe after an appropriate incubation time. 

S. Myung, S. Yoon and M. Kim, Analysis of benzene ethylamine derivatives in urine using the programmable dynamic liquid-phase microextraction (LPME) device. Analyst, 2003,128(12), 1443-1446. 

S. Pedersen-Bjergaard, T. S. Ho and K. E. Rasmussen, Fundamental studies on selectivity in 3-phase liquid-phase microextraction (LPME) of basic drugs. Journal of Separation... 

S. M. Richoll and I. Colon, Liquid-phase Microextraction (LPME) A Novel Approach to the Isolation and Quantitation of Low Level Impurities in Pharmaceutical Applications. Abstracts of Pittcon 2004. 

The analysis of ILs may afford considerable insight into the physicochemical properties underlying the rich potential interaction chemistries of ILs [14] and suggest possibilities for future applications. Simultaneously, the unique features of ILs provide some intriguing new possibilities in the area of separations that have yet to be realized. Hence, topics to be covered in this chapter include analysis of ILs by LC, applications of ILs in liquid-phase microextraction (LPME), in high-performance LC (HPLC) as mobile-phase additives, and in capillary electrophoresis (CE) as buffer additives as well as applications of surface-confined ILs (SCIL) as novel stationary phases for LC. 

Thus far, SPME methods have had only limited success in isolating polar organics (e.g., chlorophenols [30] and formaldehyde [31]) or ions [32,33] from aqueous mixtures. However, the tunable hydrophobicity and multimodal potential interaction chemistries of ILs suggest potential applications in LPME. Further, their high viscosity coupled with their minimal vapor pressure promotes stable droplet formation. Figure 5.2 illustrates the experimental setup for LPME. In addition, analyte recovery can be performed simply by injecting the droplet onto a liquid chromatographic column. 

While work in the application of ILs to LPME is thus far limited, it was reported that although the viscosity of ILs generally allows for larger droplet size, partitioning of analytes into the droplet was inhibited because of their... 

To enhance extraction efficiency in the SME system (also called single-drop microextraction [SDME]), He and Lee developed a procedure termed liquid-phase microextraction (LPME),89 which, in its static mode, resembles the SME system. 

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. 

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