Partitioning, between phases, separation techniques based

Liquid chromatography (Hostettman et al., 1986) in its many forms is a separation technique based on the polarity of the analytes and their partition between the mobile and stationary phases, and is therefore complementary to fractional distillation, which separates materials by their boiling point. The usual sequence for fractionating an essential oil or extract is to distil it first and then apply liquid chromatography to the distillation fractions as a further fractionation procedure, rather than as an analytical tool. The selectivity of the technique is achieved by choosing a stationary phase, usually from the various activities of silica gel, and varying the polarity of the mobile phase, the solvent, by mixing a non-polar component (such as hexane or pentane) with different amounts of a more polar component (such as diethyl ether, ethyl acetate or chloroform). 

In microfluidic-based systems, material is transported within microstructures (of typical dimensions of 10-500 pm) where separations, reactions, and other processes occur. Focus has been on the realization of the traditional separation techniques (electrophoresis, chromatography, isoelectric focusing, etc.) and reactions in the microchip format. The principles of separation, as in the conventional formats, are based on differences in mass and charge (thus mobility) and partitioning between phases. However, advantages associated with the small dimensions provide superior performance. For example, the higher surface to volume ratio arising from the smaller dimensions results in lower heat and mass transfer resistances and thus an improved performance. 

Chromatography is a general term applied to a variety of separation techniques based on the sample partitioning between a moving phase, which can be a gas or a liquid, and a stationary phase, which may be either liquid or solid. 

The most important class of separation techniques is based on the selective partitioning of the analyte or interferent between two immiscible phases. When a phase containing a solute, S, is brought into contact with a second phase, the solute partitions itself between the two phases. 

Capillary Electrochromatography Another approach to separating neutral species is capillary electrochromatography (CEC). In this technique the capillary tubing is packed with 1.5-3-pm silica particles coated with a bonded, nonpolar stationary phase. Neutral species separate based on their ability to partition between the stationary phase and the buffer solution (which, due to electroosmotic flow, is the mobile phase). Separations are similar to the analogous HPLC separation, but without the need for high-pressure pumps, furthermore, efficiency in CEC is better than in HPLC, with shorter analysis times. 

In CZE, differences in the viscous drag of neutral solutes, primarily as a result of size differences, can provide for their separation (3). However, these differences are usually very small and, consequently, the technique is not very useful for separating neutral compounds. With the MECC technique, a surfactant is added to the mobile phase at a concentration above its critical micelle concentration. The resulting micelles provide an effective mechanism for separating neutral compounds. Neutral solutes are separated based on their differential partitioning between an electro-osmotically-pumped mobile phase and the hydrophobic interior of the... 

The predominant mode of HPLC, reversed phase, involves the separation of material based on the partitioning between a relatively polar mobile phase and a nonpolar stationary phase. Normal phase HPLC—nonpolar mobile phase and polar stationary phase—is considered an orthogonal technique to reversed-phase HPLC when qualifying reference standards. In fact it is common for the elution order to be entirely reversed when switching an analysis from reversed to normal phase. Therefore, highly nonpolar impurities can be easily characterized by normal phase separations. 

In GLC, separation occurs based on differences in partitioning of the sample components between the carrier gas and the liquid phase. A wide selection of liquid phases makes GLC a versatile separation technique. Further, the liquid phase can be a polymer or a chemically bonded phase. In all cases, the liquid phase is film coated or chemically bonded onto a solid support surface or a column wall. Liquid-bonded phases overcome the problem of leakage of the stationary phase material into the carrier. They are used commonly in LC also, and the process to fabricate... 

Inclusion of this technique to the BOHLM has to be explained. Solvent extraction or partition of the solute between two immiscible phases is an equilibrium-based separation process. So, the membrane-based or nondispersive solvent extraction process has to be equilibrium based also. Liquid membrane separation is a rate process and the separation occurs due to a chemical potential gradient, not by equilibrium between phases [114]. According to these definitions, many authors who refer to their works as membrane-based or nondispersive solvent extraction processes are not correct. 

Micellar Electrokinetic Capillary Chromatography (MEKC). MEKC separation is based on the partition of analytes between micelles and the surrounding aqueous phase [38]. This technique can be considered a type of chromatography where the stationary phase is essentially mobile and the mobile phase is electro-osmotically... 

Separation in Micellar Electrokinetic Chromatography (MEKC) is based on partitioning of the analyte molecules between the aqueous run buffer and the core of micelles, which are contained in the run buffer. The technique is essentially a hybrid between CE and liquid chromatography (LC). The run buffer and micelles are moved through the capillary by an applied electric field. The analytes are dragged with the bulk solution. Similar to LC, the analytes partition between two phases, in this case two mobile phases, the hydrophilic run buffer and the hydrophobic micelles. Unlike other electrophoresis modes, MEKC can distinguish between different neutral compounds according to their hydrophobicity. 

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