Partition chromatography, principles

Fig. 4 Coupling the separation principles of adsorption and partition chromatography.

The most popular and versatile bonded phase is octadecylsilane (ODS), n-C18H37, a grouping that is non-polar and used for reverse phase separations. Octylsilane, with its shorter chain length, permits faster diffusion of solutes and this results in improved peak symmetry. Other groups are attached to provide polar phases and hence perform normal phase separations. These include cyano, ether, amine and diol groups, which offer a wide range of polarities. When bonded stationary phases are used, the clear distinction between adsorption and partition chromatography is lost and the principles of separation are far more complex. 

There are two types of partition chromatography that are distinguishable based upon the relative polarities of the mobile and stationary phases. In normal phase chromatography, a highly polar stationary phase is used with a relatively nonpolar mobile phase. As a general principle in normal phase chromatography, the least polar components are eluted first and, increasing the polarity of the mobile phase decreases the elution time. 

The principle of solvent extraction—the distribution of chemical species between two immiscible liquid phases—has been applied to many areas of chemistry. A typical one is liquid partition chromatography, where the principle of solvent extraction provides the most efficient separation process available to organic chemistry today its huge application has become a field (and an industry ) of its own. The design of ion selective electrodes is another application of the solvent extraction principle it also has become an independent field. Both these applications are only briefly touched upon in the chapter of this book on analytical applications (Chapter 14), as we consider them outside the scope of... 

FIGURE 6.3 Schematical diagram of the principle of partition chromatography. 

Background and principles Paper chromatography is a commonly used method for separation and identification of coloured compounds, including pigments. This technique relies on the separation of biomolecules from a mixture on the basis of partition, that is, difference in solubility and hence distribution of a given biomolecule between the stationary and mobile phase. 

The first reported use of a vapor as the mobile phase is attributed to Martin and Synge in 1941. They used the principles of partition chromatography, whereas James and Martin, in 1952, described the first application of this method, gas-liquid chromatography (GLC), for the analysis of fatty acids and amines. Gas adsorption chromatography (GSC), on the other hand, involves the use of a solid stationary phase and separation is based on an adsorptive mechanism. This technique was first described in 1947 in a doctoral thesis by Prior,under the supervision of Professor Cremer, and subsequently in their 1951 publication.f ... 

The concept of RAM combines a hydrophilic external surface and a hydrophobic internal surface in silica particles with controlled pore sizes (Fig. 9). Large biopolymers, such as proteins, are prohibited from entering the pores of the packing and are not well retained by the column. Therapeutic drugs and other small molecules permeate the pores of the column packing material, where they partition and are retained. This approach is, in principle, a combination of size exclusion and partition chromatography. In practice, to obtain a reasonable... 

In order to appreciate the forces that are responsible for chromatographic resolution, we need to review some of the principles of chromatography. The elution of a sample through a chromatography column is accomplished by a partitioning of the sample between a stationary phase and mobile phase. In GC the... 

There are two types of GC gas-solid (adsorption) chromatography and gas-liquid (partition) chromatography. The more important of the two is gas-liquid chromatography (GLC), used in the form of a capillary column. In this chapter, we describe the principles of operation of gas chromatography, the types of GC columns, and GC detectors. The principles of mass spectrometry (MS) are described, along with coupling of the gas chromatograph with a mass spectrometer (GC-MS). 

As far as liquid phase chromatography on columns is concerned it is probably true to say that the division into adsorption and partition methods is of practical, rather than theoretical, significance. The importance of adsorption varies from system to system and is mentioned briefly in connection with the different supports described below. The moving phase in partition chromatography may be a liquid or a gas, and the general principles are the same in each case. 

In principle, liquid partition chromatography (LPC) is a liquid-liquid extraction where one of the liquid phases is stationary and attached to a supporting material, and the other liquid phase is mobile. It can be carried out with either the aqueous or die organic phase stationary in the latter case the technique is referred to as reversed phase LPC. The aqueous phase can be made stationary by adsorption on silica gel, cellulose powder, etc. In order to make the organic phase stationary, beads (usually 50 - 200 pta) of PVC, teflon, Kel-F, etc., are being used. 

In liquid chromatography, separations will occur based on one of two principles partition and adsorption. With partition (liquid-liquid chromatography), a liquid stationary phase is used. This phase will be coated onto or chemically bonded onto a finely divided inert support (the latter is more common). With all forms of liquid chromatography, as the name suggests, we have a liquid mobile phase. 

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