Classifications of Chromatographic Techniques

This type of chromatography, in which the mobile phase is a liquid belongs to the oldest known form of the preparative methods of separation. This very broad category can be sub-divided depending on the retention phenomenon. 

The stationary phase is a solid medium to which the species adhere through the dual effect of physisorption and chemisorption. The physico-chemical parameter involved here is the adsorption coefficient. Stationary phases have made much progress since the time of Tswett, who used calcium carbonate or inulin (a very finely powdered polymer of ordinary sugar). 

In this technique the mobile phase is a buffered solution while the solid stationary phase has a surface composed of ionic sites. These phases allow the exchange of their mobile counter ion with ions of the same charge present in the sample. This type of separation relies on ionic distribution coefficients. 

The stationary phase here is a material containing pores whose dimensions are selected as a function of the size of the species to be separated. This method 

The stationary phase is an immobilized liquid upon an inert and porous material, which has only a mechanical role of support. Impregnation, the oldest procedure for immobilizing a liquid on a porous material, is a method now abandoned because of the elevated risk of washing out the column, which is called bleeding. 

The most common way to classify the different chromatographic techniques is by the nature of the two phases involved. The mobile phase can be a gas (gas chromatography, GC), a liquid (liquid chromatography, LC) or a supercritical fluid (SFC).  

The nature of the stationary phase can be incorporated in this nomenclature. The convention hereby is to denote the character of the stationary phase by inserting one or two letters in the middle. Hence, LSC and LLC are both forms of liquid chromatography (LC mobile phase is a liquid), while the stationary phase is a solid (S) and a liquid (L), respectively. 

Chemically bonded phases (CBP s) are very commonly used in LC, and occasionally also in GC. Such phases cannot be seen as either a solid or a liquid. The common term [201] used for LC involving such phases is bonded phase chromatography (BPC). To be consistent, the stationary phase identification should follow that of the mobile phase in defining the chromatographic system. Hence, LBPC should be used for liquid chromatography using chemically bonded stationary phases. 

A summary of the different chromatographic techniques is given in table 2.1. 

In this book the attention will mainly be focussed on the most popular chromatographic techniques, i.e. GC and LC. Some comments will be made regarding SFC in section 3.4. 

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These are arbitrary classifications of chromatographic techniques, and some types of chromatography are considered together as a separate technique, such as gas chromatography for gas-solid and gas-liquid chromatography. In every case, successive equilibria are at work that determine to what extent the analyte stays behind or moves along with the eluent (mobile phase). In column chromatography, the column may be packed with small particles that act as the stationary phase... 

A convenient classification of chromatographic techniques can be made in terms of the physical state of the phases employed in the separation process (Fig. 3) If the mobile phase is a gas, the separation techniques are known as gas-liquid chromatography (GLC) or gas-solid chromatography (GSC) when the stationary phase is a liquid or solid, respectively. GLC is the more popular separation mode and is often simply referred to as GC. If the mobile phase is a supercritical fluid, the separation technique is known as supercritical fluid chromatography (SFC) whether the stationary phase is an... 

In the rest of the chapter, various chromatographic methods will be discussed. You should recognize that no single chromatographic technique relies solely on adsorption or partition effects. Therefore, little emphasis will be placed on a classification of the techniques instead, theoretical and practical aspects will be discussed. 

Separation of the components, or solutes, of a sample results from differences in their rates of adsorption, solution, or reaction with the mobile and stationary phases. In the light of these observations distinguishing the numerous chromatographic techniques only on the basis of specification of the physical states of the stationary and mobile phases is inadequate, and a more adequate classification of these techniques must additionally also take into account (i) the nature of the separation e.g. adsorption, and (ii) the configuration of the system e.g. columnar. Table 4.4 gives a system of classification which incorporates these considerations. 

E Ruckenstein, V Lesins. Classification of liquid chromatographic methods based on the interaction forces The niche of potential barrier chromatography. In A Mizrahi, ed. Advances in Biotechnological Processes, Vol 8 Downstream Processes Equipment and Techniques. New York Alan R. Liss, 1988, pp 241-314. 

Differential scanning calorimetry (DSC) is a common technique for the classification of individual phase transitions in liquid-crystalline materials and has been applied for the phase characterization of alkyl-modified chromatographic surfaces. Hansen and Callis [187] applied DSC to investigate phase changes in Cig and C22... 

In this section a variety of analytical separations reported in the literature are reviewed to show the wide structural diversity of eluite which can be separated by RPC and to assist the reader in becoming similar with the use of this fluid chromatographic technique. The descriptions are ar-ranged according to the matrix in which an analyte is found or the area of - h istry in which the samples are generally encountered. Thus theophylline, for example, is regarded as a nucleotide and, for the most part, its analysis in food samples is found with appropriate cross references. On the other hand, the separations of pharmaceuticals found in serum, urine, and pharmaceutical samples are cited separately. It is hoped that this method of classification may serve the purposes of those wh e analytical interests are incidental to their primary research pursuits. 

In a chromatographic separation procedure the parameters of the chromatographic system (stationary phase, flow, temperature, etc.) have to be selected respectively optimized with respect to some criterion (resolution, time, etc.). In gas chromatography retention data series are published and used for the sttidy of solvent/solute interaction, prediction of the retention behaviour, activity coefficients, and other relevant information usable for optimization and classification. Several clKmometrk techniques of data anal s have been employed, e.g. PCA, numerical taxonomic methods, information theory, and j ttern recognition. 

Figure 1.2—Classification of some chromatographic techniques as a function of the polaritv of the stationary phase. The existence of a wide range of techniques permits the most suitable one to be selected for a specific analytical problem.

A general classification of oil eg crude oil, petroleum, gas oil, is often satisfactorily achieved by gas chromatographic techniques possibly coupled with mass spectrometry or infrared spectroscopy applied to a sample of the oil pollutant. The true identification invariably requires samples from potential sources for comparison with the pollutant. 

In this chapter, different techniques for exploratory analysis and classification of CE data will be discussed and supplemented with some theoretical background. Examples of the application of each technique in the CE field will also be provided, if available. If not, the technique will be illustrated with a chromatographic or spectroscopic case study, because mathematically, they deliver an output similar to electropherograms. 

Chromatographic techniques can be classified according to various criteria as a function of the physical nature of the phases of the process used or by the physico-chemical phenomena giving rise to the Nernst distribution coefficient K. The following classification has been established by consideration of the physical nature of the two phases involved (Figure 1.14). 

This section addresses two basic principles of classification—sedimentation and field-flow fractionation—and the corresponding sizing techniques. Additionally, a chromatographic technique is briefly introduced. The focus lies on sedimentation or centrifugation analysis, which corresponds to its practical relevance for the characterisation of coUoidal suspensions. 

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