Chromatographic technique types

Analytical separations may be classified in three ways by the physical state of the mobile phase and stationary phase by the method of contact between the mobile phase and stationary phase or by the chemical or physical mechanism responsible for separating the sample s constituents. The mobile phase is usually a liquid or a gas, and the stationary phase, when present, is a solid or a liquid film coated on a solid surface. Chromatographic techniques are often named by listing the type of mobile phase, followed by the type of stationary phase. Thus, in gas-liquid chromatography the mobile phase is a gas and the stationary phase is a liquid. If only one phase is indicated, as in gas chromatography, it is assumed to be the mobile phase. 

Table 7.87 shows the main features of on-line micro LC-GC (see also Table 7.86). The technique allows the high sample capacity and wide flexibility of LC to be coupled with the high separation efficiency and the many selective detection techniques available in GC. Detection by MS somewhat improves the reliability of the analysis, but FID is certainly preferable for routine analysis whenever applicable. Some restrictions concern the type of GC columns and eluent choice, especially using LC columns of conventional dimensions. Most LC-GC methods are normal-phase methods. This is partly because organic solvents used as eluents in NPLC are compatible with GC, making coupling simpler. RPLC-GC coupling is demanding water is not a suitable solvent for GC, because it hydrolyses the siloxane bonds in GC columns. On-line RPLC-GC has not yet become routine. LC-GC technology is only applicable to compounds that can be analysed by GC, i.e. volatile, thermally stable solutes. LC-GC is appropriate for complex samples which are difficult or even impossible to analyse by a single chromatographic technique. Present LC-GC methods almost exclusively apply on-column, loop-type or vaporiser interfaces (PTV).

Even though it is sometimes not thought of as a chromatographic technique, we should also include electrophoresis. In this instance, paper or a gel is the stationary phase and electricity is the mobile phase. Although all types of chromatography are in extensive use in all kinds of investigations, electrophoresis has a particular prominence today because of DNA analysis [29],... 

Various liquid chromatographic techniques have been frequently employed for the purification of commercial dyes for theoretical studies or for the exact determination of their toxicity and environmental pollution capacity. Thus, several sulphonated azo dyes were purified by using reversed-phase preparative HPLC. The chemical strctures, colour index names and numbers, and molecular masses of the sulphonated azo dyes included in the experiments are listed in Fig. 3.114. In order to determine the non-sulphonated azo dyes impurities, commercial dye samples were extracted with hexane, chloroform and ethyl acetate. Colourization of the organic phase indicated impurities. TLC carried out on silica and ODS stationary phases was also applied to control impurities. Mobile phases were composed of methanol, chloroform, acetone, ACN, 2-propanol, water and 0.1 M sodium sulphate depending on the type of stationary phase. Two ODS columns were employed for the analytical separation of dyes. The parameters of the columns were 150 X 3.9 mm i.d. particle size 4 /jm and 250 X 4.6 mm i.d. particle size 5 //m. Mobile phases consisted of methanol and 0.05 M aqueous ammonium acetate in various volume ratios. The flow rate was 0.9 ml/min and dyes were detected at 254 nm. Preparative separations were carried out in an ODS column (250 X 21.2 mm i.d.) using a flow rate of 13.5 ml/min. The composition of the mobile phases employed for the analytical and preparative separation of dyes is compiled in Table 3.33. 

Possible sources of semiochemicals in primates include the scalp, hair, axillary region, genitals, chest and/or breast, feet and skin. As possible starting points for studies on human semiochemicals, the constituents present in the effluvia, excretions and secretions of humans have been characterized. For example, a large number of constituents of normal human urine have been identified since modern gas chromatographic techniques became available for this type of analysis. The results of these earlier studies on human effluvia and urine have been reviewed by Albone [148]. 

Adsorption chromatography. This chromatographic technique is best known because of its use in the last century as a preparative method of separation. Stationary phases have made a lot of progress since Tswett. who used calcium carbonate or sugar. The separation of organic compounds on a thin layer of silica gel or alumina with solvent as a mobile phase are examples of this type of chromatography. Solutes bond to the stationary phase because of physisorption or chemisorption interactions. The physico-chemical parameter involved is the coefficient of adsorption. 

In order to determine the type and amount of free monomer present in vinyl polymer latexes (so as to ascertain the toxic and expl hazards posed by their presence) the authors report the use of a gas chromatographic technique to successfully identify and measure the amount of vinyl ethylene]... 

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