Choice of the chromatographic system

The choice of the chromatographic system depends on the chemical character of the extracts being separated. The mobile phase should accomplish all requirements for PLC determined by volatility and low viscosity, because nonvolatile components (e.g., ion association reagents and most buffers) should be avoided. It means that, for PLC of plant extracts, normal phase chromatography is much more preferable than reversed-phase systems. In the latter situation, mixtures such as methanol-ace-tonitrile-water are mostly used. If buffers and acids have to be added to either the... 

In practice the resolution will always be specified, because our goal is to separate a given mixture, and this requires a given minimum degree of resolution between each band. Thus, the value of the required number of theoretical plates is given by the choice of the chromatographic system. 

The equations which relate the parameters of the systbm are summarized in Table III. Some of these parameters are det mined by the choice of the chromatographic system the solvent viscosity t), the diffu-... 

Chapter 5 focuses on process concepts. The basis of every preparative chromatographic separation is the proper choice of the chromatographic system, as described in the previous chapters. Its implementation in a preparative process concept plays an important role in serving the different needs of substance production in terms of system flexibility and production scale. Depending on the operating mode, several features distinguish chromatographic process concepts ... 

In practice, prior to beginning the actual process of optimization of the experimental conditions of a preparative separation, extraction, or purification, it is necessary to perform the choice of the chromatographic system to be used. This requires the measurement of the most important characteristics of the performance of several combinations of stationary and mobile phase combinations. It is imperative to maximize the selectivity of the chromatographic system while making sure that its capacity is important. The feed solubility in the mobile phase must be high and the saturation capacity of the stationary phase important. The actual focus of this chapter is the optimization process following the selection of the chromatographic system (mobile and stationary phases). 

The basis for every preparative chromatographic separation is the proper choice of the chromatographic system. The most important aspects in this context are selectivity, capacity, and solubility, vhich are influenced and can be optimized by the deliberate selection of stationary and mobile phases, as is discussed extensively in Chapter 3. 

It has been shown that, in LC, the size of the distribution coefficient of a solute between the two phases determines the extent of its retention. As a consequence, the difference between the distribution coefficients of two solutes establishes the extent of their separation. The distribution coefficients are controlled by the nature and strength of the molecular interactions that takes place between the solutes and the two phases. Thus it is the choice of the phase system that primarily determines the separation that is achieved by the chromatographic system. 

In both cases, either conventional FTIR transmission or diffuse reflection detection may be used. Because TLC and the postspectroscopic evaluation are not linked directly, few compromises have to be made with regard to the choice of the solvent system employed for separation. Chromatographic selectivity and efficiency are not influenced by the needs of the detector. The TLC plate allows the separation to be made in a different site from the laboratory where the separated analytes are evaluated. The fact that the sample is static on the plate, rather than moving with the flow of a mobile phase, also puts less demand on the spectrometer. The popularity of TLC-IR derives in part from its low cost. 

This chapter describes the final configuration of the chromatographic system (column and instrument) after the optimization of the phase system (the combination of the stationary and the mobile phase) has been completed. The entire optimization process is illustrated in figure 7.1. This figure shows the different stages in the process from the moment at which it has been decided (either on the basis of literature information or on the basis of figure 2.1) which chromatographic method should be used. For example, it may have been decided that RPLC is the method of choice. It should also be decided what kind of detector will be used. For instance, we may choose to use a UV absorption detector. 

The choice of chromatographic system and the process concept are influenced by the classification of the separation problem into one of the three scenarios of Fig. 4.4. This chapter focuses on the influence of the chromatographic system, while the influence on the process concept is explained in Chapter 5.4. Here, it should be kept in mind that the elution order of the components is essential for the whole process and the elution order is determined by the chromatographic system. Especially if one component is in excess, as in scenarios (a) and (c) in Fig. 4.4, the use of thermodynamic effects like displacement or tag along are a special source for optimization as well as for severe errors and mistakes (Chapter 2.6). 

The solute is the product of the pre-purification steps. It contains the target product and the impurities. Although the solute is a part of the chromatographic system it is not a free parameter like the eluent or the adsorbent. Subsequent to previous crude separations the composition of the feedstock is fixed and the chromatographic system is completed by the choice of mobile phase and adsorbent. 

In this section we discuss the choice of a chromatographic system on the basis of, firstly, the type of stationary phase and, secondly, the composition of the eluent. A division in subsections is made from the kinds of stationary phases available, bearing in mind that the more specialized phases for ion-exchange, gel permeation (size exclusion), and bioaffinity chromatography are treated in separate chapters (4.5 to... 

The most important part of the chromatographic system is the separator column. The choice of a suitable stationary phase (see Section 1.5) and the chromatographic conditions determine the quality of the analysis. The column tubes are manufactured from inert material such as Tefzec, epoxy resins, or PEEK... 

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