Selectivity chromatographic system

Chapter 7, therefore, deals with model-based design and optimization of a chromatographic plant, where the already selected chromatographic system and concepts are applied. First, basic principles of the optimization of chromatographic processes will be explained. These include the introduction of the commonly used objective functions and the degrees of freedom. To reduce the complexity of the optimization and to ease the scale-up of a plant, this chapter will also emphasize the application of dimensionless parameters and degrees of freedom respectively. Examples for the... 

If you do not do preparative work, the goal should be not the maximal resolution, but the optimal or sufficient resolution. If you have a selective chromatographic system, you only need a small theoretical plate number for your separation. You can even afford to give away plates by increasing the temperature if you gain on time (see Tip No. 16). 

In the course of mixture separation, the composition and properties of both mobile phase (MP) and stationary phase (SP) are purposefully altered by means of introduction of some active components into the MP, which are absorbed by it and then sorbed by the SP (e.g. on a silica gel layer). This procedure enables a new principle of control over chromatographic process to be implemented, which enhances the selectivity of separation. As a possible way of controlling the chromatographic system s properties in TLC, the pH of the mobile phase and sorbent surface may be changed by means of partial air replacement by ammonia (a basic gaseous component) or carbon dioxide (an acidic one). 

Recalling that a separation is achieved by moving the solute bands apart in the column and, at the same time, constraining their dispersion so that they are eluted discretely, it follows that the resolution of a pair of solutes is not successfully accomplished by merely selective retention. In addition, the column must be carefully designed to minimize solute band dispersion. Selective retention will be determined by the interactive nature of the two phases, but band dispersion is determined by the physical properties of the column and the manner in which it is constructed. It is, therefore, necessary to identify those properties that influence peak width and how they are related to other properties of the chromatographic system. This aspect of chromatography theory will be discussed in detail in Part 2 of this book. At this time, the theoretical development will be limited to obtaining a measure of the peak width, so that eventually the width can then be related both theoretically and experimentally to the pertinent column parameters. 

By assuming that a (selectivity of the chromatographic system) can be rewritten as follows ... 

In effect, the composition of the mobile phase, and thus the selectivity of the chromatographic system, has been changed. As mentioned in the text, dynamic FAB operates effectively with lower concentrations of matrix than static FAB and although its effect may be minimal it should always be considered. Post-column addition of matrix overcomes potential problems of this nature. 

Chromatographic selectivity The degree to which compounds are separated on a particular chromatographic system. 

In partition chromatographic systems, the selectivity and degree of retention are mainly determined by the compositions of the liquid stationary and mobile phases and by the phase ratio of these two liquids. 

Solvent selectivity refers to the ability of a chromatographic system to separate two substances of a mixture. It depends on the chemistry of the adsorbent surface, such as the layer activity and type of chemical modihcation. The separation power or resolution is given by Equation 4.8 [27] ... 

The substitution of a polar component with another one can improve or decrease selectivity because of the change of interactions in the chromatographic system. This can be based on the namre and chemical reactivity of the solute. 

The discriminating power (DP) of a set of chromatographic systems is defined as the probability of identifying two randomly selected solutes in at least one of the systems. For k chromatographic systems in which N compounds are investigated, the DP is given by Equation 4.33 [18] ... 

Procedures used vary from trial-and-error methods to more sophisticated approaches including the window diagram, the simplex method, the PRISMA method, chemometric method, or computer-assisted methods. Many of these procedures were originally developed for HPLC and were apphed to TLC with appropriate changes in methodology. In the majority of the procedures, a set of solvents is selected as components of the mobile phase and one of the mentioned procedures is then used to optimize their relative proportions. Chemometric methods make possible to choose the minimum number of chromatographic systems needed to perform the best separation. 

The optimization of preparative and even micropreparative chromatography depends on the choice of an appropriate chromatographic system (adsorbent and eluent), sample application and development mode to ensure high purity, and yield of desirable compounds isolated from the layer. For the so-called difficult separations, it is necessary to perform rechromatography by using a system with a different selectivity. But it should be taken into account that achievement of satisfactory results frequently depends on a compromise between yield and the purity of the mixture component that is being isolated. 

The selectivity of a chromatographic system is the main critical parameter in the result of separation in analytical and preparative chromatography. For a pair of substances, selectivity is characterized quantitatively by the separation coefficient (a = k, /k[i for the compounds I and II) for a large number of substances the correlations (log vs. log ) are the characteristics of the selectivity... 

FIGURE 11.7 Characteristic cases for log k, vs. log kntR i vs. R i]) correlations (a) identical selectivity of system I and II (b) similar selectivity of both systems, system I more selective (higher A log k, = AR values) (c) system I selective, system II nonselective (d) system II selective, system I nonselective (e) different selectivity of both systems and (f) lack of correlation, strong diversity of retention. (From Soczewiriski, E., Preface, in Waksmundzka-Hajnos, M., Chromatographic Dissertations, Medical University, Lublin, Poland, 2, 9-12, 1998.)... 

Chromatographic systems were finally coupled with relatively inexpensive, yet powerful, detection systems with the advent of the quadrupole mass selective detector (MSD). The operational complexity of this type of instrumentation has significantly declined over the last 15 years, thus allowing routine laboratory use. These instruments... 

ABC Laboratories Model SP 1000 gel permeation chromatograph system equipped with a 31.0 X 2.5-cm glass column of Envirobeads SX-3 select 200 00 mesh (ca 60 g) preconditioned with ethyl acetate-cyclohexane (1 1, v/v)... 

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