Column evaluation reduced parameters

The reduced parameters are also helpful in evaluating column performance. The best columns have a reduced plate height of 2 to 5—a number that can be thought of as representing the number of particles between sorptions—and 2 is a practical minimum. The reduced velocity represents the ratio between the flow velocity and the diffusion rate over one particle diameter typical values should be in the range of 3 to 20. 

Method development remains the most challenging aspect of chiral chromatographic analysis, and the need for rapid method development is particularly acute in the pharmaceutical industry. To complicate matters, even structurally similar compounds may not be resolved under the same chromatographic conditions, or even on the same CSP. Rapid column equilibration in SFC speeds the column screening process, and automated systems accommodating multiple CSPs and modifiers now permit unattended method optimization in SFC [36]. Because more compounds are likely to be resolved with a single set of parameters in SFC than in LC, the analyst stands a greater chance of success on the first try in SFC [37]. The increased resolution obtained in SFC may also reduce the number of columns that must be evaluated to achieve the desired separation. 

A possibility to reduce the influence of column efficiency on the results obtained by the ECP method is to detect the position of the peak maximum only, which is called the peak-maximum or retention-time method. Graphs like Fig. 6.23 are then achieved by a series of pulse injections with different sample concentrations. The concentration and position of the maximum is strongly influenced by the adsorption equilibrium due to the compressive nature of either the front or the rear of the peak (Chapter 2.2.3). Thus, the obtained values are less sensitive to kinetic effects than in the case of the ECP method. The isotherm parameters can be evaluated in the same way as described in Section 6.5.7.6, but the same limitations have to be kept in mind. For some isotherm equations, analytical solutions of the ideal model can be used to replace the concentration at the maximum (Golshan-Shirazi and Guiochon, 1989 and Guiochon et al., 1994b). Thus, only retention times must be considered and detector calibration can be omitted in these cases. 

Thus the evaluation of the basic process parameters reduces to the determination of the amount of the element contained in the initial and resultant components of the raw material and the end product. Such values can be established in different ways, some of which were discussed by Pankov and Khripin [175], To obtain a value proportional to the amount of carbon in the key component, the zones of compounds separated in a chromatographic column are converted into carbon dioxide or methane. 

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