Aspects of Gradient Optimization

Stavros Kromidas, Frank Steiner, and Stefan Lamotte 

Gradients are versatile and, therefore, find wide application. For example, gradients are just as essential in method development of unknown samples as for quantification at trace levels. The theoretical background of gradient elution is quite complex, because what happens in the column during gradient elution, compared with isocratic separations, is affected by more factors these sometimes act in opposite directions or multiplicative. 

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Aspects of Gradient Optimization 1173 AsceutivExpi-ess 20-70%MeOEi ill 15 min 20 Gi fld C Kiiss Jamiar Gi adieiitenlest492.kd... 

Chapter 3 is devoted to the gradient elution. Stavros Kromidas, Frank Steiner, and Stefan Lamotte discuss about aspects of gradient optimization in a dense form in the first part and offer simple to-do rules. In the second part, Hans-Joachim Kuss shows that predictions of gradients runs with excel can be very unerring and that the often used linear model represents a simplified approximation. 

Heat-Transfer Analysis Thermal-Capillary Models. Numerous analyses of various aspects of heat transfer in the CZ system have been reported many of these are cited by either Kobayashi (143) or Derby and Brown (144). The analyses vary in complexity and purpose, from the simple one-dimensional or fin approximations designed to give order-of-magni-tude estimates for the axial temperature gradient in the crystal (98) to complex system-oriented calculations designed to optimize heater design and power requirements (145,146). The system-oriented, large-scale calculations include radiation between components of the heater and the crucible assemblies, as well as conduction and convection. 

In this section, the important aspects of the mathematical basis for optimization methods are described. This will provide the necessary background to understand the most widely used method, LP. Then descriptions of two more effective NLP methods are outlined the generalized reduced gradient method and the successive LP method. Then methods for mixed-integer and multicriteria optimization problems are summarized. 

Lodi, G. Betti, A. Menziani, E. Brandohni, V. Tosi, B. Some aspects and examples of automated multiple development (AMD) gradient optimization. J. Planar Chromatogr. 1991, 4 (3/4), 106-110. 

The aim of Part 1 is to provide an overview of important aspects of optimization in HPLC from various viewpoints. In Chapter 1.1 (Stavros Kromidas), the principles of optimization are illustrated using RP-HPLC as an example, and recommendations for method development are made. Fast gradients on short columns lead more often than one might think to sufficient resolution in the shortest of analysis times, and this topic is discussed in Chapter 1.2 (Uwe D. Neue). For the separation of polar/ionic substances, pH is by far the most important factor in optimization procedures. The next two chapters (1.3 Uwe D. Neue, 1.4 Michael McBrien) are devoted to this aspect. Optimization means more than merely the correct choice of method parameters. Efforts to obtain as much information as possible, or at least the necessary information, are also a part of optimization. In this context, the evaluation of chromatographic data and calibration take on a special significance. These topics are dealt with in Chapter 1.5 Hans-Joachim Kuss) and Chapter 1.6 St an Schomer). 

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