Selection of optimization criteria

In chromatographic terms the purpose of this paper is to find a mixture composition which results in a good separation of the solutes, both under standard environmental conditions and for different temperatures and relative humidities. In this section this chromatographic purpose is combined with the Taguchi approach to robustness. This results in mathematical expressions which quantify the separation and the robustness against environmental influences. 

The purpose of the optimization is to find mixture compositions which result in robust separations. Therefore the separation power of a mobile phase at all combinations of temperature and relative humidity should be examined. The question is how all these values should be combined. 

There are several advanced mathematical techniques available to evaluate several responses, such as the resolutions at different temperatures and relative humidities [22]. We have chosen for a technique called Pareto Optimality since this method is simple and graphical techniques can be used to display the results. 

One of the more important criteria in the estimation of robustness is the occurrence of different spot orders at different temperatures or relative humidities. This is called spot cross-over. A spot cross-over clearly implies a lack of resolution and an interpretation problem (which spot is which compound). The number of spot cross-overs should be minimal and preferably zero. The number of cross-overs can easily, be determined when at fixed settings the elution order on the plates is examined. 

A second criterion should express the robustness of the separation more subtle than the number of spot cross-overs. The minimum resolution at different temperatures and relative humidities can be combined to an average and a minimum value. 

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