Selection of the separation mode

In chromatography, the separation efficiency of any single separation method is limited by the efficiency and selectivity of the separation mode, that is, the plate count of the column and the phase of the selected system. Adding more columns will not overcome the need to identify more components in a complex sample, due to the limitation of peak capacities. The peak capacity in an isocratic separation can be described, following Grushka (1970), as given in Equation (17.1) ... 

Two of the most important steps in the development of an analytical method are the selection of the separation mode and the appropriate column packing. The columns most commonly encountered have an internal diameter (i.d.) of... 

Another decision for the design engineer is the selection of the operation mode, whereby he can choose between the single- and cascade-mode of operation. In principle this is valid for multipurpose plants of medium scale, such as plants to extract spices and/or herbs. Fig. 8.1-6 gives data on the different production costs for a plant with a total extraction volume of 600 1, operated in different modes, but with the same capacity. The investigation is based on equal batch times (in our case 4 hours), equal mass-flow per kg of raw material and, of course, equal extraction- and separation conditions. 

The majority of CEC separations reported to date have been performed in the reversed-phase mode. Under these conditions, the hydrophobicity of the stationary phase determines the selectivity of the separation, and retention can easily be controlled by adjusting either the composition of the mobile phase or the hydrophobicity of the surface, with the first option being easier to implement. However, in contrast to the rich variety of solvents available for use in HPLC, acetonitrile-based solvent systems are employed in most CEC applications due to their high dielectric constant and low viscosity [30,35,51,64,70],... 

A fluorescence monitor can conveniently confirm and support data obtained with an absorbance detector. However, any comparison of the relative sensitivity/selectivity of the absorbance mode vs the fluorescence mode depends on the spectrochemical nature of both the pesticide itself, and the co-extracted co-elutants found in the agricultural product extracted. A judicious selection of mobile phase is required to optimize separation of the pesticide and co-extractives on a CN-bonded polar stationary phase. 

There are two general approaches for the separation of enantiomers [1-4,28-32]. The direct method is based on the formation of transient diastereomer association complexes with a chiral selector immobilized in the stationary phase, or added to the mobile phase. The former approach requires the use of special stationary phases (section 10.4) while the later uses conventional stationary phases with special additives included in the mobile phase (section 10.5). When preparative applications are contemplated the use of immobilized chiral selectors is the more common approach. Method selection also depends on the choice of the separation mode. Table 10.2. While chiral stationary phases are the only choice for gas chromatography [16,28,33-38], and are used almost exclusively for supercritical fluid chromatography [39-43] and capillary electrochromatography [44-47], they also dominate the practice of liquid chromatography... 

A separate output switch cabinet installed by the customer allows the selection of the operation mode Dual Mode (operation with both frequency converters, one per winding). Single Mode 1 (operation with converter 1—Both stator windings are connected in parallel or... 

Regarding development, normal sdica gel layers are tried first if not otherwise indicated. Selection of the best developing solvent has to be performed systematically, and its preparation should be proper. Various modes of development and separation chambers, both of which can influence the separation, have to be considered for development, with focus on the classical method of chromatogram development. Aspects of the chamber climate are helpful as background information and can improve resolution. 

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