Gradient switch materials

Chemotaxis Assay for Differentiated HL-60 Ceiis with Gradient Switch Materials ... 

In the rest period or quiescent stage, the stirrer is switched off for perhaps 30 s but the electrolysis potential is held. This permits the concentration gradient of material within... 

The selectivity of different stationary phase materials can be applied using columns in sequence to provide high-speed isocratic separations instead of gradient elution. An example for amino acids analysis is shown later in Figure 4.15, where the same eluent was used for all of the separations and the fraction containing the sample components of interest was switched from one column to another. 

A sample of povidone or copovidone is analyzed after dissolution in a water/ methanol mixture using HPLC reverse phase chromatography (gradient run). The polymeric material is kept away from the chromatographic system using a precolumn and switching technique. 

As illustrated by these and other examples, increased chromatographic retention is one way to reduce the impact of ion suppression. Switching from isocrahc to gradient elution can also help and provide a wide difference in k values for the analyte and weakly retained matrix materials. In cases where ion suppression is pronounced, more selective sample cleanup may be necessary. In addition, careful consideration should be given to internal standard selechon. A procedure for the assessment of the ability of an internal standard to track the analyte was recently published by Avery [127]. In this procedure, fluctuation in the peak-area ratio of analyte to internal standard was examined for different lots of control plasma. As one would expect, this degree of fluctuation was not only related to the internal standard selechon but also to the method used for extrachon. 

In most cases, the flow properties of polymers in solution or in a molten state are Newtonian, pseudoplastic, or a combination of both. In the case of liquid crystal polymer solutions, the flow behavior is more complex. The profound difference in the rheological behavior of ordinary and liquid crystalline polymers is due to the fact that, for the flrst ones, the molecular orientation is entirely determined by the flow process. The second ones are anisotropic materials already at equilibrium (Acierno and Brostow 1996). The spontaneous molecular orientation is already in existence before the flow and is switched on, varying in space, over distances of several microns or less (polydomain). If one ignores the latter, one can discuss the linear case (slow flow) as long as the rate of deformation due to flow (the magnitude of the symmetric part of the velocity gradient) is lower than the rate at which molecules rearrange their orientational spread by thermal motions. 

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