Gradient application

Diffusion The mixing of substances by molecular motion to equalize a concentration gradient. Applicable to gases, fine aerosols and vapors. (See Brownian diffusion.)... 

For pump blending or gradient applications, thorough mobile phase degassing is critical. 

For gradient applications at low flow rates (<0.2mL/min), piston size and mixing volumes of the LC pumps can be the limiting factors. Use specialized pumps for microbore and capillary columns (see the pump section of Chapter 3). 

Figure 5. Reactor and catalyst gradients application to a heterogeneous catalyst.

Fig. 9.1.9 [Blal ] Timing diagrams for rf and gradients applicable to spatially resolved 2D spectroscopy, (a) Spatially localized COSY, (b) Spatially localized NOESY.

The scope of this chapter is bulk liquid membrane processes driven mainly by the concentration gradient. Application of a pressure difference, an electric field, or temperature considerably intensifies the process, but these special methods are beyond the scope of this overview. 

Whenever buffers or other mobile-phase additives are used, you should carefully check the solubility in the mobile phase. This is especially true for gradient applications or when the mobile-phase composition is generated automatically by the HPLC instrument. Buffer predpitation is one the most common column problems. A predpitated buffer can be redissolved by slowly purging the column with a mobile phase, in which the buffer is soluble. 

Robert de Saint Vincent M, Delville JP (2012) Thermocapillary migration in small-scale temperature gradients application to optofluidic drop dispensing. Phys Rev E 85 026310... 

Details of Gradient Application to the DEQUEST Preparative Purification... 

A typical gradient application of lonPac CS17 is the analysis of alkanolamines, including mono-, di-, and triethanolamine. Alkanolamines are most commonly used, individually or in combination, to optimize the efficiency of the scrubber treatment for a specific chemical process. The CS17 column resolves all combinations of these scrubber amines using a methanesulfonic acid gradient and elevated column temperature, as illustrated in Figure 4.23. 

Deterministic optimization has been the common approach for batch distillation operation in previous studies. Since uncertainties exist, the results obtained by deterministic approaches may cause a high risk of constraint violations. In this work, we propose to use a stochastic optimization approach under chance constraints to address this problem. A new scheme for computing the probabilities and their gradients applicable to large scale nonlinear dynamic processes has been developed and applied to a semibatch reactive distillation process. The kinetic parameters and the tray efficiency are considered to be uncertain. The product purity specifications are to be ensured with chance constraints. The comparison of the stochastic results with the deterministic results is presented to indicate the robustness of the stochastic optimization. 

The compounds taurine (2-aminoethanesulfonic acid) and CAPS [3-(N-cyclo-hexylamino)-l-propanesulfonic acid] used by Irgum are commercially available, but have to be thoroughly purified [150,151] for gradient applications. For now, it is not known to what extent the eluant ion concentration, necessary for eluting strongly-retained anions, is limited by the slow transport of zwitterions through the membrane. 

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