Kinetic optimization

Mobile phase modifications of stationary phase properties, that are also expected to vary with density, are a further complication [90-96]. The supercritical fluid 

The general approach for kinetic optimization of open tubular columns has been to adopt the familiar Golay equation (section 1.5.2) and to assume that the mobile phase can be approximated by an incompressible fluid with ideal gas properties [93,99-104]. Circumstances that are approximate at best, but serve adequately to demonstrate some of the fundamental characteristics of open tubular columns operated at low fluid densities. The column plate height equation can be written in the form given by Eq. (7.1) 

Influence of column internal diameter on efficiency and separation time for open tubular columns in supercritical fluid chromatography 

Column diameter (M-m) Optimum plate height and velocity at two retention factors k=l k=5  

A number of theoretical studies concluded that a pressure drop greater than 20 bar would cause a severe efficiency losses in packed colunrn supercritical fluid chromatography [1,2,5,6,82]. With such small a pressure drop, the maximum column efficiency would be limited to approximately 20,000 plates. In practice packed columns providing over 100,000 plates per meter with a total plate number more than 200,000 and operated with a column pressure drop of about 160 bar have been demonstrated [86,109-112]. Coupled columns providing the highest total plate number were obtained with mixed mobile phases of low compressibility and demonstrate that the theoretical performance limit proposed for packed column supercritical fluid chromatography is too pessimistic. 

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Knox, J. H. and Gilbert, M. T., Kinetic optimization of straight open-tubular hquid-chromatography. Journal of Chromatography 186(Dec), 405 18, 1979. 

In summary, at nanostructured tin-oxide semiconductor-aqueous solution interfaces, back ET to molecular dyes is well described by conventional Marcus-type electron-transfer theory. The mechanistic details of the reaction, however, are remarkably sensitive to the nature of the semiconductor-dye binding interactions. The mechanistic differences point, potentially, to differing design strategies for kinetic optimization of the corresponding liquid-junction solar cells. 

An example of kinetic optimization is the nanosize barium titanate powder with specific surface area of 38 0 m /g and particle size of 25-30 nm. According to electron microscopy the dispersion range was a little broader (around 8-40 nm,... 

Kinetic optimization. The primary process must be sufficiently fast to compete with energy waste due to backtransfer to the antenna. The efficient route will involve pseudoactivationless ET, (nearly) optimizing F. 

Unistep superexchange ET requires (i) AG is small (or negative) violating kinetic optimization for the sequential mechanism and (ii) large values of Vpg and Vbj. ... 

At this point, the stationary phase particle diameter is extremely important for the kinetic optimization of separations. A smaller particle diameter reduces the distance for the necessary radial diffusion of analyte molecules on the one hand, but increases the geometrical radial concentration gradient that drives the diffusion. Both effects are synergistic for an efficient analyte transport and this is the physicochemical foundation for the decrease of the C-term with the squared particle diameter (dp ). This will be used effectively in the speed optimization strategy. 

Temperature increase always helps for a kinetic optimization of separations (it does, however, require prerequisites regarding the column thermostatting and stationary phase temperature stability). 

The fact that temperature increase reduces retention has nothing in common with temperature-based kinetic improvement or speed-up of separations. The related reduction of retention may even be a risk for the method robustness or resolution. Retention in LC can be much more effectively reduced with mobile phase changes, and it is a purely thermodynamic consideration that has no place in kinetic optimization. 

We have seen with the speed-optimization in Section 2.3.4 that the particle size dp is the predominant parameter to influence the plate number, N, being the primary descriptor for efficiency to enable kinetic improvement of HPLC methods. Temperature can also change kinetic parameters in HPLC, but to a smaller extent than particle size and it simultaneously changes thermodynamic parameters such as retention and can sometimes even alter the selectivity. Kinetic optimization is mostly about increasing speed of analysis by providing a method that generates the same A in a shorter time. We now see how an increase of N through column parameters can improve resolution and what considerations have to be made. 

Nevertheless, the history of supports for HPLC does not stop at this point, and there are several further developments aimed at either a kinetic optimization of the materials (see Chapter 3), an extension of the usable pH range, or the pressure stability of the silicas. In the course of kinetic optimization of the materials, essentially the diffusion paths of the analytes interacting with the stationary phase are shortened. This is done primarily by reducing the particle diameter of the supports. 

Scott, R.P.W. Kucera, P. Mode of operation and performance characteristics of microbore columns for use in liquid chromatography. J. Chromatogr. 1979,169,51-72. Knox, J.H. Gilbert, M.T. Kinetic optimization of straight open-tubular liquid chromatography. J. Chromatogr. 1979, 186,405 18. 

As discussed in the previous sections, online monitoring polymerization reactions have multiple benefits, providing comprehensive, quantitative data for understanding fundamental mechanisms and kinetics, optimizing reaction processes, and, ultimately, precisely controlling reactions to yield desired products. 

Figure 5. A schematic description of the kinetic optimization and kinetic stability in bacterial photosynthesis at 300K. The (VpB,AGj) range marked constitutes the...

Wang XJ, Ching CB. A systematic approach for preferential crystallization of 4-hydroxy-2-pyrrolidone thermodynamics, kinetics, optimal operation and in-situ monitoring aspects. Chem. Eng. Sci. 2006 61 2406-2417. 

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