Band broadening processes

Since the exact profile of the mobile phase flow through a packed bed is unknown, only an approximate description of the ] and broadening process can be attained. For packed column gas chronatography at low mobile phase velocities, equation (1.35) provides a reasonable description of the band broadening process [70,82,83]. 

Band broadening processes and particle size of packing... 

A depends on band-broadening processes such as multiple flow paths that... 

Dingenen [9], who studied the effect of the mobile phase velocity on the height equivalent to a theoretical plate (HETP) at different temperatures for benzotriazole derivatives, obtained the results shown in Figure 22, which represents the HETP values found for methanol and the hexane-ethanol mixture. Both curves of Figure 22 clearly demonstrate that the kinetic circumstances are less favorable at low temperatures. A slow mass transfer between the two phases clearly determines the band-broadening process at temperatures below 20°C. This... 

EXAMPLE 24-1 The standard deviations in a chromatographic band resulting from several independent band-broadening processes are 0.013, 0.0024, 0.0067, and 0.047 cm. 

Figure 4.1 Band-broadening processes in porous irregular microparticles, (a) eddy diffusion analyte molecules take different routes to circumnavigate the particles. They also move more quickly through wide channels than through narrow channels, (b) diffusion in the mobile phase. The short bracket indicates initial band width, the long bracket indicates final band width, (c) mass transfer. On the left is shown mass transfer in stagnant mobile phase in pores, and that due to the adsorption/desorption process. The narrow band represents initial band width, the broad band final band width. On the right is shown mobile phase mass transfer caused by laminar flow.

The peak volume is also an important consideration when determining the importance of extra-column band broadening processes. It is well known that... 

In 1952, Martin and Synge were the first to describe column efficiency in terms of H, an equilibrium step or HETP (height equivalent to a theoretical plate) and peak broadening or variance in the elution process [2,3]. However, in 1956, van Deemter et al. realised that most band broadening occurs during the elution process in the column [4]. Column band broadening processes are the subject of the next section. 

For a given column length, optimum column efficiency is obtained when the equilibrium step or plate height is at a minimum, that is, the column band broadening processes described by the van Deemter equation (see section 2.5.1) are minimised by selecting the optimum velocity, t>f the mobile phase. Figure 5.3 shows the van Deemter plot of H against n for... 

If gradient and isocratic elution represent different versions of the same retention and band-broadening processes, can data from one elution mode be used to predict data for the alternate elution mode What errors or complications can arise in such interconversions of data ... 

Rate theory describes the contribution of different band broadening processes as a function of mobile-phase flow rate, The original rate theory developed by van Deemter in 1956 [1], relates the plate height to the three major band broadening terms. This theory is used to minimize peak width in terms of plate height (//) and was further refined to the Hawkes equation, which is shown in Eq. 8. 

To keep the band broadening process as low as possible, particles must exhibit narrow size-dispersity. Sophisticated polymerization reactors or advanced procedures for sizing, size-fractionation of particles are employed by the column packing producers to reach this goal. 

Safety devices such as air sensors and pressure transducers are built into preparative HPLC units, together with a series of valves. These devices create dead volume and contribute to the extra column volume. A large-scale chromatography unit is composed of valves for selection of buffers and feed solutions, at least two pumps, the separation column, and, in most cases, at least one detector. Instead of a fraction collector, a combination of valves is often used. These sources of dead volume create typical washout kinetics, which contribute exponentially to the band-broadening processes. For the industrial scale, the equipment is mainly customer designed. For medium scale, modular units are available [51]. Attention should be paid to extra column volume when systems are compared. Extra column effects are an important parameter of the quality of a system and should be considered when a system is purchased. 

These mathematical models enable prediction of isothermal or temperature-programmed retention times with very good accuracy, and so chromatographers can estimate the effects of changing conditions on peak elution sufficiently well to provide a good basis for optimization. These models do not take into account any of the band-broadening processes that determine peak shapes, and therefore alone they cannot predict peak resolution, Trennzahl or separation number, or any other measurement of chromatographic quality. 

A term The component of the van Deemter and Knox ecpiations that describes the impact that eddy dispersion has on the band broadening process. 

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