Plate, effective plates height, reduced

In 1972-1973 Knox et al. [3, 4, 5] examined, in considerable detail, a number of different packing materials with particular reference to the effect of particle size on the reduced plate height of a column. The reduced plate height (h) and reduced velocity (v) were introduced by Giddings [6,7] in 1965 in an attempt to form a rational basis... 

The curves represent a plot of Log.(/V),(Reduced Plate height)against Log.(v), (Reduced Velocity). The lower the Log.(/7) curve versus the Log.(v) curve the better the column is packed. At low velocities the (B) term dominates and at high velocities the (C) term dominates as in the Van Deemter equation. The best column efficiency is achieved when the minimum is about 2 particle diameters and thus, Log (.ft) Is about 0.35. The minimum value for (H) as predicted by the Van Deemter equation has also been shown to be about two particle diameters. The optimum reduced velocity is in the range of 3 to 5 that is Log.(v ) takes values between 0.3 and 0.5. The Knox equation is a simple and effective method of examining the quality of a given column but, as stated before, is not nearly so useful In column design due to the empirical nature of the constants. 

Research into the effectiveness of separator internals for foam breakdown is warranted. Parallel plates do reduce foam height, but there is need for further improvement... 

Let us take a better look at the effect of the terms of Equation 2.80 on plate height. The contribution of the 2Adp term can be decreased by reducing the particle size. However, as the particle size becomes very small, the pressure drop through the... 

Eqn.(7.17) shows that an increase in the column efficiency by way of a decrease in the reduced plate height (h) has a positive effect on the sensitivity. Therefore, well-packed columns should be used at (or just above) the optimum flow rate. If that is the case, then h may be considered as a constant (2 < h < 3), i.e. considered to be independent of the column diameter and the particle size. 

As is evident from the preceding discussion, the retention behavior of a polypeptide or protein P- expressed in terms of the capacity factor k is governed by thermodynamic considerations. Peak dispersion, on the other hand, arises from time-dependent kinetic phenomena, which are most conveniently expressed in terms of the reduced plate height he, . When no secondary effects, i.e., when no temperature effects, conformational changes, slow chemical equilibrium, pH effects, etc. occur as part of the chromatographic distribution process, then the resolution Rs, that can be achieved between adjacent components separated under these equilibrium or nearequilibrium conditions can be expressed as... 

Unfortunately, the results proved to be rather ambiguous. While the beneficial effects of pore flow on separation efficiency could be demonstrated, the increase in efficiency was only approximately 30% at conditions at which fully perfusive behavior can be expected. The highest efficiency that was reported was a reduced plate height of 1.3. This improvement is much less as predicted. 

To illustrate more clearly the effect of these variables on analysis time, reduced parameters can be used for the plate height and velocity. Reduced parameters effectively normalize the plate height and velocity for the particle diameter and the diffusion coefficient to produce dimensionless parameters that allow comparison of different columns and separation conditions. The reduced plate height and reduced velocity are expressed, respectively, as... 

The change of the selectivity term in the resolution is directly expressible by im. Interestingly, the effect of the EOF on the dispersion effects, expressed by the plate height H, also depends directly on im. For longitudinal diffusion. Joule self-heating, and concentration overload, the variation of the plate height in the presence of the EOF is directly dependent upon this reduced mobility according to... 

The Stationary Phase Mass-Transfer Term C u When the stationary phase is an immobilized liquid, the mass-transfer coefficient is directly proportional to the square of the thickness of the film on the support particles, d, and inversely proportional to the diffusion coefficient, D, of the solute in the film. These effects can be understood by realizing that both reduce the average frequency at which analyte molecules reach the interface where transfer to the mobile phase can occur. That is, with thick films, molecules must on the average travel farther to reach the surface, and with smaller diffusion coefficients, they travel slower. The consequence is a slower rate of mass transfer and an increase in plate height. 

In open tubular colvunns, because the thin hquid film is deposited directly on the wall of the column rather than the sohd supports, the A term is zero, therefore ehminating one of the major contributor to zone broadening. Comparing to packed columns, the resistance to mass transfer is also reduced in both the hquid phase due to the apphcation of very thin film of the stationary phase, and in the mobile phase due to the apphcation of very narrow internal diameter columns. The typical open tubular colmnns have an internal diameter of 0.25 mm and a film thickness of 0.25 pm. A combination of ah these factors makes for the fact that capillary GC columns have much lower plate height value and substantiaUy more theoretical plates. The effect of carrier gas and hnear velocity on capihary column efficiency is illustrated in Figure 4, which shows a family of van Deemter plots for common carrier gases. 

A more effective way to increase the efficiency in HPLC is by reducing the stationary phase particle size, as predicted by the well-known van Deemter equation [12], which describes the plate height in HPLC as the sum of different contributions. In this respect, submicron particles have been applied and efficiencies up to 730,000 plates/m have been reported for small molecules [13,14]. 

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