Plate height value

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. 

These columns are similar to normal HPLC colunms except that they have an internal diameter of around 1 mm. The packing materials consist of small diameter particles (3-30 /im) packed into stainless steel colunms by normal slurry packing techniques. An examination of the effect of internal diameter on column efficiency using 1 m columns packed with a silica gel stationary phase demonstrated that the lowest plate height values (//) were obtained with an internal diameter of approximately 1.02 mm (Scott and Kucera, 1979 McGuf-... 

By comparing the two Equations 5.3 and 5.5, it can be seen that the linear velocity u is proportional to dp in a pressure-driven system while it is independent of the particle diameter in an electrically driven system. Since plate height values are generally lowered as a result of using small diameter particles, it is possible in electrically driven systems to use very small diameter packing materials and still maintain high linear velocities to yield rapid and very efficient separations. 

Generally the H-u or h-v plots are used to compare the kinetic efficiency of the chromatographic columns. Figure 5.1 demonstrates an example of h-v plots of different type modern stationary phases. The characteristic properties of these plots are the minimum plate height value (Hmin or fimin) and the observed optimum in linear velocity Uopt or Vopt). The position of the minimum on the H-u, or h-v curve, and the optimum linear velocity can be determined by the use of differential calculus. The optimum linear velocity occurs when the slope of the H (or h) versus u (or v) curve is zero—that is, when dH/du = 0 (or dh/dv = 0). This condition is satisfied in the van Deemter form when ... 

V = 4.5. These are optimum values from a graph of reduced plate height versus reduced... 

The curves represent a plot of log (h ) (reduced plate height) against log (v) (reduced velocity) for two very different columns. The lower the curve, the better the column is packed (the lower the minimum reduced plate height). At low velocities, the (B) term (longitudinal diffusion) dominates, and at high velocities the (C) term (resistance to mass transfer in the stationary phase) dominates, as in the Van Deemter equation. The best column efficiency is achieved when the minimum is about 2 particle diameters and thus, log (h ) is about 0.35. The optimum reduced velocity is in the range of 3 to 5 cm/sec., that is log (v) takes values between 0.3 and 0.5. The Knox... 

Employing the value (N) obtained for the plate efficiency, again, in an analogous manner to GC and LC the plate height of a TLC plate (Htlc) is given by... 

The contribution to the plate height from molecular diffusion in the mobile phase arises from the natural tendency of the solute band to diffuse away from the zone center as it moves through the column [59,60,63,64]. Its value is proportional to the diffusion coefficient and the. time the sample spends in the column. Its contribution to the total plate height is given by... 

The A term represents the contribution from eddy diffusion, the B term the contribution from longitudinal diffusion, and the C terms the contributions from mass transfer in the mobile and stationary phases to the total column plate height. By differentiating equation (1.31) with respect to the mobile phase velocity and setting the result equal to zero, the optimum values of mobile phase velocity (u ) and plate height (HETP ) can be obtained. 

The possibility of obtaining significant improvements in performance by using semi-packed and open tubular columns is clearly illustrated by the values for the separation impedance in Table 1.17. Variation of the reduced plate height with the reduced velocity for an open tubular column is given by equation (1.82), assuming that the resistance to mass transfer in the stationary phase can be neglected... 

The reduced plate height is reasonably constant, independent of the capacity factor for a packed column, while equation (1.82) indicates Ithat the reduced plate height, at least for small values... 

Figure 7.4 Change In peak width at half height as a function of nigration distance for a typical test nixture on a high perfomance TLC plate. The values for b, and bf are calculated by extrapolation using linear regression.

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