The Minimum Plate Height

The expression that gives the minimum plate height is given by equation (29), chapter 12, and is reiterated here. 

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Further, by substituting for (uopt.) in equation (3), it can be seen that the minimum plate height (Hmin.) is given by,... 

Now, the column length (L) can be defined as the product of the minimum plate height and the number of theoretical plates required to complete the separation as specified by the Purnell equation. 

It must be emphasized that equation (4) only applies to the separation and conditions defined in Table 1. The effect of the magnitude of the separation ratio on the minimum plate height is shown in Figure 4. 

The optimum velocity changes linearly from about 1 cm/sec. to 800 cm/sec. and the minimum plate height from 200 pm to about 10 pm over the same separation ratio... 

Of course it is impossible to carry out a given separation at the critical pressure, APe, since the analysis time would be infinite. It is easy to show that if A is zero in Eq. (40), the pressure, AP , required to obtain the minimum plate height, hn, is given by Eq. (43) ... 

Because of its open, rigid structure, solvent flows through the monolithic column with relatively little resistance. The same pressure required to obtain a flow rate of 1 ml./min with 3.5-pm spherical particles provides a flow rate of 9 mL/min in the monolithic column. At 9 mL/min, the plate height in the monolithic column is only 50% greater than the minimum plate height observed at 2 mL/min. 

To find the correct flow velocity, v, which gives the minimum plate height, we take the first derivative of Equation 2.74 and set dh/dv equal to 0. This results in... 

For GC, the optimum velocity corresponds to the minimum plate height, which occurs at i = (B/C)... 

The first notable point Is that the minimum plate heights are the same. This Is reasonable and predictable since the minimum plate height (HETP j n) In a van Deempter curve for a packed column Is a function of the packing particle diameter (dp) and the analyte capacity factor (k ) ... 

For forced flow separations a constant plate height independent of the solvent-front migration distance is obtained. Figure 6.3. The minimum plate height for capillary flow is always greater than the minimum for forced flow. This is an indication that the limited range of capillary flow velocities is inadequate to realize the optimum kinetic performance for the layers. At the mobile phase optimum velocity, forced flow affords more compact zones and shorter separation times compared with capillary flow. As expected the intrinsic efficiency increases with a reduction of the average particle size for the layer. 

This can now give a number of useful interpretations of the performance of the capillary column. From the minimum of the Golay curve, the minimum plate height can be derived ... 

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