Plate height separation time

Recovery factor Reduced column length Reduced plate height Reduced velocity Relative retention ratio Retardation factor d Retention time Retention volume Selectivity coefficient Separation factor... 

Upon substitution of the reduced parameters given above the separation time for a packed column and an open tubular column would be Identical if d 1.73 dp given the current limitations of open tubular column technology the column diameter cannot be reduced to the point %diere these columns can compete with packed columns for fast separations. This is illustrated by the practical txanple in Figure 6.3 (57). Ihe separation speed cannot be Increased for an open tubular column by increasing the reduced velocity since the reduced plate height is increased... 

The Poppe plot is a log-log plot of H/uq = t(JN versus the number of plates with different particle sizes and with lines drawn at constant void time, t(). H is the plate height, Vis the number of plates, and u() is the fluid velocity (assumed equal to the void velocity). The quantity H/u() is called the plate time, which is the time for a theoretical plate to develop and is indicative of the speed of the separation, with units of seconds. In the Poppe plot, a number of parameters including the maximum allowable pressure drop, particle diameter, viscosity, flow resistance, and diffusion coefficient are held constant. 

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) ... 

From this equation it can be seen that the efficiency per unit length is inversely proportional to the capillary diameter. Decreasing the diameter of the capillary will decrease the height equivalent to a theoretical plate. The efficiency per unit length increases. Therefore, smaller-diameter capillaries can be used at shorter lengths, which ultimately decreases the separation time. 

So far, the separation efficiencies reported with the submicron packed beds have not offered a significant improvement over those obtained with particle diameters in the 1 pm range [66,119-121]. Fig. 4.17 depicts the separation of a test mixture obtained in a packed bed with particles of about 0.5 pm in diameter. As reported by Luedtke, et al. [121], plate heights of about three times the particle diameter (H = 3dp) are achieved. This has been attributed to band dispersion due to temperature effects and instrumental limitations, such as the maximum electric field that can be applied with existing units and detection systems [121], Plots of plate height versus linear... 

Given a certain relative dispersion in flow velocities, Eq. 9.12 shows that the plate height increases with the mean velocity V of the solute. Clearly then, H can be reduced by decreasing the flowrate. However a reduction in flow will lead to longer retention times and slower separation. Therefore the choice of flow velocity requires a compromise between separation speed and efficiency. This subject has been treated in some detail for chromatography [2] we will say more about it in Chapter 12. 

Many chromatographic systems are run at or above the optimum flow velocity vopt to achieve faster separation. In this case Eq. 12.57 is no longer valid for plate height. However, for purposes of the present discussion, in which we are seeking ways to reduce plate height but not time, we will assume that the first step, velocity optimization, has been taken, and that Eq. 12.57 is applicable. 

The best separation will occur at the a- value where the highest a occurs. If two or more windows have approximately the same a value, the best one will be the smallest one since it will usually provide the shortest analysis time. The length of column needed to get a given resolution can be calculated with the equation for nreq presented in Chapter 4 and the assumption of a reasonable plate height. 

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... 

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