Plate height resolution

Separation Parameters Retention Ratio, Plate Height, Resolution,... 

Elution volume, exclusion chromatography Flow rate, column Gas/liquid volume ratio Inner column volume Interstitial (outer) volume Kovats retention indices Matrix volume Net retention volume Obstruction factor Packing uniformity factor Particle diameter Partition coefficient Partition ratio Peak asymmetry factor Peak resolution Plate height Plate number Porosity, column Pressure, column inlet Presure, column outlet Pressure drop... 

Another approach to improving resolution is to use thin films of stationary phase. Capillary columns used in gas chromatography and the bonded phases commonly used in HPLC provide a significant decrease in plate height due to the reduction of the Hs term in equation 12.27. 

The important parameters to consider are the selectivity (dKJdlogR), the ratio of pore volume, Vp, over void volume, Vq, the plate height, H, and the column length, L. The distribution coefficient, Kq, has a slight effect on resolution (with an optimum at Kp 0.3-0.5). In addition to this, extra column effects, such as sample volume, may also contribute to the resolution. 

Plate height determining the length of the column for a desired resolution... 

In order to evaluate the efficacy of the expanded bed technique the plate height (HETP), plate number (N), resolution (Rs), Bodenstein number (Bo), particle Peclet number (Pep) and axial dispersion coefficient (DJ have been calculated and compared with the corresponding values of a traditional HPLC column. N can be expressed by... 

A second reason why very high resolution can be obtained in chromatography is that very large numbers of theoretical plates are readily achieved. If the column is well packed with particles having a narrow spread of sizes, the plate height is about twice the particle diameter 9, n A typical large-scale GC or LC column will contain of 103-104 plates. 

For validation of columns that will be used for an official assay and to provide an unambiguous standard for qualifying future media lots, it is useful to employ more measurable comparative criteria than a simple overlay. Resolution, plate heights, and peak symmetry, as calculated by the classical formulae, should match very closely among test and reference columns (Figure 6.1, Figure 6.2). 

Substances A and B were found to have retention times of 17-30 and 19-92 minutes respectively on a 25-0 cm column. The widths (at the base) for A and B were 1-10 and 1-22 minutes respectively. The average number of plates in the column and the plate height calculated resolutions. 

In packed columns, all three terms contribute to band broadening. For open tubular columns, the multiple path term, A, is 0, so bandwidth decreases and resolution increases. In capillary electrophoresis (Chapter 26), both A and C go to 0, thereby reducing plate height to submicron values and providing extraordinary separation powers. 

Particles in a packed column resist flow of the mobile phase, so the linear flow rate cannot be very fast. For the same length of column and applied pressure, the linear flow rate in an open tubular column is much higher than that of a packed column. Therefore, the open tubular column can be made 100 times longer than the packed column, to give a similar pressure drop and linear flow rate. If plate height is the same, the longer column provides 100 times more theoretical plates, yielding VTOO = 10 times more resolution. 

Helium is the most common carrier gas and is compatible with most detectors. For a flame ionization detector, N2 gives a lower detection limit than He. Figure 24-11 shows that H2, He, and N2 give essentially the same optimal plate height (0.3 mm) at significantly different flow rates. Optimal flow rate increases in the order N2 < He < H2. Fastest separations can be achieved with H, as carrier gas, and H2 can be run much faster than its optimal velocity with little penalty in resolution.11 Figure 24-12 shows the effect of carrier gas on the separation of two compounds on the same column with the same temperature program. 

Capillary electrophoresis provides unprecedented resolution. When we conduct chromatography in a packed column, peaks are broadened by three mechanisms in the van Deemter equation (23-33) multiple flow paths, longitudinal diffusion, and finite rate of mass transfer. An open tubular column eliminates multiple paths and thereby reduces plate height and improves resolution. Capillary electrophoresis reduces plate height further by knocking out the mass transfer term that comes from the finite time needed for solute to equilibrate... 

Zhang et al. [106] resolved rhodaminel23 and fluorescein by using NCE. The migration times, theoretical plate numbers resolution, back flow rate, and peak heights of rhodaminel23 and fluorescein are given in Table 3.5. 

TABLE 3.5 Effects of Reservoir Levels on Migration Time, Theoretical Plate Numbers, Resolutions, and Peak Heights of Rhodaminel23 and Fluorescein in NLC ... 

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