Plate height ratio

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... 

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

Now t[i is a minimum when k = 2, that is, when = 3t . There is little increase in analysis time when k lies between 1 and 10. A twofold increase in the mobile-phase velocity roughly halves the analysis time (actually it is the ratio Wu which influences the analysis time). The ratio Wu can be obtained from the experimental plate height/velocity graph. 

The reduced velocity compares the mobile phase velocity with the velocity of the solute diffusion through the pores of the particle. In fact, the mobile phase velocity is measured in units of the intraparticle diffusion velocity. As the reduced velocity is a ratio of velocities then, like the reduced plate height, it also is dimensionless. Employing the reduced parameters, the equation of Knox takes the following form... 

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. 

Figure 4. Graph of Minimum Plate Height against Separation Ratio of the Critical Pair...

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... 

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. 

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

As in chromatography, the most common variable to describe dispersion in FFF is the plate height (H), defined as the ratio between the spatial variance (Oj) of the band and its mean position (s) as it migrates inside the separation medium of length (L) ... 

Calculate the plate height contributed by sorption-desorption mass transfer (nonequilibrium) through a uniform liquid layer (configuration factor q = 2/3) of thickness 1.0 x 10 3 cm coated on the inside of an open tubular (capillary) column. The gas velocity v is 10 cm/s. The solute retention ratio is 0.10 and its diffusion coefficient Ds through the stationary liquid is 1.0 x 10 5 cm2/s. 

What length column is needed to separate at unit resolution components A and B where the distribution coefficients are Ka = 110, Kb = 120, and where the ratio 0 = VJVS = 20 The plate height H for both components is approximately 0.050 cm. 

Partition coefficient, 9, 10 Partition ratio, 11 time optimization of, 57-58 Peak, definition of, 69 Peak capacity, 18, 19 Pellicular supports, 157 Permeability, 63-64 Phase selection diagrams, 218-219 Phase volume ratio, 11 Pinkerton (ISRP) columns, 225-226 Plate height, 17 Plate number, 14-16 Plate theory, 3, 28 Polarity index, 210, 211 Pore size of LC supports, 157 Porosity, 27 Precision, 99-100 Preparative scale ... 

The reduced parameters are also helpful in evaluating column performance. The best columns have a reduced plate height of 2 to 5—a number that can be thought of as representing the number of particles between sorptions—and 2 is a practical minimum. The reduced velocity represents the ratio between the flow velocity and the diffusion rate over one particle diameter typical values should be in the range of 3 to 20. 

Unfortunately, Eq. (8) is not fully satisfactory since it results in negative values for the Cs term at high pore flow (to + k" >1). Such high flow ratios may be created when a pressure-driven flow is directed against the electroosmotic flow. Also, when the interior of the particles has a much higher surface potential than the exterior, surface flow ratios > 1 may be expected. Of course, negative contributions to the total plate height are physically impossible and Eq. (9) cannot be valid. 

Figure 12 Plate height obtained with 7-pm reversed-phase particles as a function of pore flow ratio to.

When the pore size, ionic strength, and electrical field strength are optimized for a high pore flow velocity and a high pore-to-interstitial flow ratio, reduced plate heights well below unity can be achieved in CEC of low-molecular-mass compounds. When such conditions can be created in combination with the use of small particles (dp < 1 pm), plate heights below 1.0 pm will be possible. 

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