Plate, effective plates height

However, Increasing the protein concentration further to 5 mg In 0.1 ml increased the effective plate height from about 0.6 mm to 0.82 mm. This change In plate height corresponds to an Increase In R and a corresponding... 

McReynold s constant for 2-melhylpentanol-2 Effective plate height (height equivalent to one effective plate) Isoteric heat of adsorption Retention index... 

Band Asymmetry. The peak asymmetry factor AF is often defined as the ratio of peak half-widths at 10% of peak height, that is, the ratio b/a, as shown in Fig. 11.2. When the asymmetry ratio lies outside the range 0.95-1.15 for a peak of k =2, the effective plate number should be calculated from the expression... 

Equations 12.21 and 12.22 contain terms corresponding to column efficiency, column selectivity, and capacity factor. These terms can be varied, more or less independently, to obtain the desired resolution and analysis time for a pair of solutes. The first term, which is a function of the number of theoretical plates or the height of a theoretical plate, accounts for the effect of column efficiency. The second term is a function of a and accounts for the influence of column selectivity. Finally, the third term in both equations is a function of b, and accounts for the effect of solute B s capacity factor. Manipulating these parameters to improve resolution is the subject of the remainder of this section. 

In 1972-1973 Knox et al. [3, 4, 5] examined, in considerable detail, a number of different packing materials with particular reference to the effect of particle size on the reduced plate height of a column. The reduced plate height (h) and reduced velocity (v) were introduced by Giddings [6,7] in 1965 in an attempt to form a rational basis... 

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 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. 

FIGURE 10.1 I Effect of linear velocity on plate height for proteins. Column SynChropak GPC 500, 250 X 4.6 mm i.d. Mobile phase 0.1 M potassium phosphate, pH 7. (Reprinted from Ref. 4 by courtesy of Marcel Dekker, Inc.)... 

The point of all this is simply that we must not use the apparent plate height or the apparent plate number as performance criteria in the unified chromatography techniques on the justification that they already work well for LC and that they work well for GC when a pressure correction is applied. A considerable expansion of theory and an effective means for evaluating equations (7.4) or (7.5) are required first. Likewise, as we consider multidimensional chromatography involving techniques existing between the extremes of LC and GC, we must not build judgments of the multidimensional system on unsound measures of the individual techniques involved. 

J. C. Giddings, Plate height of nonuniform cliromatographic columns. Gas compression effects, coupled columns and analogous systems . Anal. Chem. 35 353-356 (1963). 

The efficiency of an open tubular column can be measured in several ways the most widely used methods are the number of theoretical plates (n), the nu3aber of effective theoretical plates (N), the height equivalent to a theoretical plate (HETP) or effective plate, the coating efficiency and the separation number (SM). No single method is ideal, standardization is lacking and... 

The plate height, and thus the total number of theoretical or effective plates, depends on the average linear carrier gas velocity (van Deemter relationship) and, for a particular carrier gas, the efficiency will maximize at a particular flow rate. Only at the optimum carrier gas flow rate are n, N, and HETP Independent of the column length. The efficiency will also depend on the column diameter (see section 1.7.1) where typical values for n, N, and HETP for different column types can also be found. Values for n, N, and HETP are reasonably independent of temperature but may vary with the substance used for their determination, particularly if the test substance and statioKary phase are not compatible. 

The diffusivities thus obtained are necessarily effective diffusivities since (1) they reflect a migration contribution that is not always negligible and (2) they contain the effect of variable properties in the diffusion layer that are neglected in the well-known solutions to constant-property equations. It has been shown, however, that the limiting current at a rotating disk in the laminar range is still proportional to the square root of the rotation rate if the variation of physical properties in the diffusion layer is accounted for (D3e, H8). Similar invariant relationships hold for the laminar diffusion layer at a flat plate in forced convection (D4), in which case the mass-transfer rate is proportional to the square root of velocity, and in free convection at a vertical plate (Dl), where it is proportional to the three-fourths power of plate height. 

Neither the capacity factor nor the separation factor take into account the effect of dispersion, which is measured by the plate number or the plate height of the column. These were defined in Section 2.3.2. 

Figure 4.3 Effect of particle diameter on plate height. (Reproduced from Lichrospher Lichroprep Sorbents Tailored for Cost Effective Chromatography, EM Separations, Gibbstown. With permission from Merck kGaA, Darmstadt, Germany, and EMD Chemicals, Inc.)...

A less obvious effect of elevated temperature is on plate height. The equation describing plate height (H) can be written as follows [14] ... 

FIGURE 9.2 Theoretical plate-height curves for 5 pm particles, illustrating the effect of temperature on plate height and linear velocity. (Reprinted from Lestremau, F. et al., J. Chromatogr., 1138, 120, 2007. Copyright 2007. With permission from Elsevier.)... 

It follows, that if the detector was to be effective and produce the true Gaussian form of the eluted peak then, (Ca), must at all times be unity and consequently the detector must have the same plate capacity as that of the column. This means that the detector must employ the same absorbant, have the same geometry and be packed to give the same plate height as the column. It is obvious to accomplish this, the column must also be the detecting cell and the temperature sensing element must be placed in the column packing itself. 

The curves represent a plot of Log.(/V),(Reduced Plate height)against Log.(v), (Reduced Velocity). The lower the Log.(/7) curve versus the Log.(v) curve the better the column is packed. At low velocities the (B) term dominates and at high velocities the (C) term dominates as in the Van Deemter equation. The best column efficiency is achieved when the minimum is about 2 particle diameters and thus, Log (.ft) Is about 0.35. The minimum value for (H) as predicted by the Van Deemter equation has also been shown to be about two particle diameters. The optimum reduced velocity is in the range of 3 to 5 that is Log.(v ) takes values between 0.3 and 0.5. The Knox equation is a simple and effective method of examining the quality of a given column but, as stated before, is not nearly so useful In column design due to the empirical nature of the constants. 

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