Packed columns reduced parameters

Having chosen the test mixture and mobile diase composition, the chromatogram is run, usually at a fairly fast chart speed to reduce errors associated with the measurement of peak widths, etc.. Figure 4.10. The parameters calculated from the chromatogram are the retention volume and capacity factor of each component, the plate count for the unretained peak and at least one of the retained peaks, the peak asymmetry factor for each component, and the separation factor for at least one pair of solutes. The pressure drop for the column at the optimum test flow rate should also be noted. This data is then used to determine two types of performance criteria. These are kinetic parameters, which indicate how well the column is physically packed, and thermodynamic parameters, which indicate whether the column packing material meets the manufacturer s specifications. Examples of such thermodynamic parameters are whether the percentage oi bonded... 

Reduced parameters (section 1.7.10) can -be used to compare the potential of open tubular and packed columns to deliver a certain separation potential in SFC [8,43,53-56]. The Golay equation, equation 6.1, can be rewritten as... 

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 measurement of effluent volume is not very reliable because of the effect of the geometry and packing characteristics of any column. It is often more useful to use a reduced parameter, such as V/V0, which is not so dependent upon column characteristics and is comparable with the calculation for RF values in thin-layer chromatography. 

The best values of the parameter Cj are 1.51, 1.36, and 2.01 for no mass transfer, d and c direetion of transfer respectively. The product af is considered as the agitation variable in the equation, since the fit could not be improved if a and / were treated separately. The average absolute value of the relative deviation in the predicted values of d 2 from the experimental points is 16.3%. Even in packed columns, the separation can be substantially improved by pulsing of the continuous phase resulting from greater shear forces that reduce the drop size and increase the interfacial area [1, Chapter 8]. 

MECC separations are conducted in open capillaries, hence eddy diffusion is not problematic. However, the columns behave in many ways like packed columns, with the micelles functioning as uniformly sized and evenly dispersed packing particles. In packed columns, resistance to mass transfer in the mobile phase is reduced (i.e., efficiency improved) when smaller particles are used because the "diffusion distance" between particles is decreased. Average inter-micellar" distance is the analogous parameter in MECC. This distance can be decreased by increasing surfactant concentration. 

The reduced plate height, h, is independent of the particle diameter. The reduced fluid velocity (u), a concept conceived by Giddings, is a measure of the rate of flow over a particle relative to the rate of diffusion of solute within the particle. Since both reduced parameters, h and V, are normalized for the particle diameter, when h is plotted against v, the different size fractions of the same packing materials should give similar curves. This has been confirmed in practice, so that use of h vs v curves is preferred over that of plate height curves (H versus mobile phase velocity) when comparison of the efficiency of different columns is to be carried out. 

The major advantage of using reduced parameters (h, v, (]>) in place of the absolute parameters (H, u, Ko) is that results obtained from columns containing packing... 

To reduce or eliminate polymer solute/glass packing interactions the following parameters were optimized a) pH, ionic strength and concentrations of additives such as nonionic surfactants, b) selection of pore sizes in a column combination. 

In eluent compositions beyond those that are typically part of the routine adjustments carried out by the analyst to meet system suitability parameters. As expected, the use of short columns packed with 3-um particles results in reduced solvent consumption and reduced mobile phase re-equilibration time in gradient elution. 

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