Column independent time factors

Only the first factor is influenced by the physico-chemical separation process (the selectivity), while the other two factors are determined by the column and the operating conditions, respectively. If C is a continuous criterion (see table 4.7), then both C and C, can be transferred from one column to another. Both column dimensions and flow rate have trivial effects on the analysis time tm. However, if the final analysis is to be run on a different (optimized) column, then it is more logical to use the dimensionless, column-independent factor (1 + km) in eqn.(4.31) instead of tm ... 

Capacity factor, k This is a more useful measure of peak retention that retention time, as it is independent of column length and flow rate. To calculate k you need to measure column dead time, to- This is the time it takes an unretained component to pass through the column without any interaction with the stationary phase. It is the time taken from the point of sample injection until the first disturbance in the base line caused by the... 

Figure 3.1 shows the effect of various mobile phase compositions on the selectivity factors for the separation of phenol/aniline and toluene/benzene pairs on 1.0, 2.1, 3.0, and 4.6 mm i.d. columns operating at a given velocity. The selectivity factors for the four columns are practically the same, even though retention times were not corrected to compensate for the differences in extra-column migration times. This finding is in agreement with Eq. (3.4) that shows that selectivity factors are independent of extra-column volume and column internal diameter. 

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. 

The factor k, which is independent of the flow rate and length of the column, can vary with experimental conditions, k is the most important parameter in chromatography for determining the behaviour of columns. The value of k should not be too high, otherwise the time of analysis is unduly elongated. 

Figures 9 and 10 illustrate changes in two dependent variables dynamic N2 adsorption capacities and CH4/N2 separation factors. Independent variables are column temperature, operating pressure, and time allowed for vacuum regeneration. This experimental series used a constant feed rate of 6.0 1/min over a time of 1.00 min into a 1" dia. x 24" long adsorber filled with 180g of zeolite. Column depressurization took place for 1.00 min. and this was followed by a variable length vacuum regeneration.

Chromatographic Retention The retention of a drug with a given packing material and eluent can be expressed as a retention time or retention volume but both of these are dependent on flow rate, column length, and column diameter. The retention is best described as a column capacity ratio (k ) which is independent of these factors. The column capacity ratio of a compound (A) is defined by... 

The capacity factor is independent of the equipment being used, and is a measure of the column s ability to retain a sample component. Small values of k imply that the respective component elutes near the void volume thus the separation wall be poor. High values of k, on the other hand, are tantamount to long analysis times, associated peak broadening, and a decrease in sensitivity. 

In ideal chromatography, we assume that the column efficiency is infinite, or in other words, that the axial dispersion is negligibly small and the rate of the mass transfer kinetics is infinite. In ideal chromatography, the surface inside the particles is constantly at equilibrium with the solution that percolates through the particle bed. Under such conditions, the band profiles are controlled only by the thermodynamics of phase equilibria. In linear, ideal chromatography, all the elution band profiles are identical to the injection profiles, with a time or volume delay that depends only on the retention factor, or slope of the linear isotherm, and on the mobile phase velocity. This situation is unrealistic, and is usually of little importance or practical interest (except in SMB, see Chapter 17). By contrast, nonlinear, ideal chromatography is an important model, because the profiles of high-concentration bands is essentially controlled by equilibrium thermodynamics and this model permits the detailed study of the influence of thermodynamics on these profiles, independently of the influence of the kinetics of mass transfer... 

The column is not capable of separating any quantity of substances. If too much material is injected, then the retention factor and the peak width cease to be independent of sample size (see Figure 2.23). The peaks become wider and asymmetric and the retention time changes at the same time. Tailing is accompanied by decreased retention and fronting by an increase in k. 

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