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the solutes are slowed down by the more retentive layer and are thus focused as a sharp band at the front of the plate. As development proceeds, the solutes separate in the normal high retentive layer in the usual manner. This procedure has other advantages. If the sample is contaminated with salts or biological polymers, these will be trapped in the concentration zone and, thus, will not pass onto the separation region of the plate and effect the quality of the separation. Band applicators operate differently and are usually fully automated. The sample is atomized in a stream of air or nitrogen depending on the nature of the sample and its tendency to oxidation. A diagram of the type of atomizer used in band application is shown in figure 35. Figure 34 TLC Plate with Sample Concentrating Zone

The width of the band of an eluted solute relative to its proximity to its nearest neighbor determines whether two solutes are resolved or not. The ultimate band width as sensed by the detector is the result of a number of individual dispersion processes taking place in the chromatographic system, some of which take place in the column itself and some in the sample valve, connecting tubes and detector (see Extra Column Dispersion ). In order to determine the ultimate dispersion of the solute band it is necessary to be able to calculate the final peak variance. This is achieved by taking into account all the individual dispersion processes that take place in a chromatographic system. It is not possible to sum the band widths (standard deviation or (s)) resulting from each individual dispersion process to obtain the final band width, but it is possible to sum all the respective variances. However, the summation of all the variances resulting from each process is only possible if each

parabolic velocity profile that occurs under conditions of Newtonian flow, (i.e. when the velocity is significantly below that which produces turbulence). Under condition of Newtonian flow, the distribution of fluid velocity across the tube adopts a parabolic profile as shown in figure 1. The velocity at the walls is virtually zero and that at the center a maximum. This situation is depicted diagramatically in Figure 1. Figure 1. The Parabolic Velocity Profile of a Solute Band Passing Through a Tube Due to the relatively high velocity at the center of the tube and the very low velocity at the walls, the center of the band of solute passing down the tube will move ahead of that situated at the walls. The resulting effect of band dispersion is depicted in figure 2

nbsp;   The different excitation frequencies can be used in various types of analytical applications some of which are broadly summarized in table 1.   The bands given in table 1, for the most part, comprise all of the analytically useful bands in the ultra violet-visible region. The energy associated with a specific transition, theoretically, should produce a single sharp absorption band.   Table 1 Application Areas for Absorption at Different UV Wavelengths   Adsorption Band- Substance Analytical Application 180-250 nm bands of most aromatic hydrocarbons Excellent for trace analysis and characterization 160-180 nm bands of single olefins Good for trace analysis with an appropriate spectrophotometer 250 nm bands of benzene Reasonable for trace analysis good for characterization 260-290 nm bands of saturated aldehydes and ketones Poor for

with the benzene. In fact, as a result of solute–solute interaction the benzene is, in effect, partially eluting itself. The effect of the mass overload of benzene on the other solutes is also clearly demonstrated. The presence of the high concentration of benzene in the mobile phase increases the elution rate of both the naphthalene and the anthracene. Its also seen, however, that the effect of the high concentration of the benzene on the closer eluting peak naphthalene is to produce band dispersion, whereas the anthracene band does not suffer significant dispersion and the retention of both the front and the rear of the anthracene peak appear to be linearly reduced with sample mass. The  chromatograms shown in figure 7 also show that the anthracene peak maintains its symmetry throughout all sample sizes. The impact of the high concentration of benzene on the elution of the other solutes only occurs while the solutes are still in contact with one another at the beginning of

Thus, assuming there are (n) non-interacting, random dispersive processes occurring in the chromatographic system, then any process (p) acting alone will produce a Gaussian curve having a variance  ,                 Hence,         where,  () is the variance of the solute band as sensed by the detector. The above equation is the algebraic enunciation of the principle of the summation of variances and is fundamentally important. If the individual dispersion processes that are taking place in a column can be identified, and an expression for the variance arising from each dispersion process evaluated, then the variance of the final band can be calculated from the sum of all the individual variances. This is how the Rate Theory provides an equation for the final