Layers band broadening

Pelander et al. [81] developed a computer program for optimization of the mobile phase composition in TLC. They used the desirability function technique combined with the PRISMA model to enhance the quahty of TLC separation. They apphed the statistical models for prediction of retardation and band broadening at different mobile phase compositions they obtained using the PRISMA method the optimum mobile phase mixtures and a good separation for cyanobacterial hepatotoxins on a normal phase TLC plate and for phenolic compound on reversed-phase layers. 

Laminar flow (LC) 514 Lattice statistical thermodynamic theory (LC) 401 Layers (TLC) 671 alumina 673 band broadening 662 bilayer 671 binders 671... 

Chromatographic methods were developed to separate a few of the citrus flavonoids from the complex mixture of citrus flavonoids. The early paper chromatographic methods for flavanones (22, 23) were difficult to quantitate because of band broadening and uneven solvent development. Several thin layer chromatographic (TLC) methods were developed to separate the bitter from the nonbitter flavanone glycosides (24, 25, 26, 27). 

In the preparation of standards, diffusional techniqnes are of special importance. Using snch techniqnes diffusion layers are formed. With a special modification by use of wedge-type samples, these layers are broadened and are thus better accessible for the microprobe.The samples consist of several diffusion bands because of the various phases in the respective carbide system. Usually, however, the samples should be single phase and homogeneous in order to make chemical gross compositional analysis applicable. Only with a comparison of different techniques a real standardization is possible. Such transition metal carbide standards have been made from the phases VC, NbC, TaC and Cr3C2. ... 

Strong band broadening upon adsorption in the hydrogen region is observed for both Pt(lll) (Fig. 21) and Pt(llO) (Fig. 22) surfaces. On Pt(lll) much less band broadening is observed for CO/H2O than for CO/H co-adsorption. In the case of CO adsorbed on Pt(llO) in the double layer region, the band splitting makes it diffi-... 

Pellicular or controlled surface porosity particles were introduced in the late 1960s these have a solid inert impervious spherical core with a thin outer layer of interactive stationary phase, 1-2 pm thick [13]. Originally, the inner sphere was a glass bead, 35-50 pm i.d., with a thin active polymer film or a layer of sintered modified silica particles on its surface. Such particles were not very stable, had very low sample load capacities because of low surface areas and are not used any more. Nowadays, this type of material is available as micropellicular silica or polymer-based particles of size 1.5 to 2.5 pm [14]. Micropellicular particles are usually packed in short columns and because of fast mass-transfer kinetics have outstanding efficiency for the separation of macromolecules. Because the solutes are eluted as very sharp narrow peaks, such columns require a chromatograph designed to minimise the extra-column contributions to band broadening. 

Jaulmes and Vidal-Madjar [51] studied the influence of the mass transfer kinetics on band profiles, using a Langmuir second-order kinetics, and a constant axial dispersion coefficient, D. They derived numerical solutions using a finite difference algorithm. The influence of the rate constant on the band profile at various sample sizes is illustrated in Figure 14.18. As the mass transfer kinetics slows down, the band broadens and the shock layer becomes thicker. When the sample size increases, however, the influence of thermodynamics on the profile becomes more dominant, as shown by the change in shock layer thickness which decreases with increasing sample size. 

Efficiency, N The column efficiency characterizes the combined effects of the sources of band broadening due to axial dispersion and mass transfer resistance. It is derived from the width of the elution peak observed as the response to the injection of a small, narrow pulse of a dilute solution of a compoimd. It is difficult to correct for the contribution of the extracolumn sources of band broadening which have to be kept small. In preparative and nonlinear chromatography, there is a correlation between the colmnn efficiency and both the steepness of the shock layer and the duration of the band beyond the retention time However, the column efficiency is essentially a concept of linear chromatography, and it is difficult to extend to and use in nonlinear chromatography, except through the shock layer thickness concept. 

It was of interest to compare the results described above with those obtained for aniline molecules adsorbed from the vapor phase on to sublimed KCl layer in vacuo (47). The spectrum shows even at a low coverage (corresponding to 0.1 mm Hg vapor pressure) the 280 m/x band broadened and shifted to the lower frequencies by about 5000 cm-i. This is a kind of chemisorption, since, in contrast to pyridine, evacuation at 20°C is not sufficient to remove the adsorbed molecules, and for the disappearance of this spectrum heating is additionally required. 

On ion-exchange resins the rate of diffusion through the stationary phase, to and from the ion-exchange site, is relatively slow. This slow mass transfer results in a large contribution towards band broadening. Pellicular materials with only a thin layer of resin were evolved to... 

The flow profile of the EOF has the form of a plug (Fig. 3.4). The flow velocity is identical over the whole capillary diameter, except for the slower moving diffuse layer close to the capillary wall. This homogeneous velocity distribution minimises band broadening and, thus, increases separation efficiency. A radically different situation occurs with the pressure driven flow used in liquid chromatography. Here, the flow profile is parabolic the flow velocities have a large distribution over the column diameter. Analytes in the middle flow considerably faster than analytes... 

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