Chromatographic zone broadening

The simplest picture of chromatographic zone broadening comes from the random walk model. The random walk model for chromatography will be developed at some length in Chapter 11 here we present a synopsis. 

TABLE 1 Typical Values of Parameters Important in Chromatographic Zone Broadening... 

Chromatographic zone broadening, however, is also influenced by finite mass transfer rates, or porous diffusion. Selection of the proper relationship between n and c is therefore needed. 

As correctly stated by Ettre [13], the idea of making a column with a porous sorbent layer on the inner capillary walls was implied in an early version of Golay s equation describing chromatographic zone broadening in an open capillary column, depending on the carrier gas velocity (the Amsterdam Symposium, 1958). 

This chapter Fundamentals of gas-solid chromatography comprises four sections 3.1. Retention in gas-solid chromatography 3.2. Chromatographic zone broadening in ALOT columns 3.3. Dependence of column efficiency on sample size and 3.4. Resolution of analyzed compounds. From a theoretical point of view, these topics are mainly of interest for capillary gas-solid chromatography. 

Similar to all chromatographic processes the band of solute that emerges from the column can be broadened by a number of processes, including contributions from the apparatus, flow of the solution through the packed bed of gel particles, and the permeation process. Corrections for this zone broadening may be made empirically it generally becomes unimportant when the sample has... 

FLOW. The rate at which zones migrate down the column is dependent upon equilibrium conditions and mobile phase velocity on the other hand, how the zone broadens depends upon flow conditions in the column, longitudinal diffusion, and the rate of mass transfer. Since there are various types of columns used in gas chromatography, namely, open tubular columns, support coated open tubular columns, packed capillary columns, and analytical packed columns, we should look at the conditions of flow in a gas chromatographic column. Our discussion of flow will be restricted to Newtonian fluids, that is, those in which the viscosity remains constant at a given temperature. 

The main cause of zone broadening (i.e., loss of efficiency) in a chromatographic bed is diffusion. Diffusion perpendicular to the flow is restricted by the walls of the column. Therefore, longitudinal diffusion is the primary factor contributing to zone broadening. While a protein is adsorbed to the medium, little or no diffusion takes place, but once the protein is unbound, diffusion begins. The amount of diffusion that occurs is proportional to the time required for the material to emerge from the... 

Despite such complications of detail, the random-walk model describes the essence of chromatographic zone spreading. It properly accounts for the way in which all major experimental parameters influence the broadening process. 

The process of analyte retention in high-performance liquid chromatography (HPLC) involves many different aspects of molecular behavior and interactions in condensed media in a dynamic interfacial system. Molecular diffusion in the eluent flow with complex flow dynamics in a bimodal porous space is only one of many complex processes responsible for broadening of the chromatographic zone. Dynamic transfer of the analyte molecules between mobile phase and adsorbent surface in the presence of secondary equilibria effects is also only part of the processes responsible for the analyte retention on the column. These processes just outline a complex picture that chromatographic theory should be able to describe. 

HPLC theory could be subdivided in two distinct aspects kinetic and thermodynamic. Kinetic aspect of chromatographic zone migration is responsible for the band broadening, and the thermodynamic aspect is responsible for the analyte retention in the column. From the analytical point of view, kinetic factors determine the width of chromatographic peak whereas the thermodynamic factors determine peak position on the chromatogram. Both aspects are equally important, and successful separation could be achieved either by optimization of band broadening (efficiency) or by variation of the peak positions on the chromatogram (selectivity). From the practical point of view, separation efficiency in HPLC is more related to instrument optimization, column... 

The most rigorous discussion of the formation of chromatographic zone and the mathematical description of zone-broadening is given in reference 1. Here only practically important and useful equations will be discussed. 

This process governs the rate of deposition of the molecules of nonvolatile compounds on the surface of gas ducts, and contributes to broadening of the chromatographic zones. Being of the order of 0.1 pm at STP, the mean free path of molecules, which is inversely proportional to pressure, reaches 1 cm only at about 0.01 mmHg. In dense enough gas, in the absence of convective flow, the macroscopic picture of migration of molecules (as well as of aerosol particulates) is described by the equations of diffusion. The mean squared diffusional displacement z2D of molecules, the time of diffusion t and the mutual diffusion coefficient >i 2 are related by ... 

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