Frontal analysis by characteristic point

One of the most important applications of frontal chromatography is the determination of equilibrium adsorption isotherms. It was introduced for this purpose by Shay and Szekely and by James and Phillips [4,5], The simplicity as well as the accuracy and precision of this method are reasons why the method is so popular today and why it is often preferred over other chromatographic methods e.g., elution by characteristic points (ECP) or frontal analysis by characteristic points (FACP) [6,7]. Frontal chromatography as a tool... 

It has to be noted that the half-height and inflection-point methods do not give reliable results if the isotherm is concave upward and ascending concentration steps are performed. The same is true for a convex upward isotherm and descending concentration steps. The reason for this is that, in these cases, a diffuse breakthrough profile is obtained and, consequently, errors are made in the accurate determination of the retention volumes when they are derived from the half-height or the inflection point. The diffuse profile can, however, be used for the determination of isotherms by the frontal analysis by characteristic points method (FACT). 

The dynamic methods are based on direct chromatography and are popular because they are faster and easier to automate. Four direct chromatographic methods that are available for determination of adsorption isotherms are frontal analysis (FA) [13, 109] frontal analysis by characteristic points (FACP) [109], elution by characteristic points (ECP) [109] and the perturbation peak (PP) method [118-121], The FACP and ECP methods have... 

Analysis of disperse fronts ECP (elution by characteristic point) FACP (frontal analysis by characteristic point) Pulse or step injection (high concentration) Slope of dispersive front Single component Small sample amounts Highly efficient columns and small plant effects necessary Phase equilibrium is required (sensitive to kinetics) Precise detector calibration necessary... 

The primary use of isotherm data measurements carried out on single-component elution profiles or breakthrough curves is the determination of the single-component adsorption isotherms. This could also be done directly, by conventional static methods. However, these methods are slow and less accurate than chromatographic methods, which, for these reasons, have become very popular. Five direct chromatographic methods are available for this purpose frontal analysis (FA) [132,133], frontal analysis by characteristic point (FACP) [134], elution by characteristic point (ECP) [134,135], pulse methods e.g., elution on a plateau or step and pulse method) [136], and the retention time method (RTM) [137]. 

A number of experimental techniques have been described for the determination of isotherms based on frontal analysis, frontal analysis by characteristic point, elution by characteristic point, and perturbation methods [12,21,27,169,176-179]. Most authors report single-component isotherm results. Multiple-component isotherm data are more complicated because all components are simultaneously in competition for the sorption sites on the stationary phase. The retention time and peak shapes of any solute is dependent on the concentration and properties of all other solutes in the mixture [12,170,180]. For multicomponent mobile phases in liquid and supercritical fluid chromatography this includes each component of the mobile phase. 

Analysis of disperse fronts elution by characteristic point (ECP), frontal analysis by characteristic point (FACP)... 

Danner et al." tested the frontal analysis by characteristic point (FACP) technique to measure thermodynamic data for polymer-solvent systems at finite concentrations. In the FACP technique, a complete isotherm can be derived from the shape of one breakthrough profile. A point on an isotherm is obtained by measuring the retention volume of the characteristic point at the corresponding concentration. The methods to determine thermodynamic data by FACP technique were discussed in detail by Conder and Young. ... 

As mentioned, there are different variations of chromatographic methods. The most well known are [2,4-6] frontal analysis, frontal analysis by characteristic point, perturbation method, elution method by characteristic point, and inverse chromatography. 

Direct determination of the column saturation capacity requires measurement of the adsorption isotherm. Use of methods such as frontal analysis, elution by characteristic point are classical techniques. Frontal analysis and elution by characteri.stic point require mg or gram quantities of pure product component. It is also possible to estimate the column saturation capacity from single-component overloaded elution profiles using the retention time method or using an iterative numerical method from a binary mixture [66J. 

Apparatus for obtaining thermodynamic data from gas chromatography has been described by a number of authors. Conder and Purnell described a normal pressure apparatus suitable for working at finite concentrations. The apparatus could be operated in three modes, viz. (1) frontal analysis by a characteristic point, (2) frontal analysis, and (3) elution on a plateau. The first method is the most rapid a complete isotherm can be measured and calculated... 

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