Equilibrium constants frontal analysis

An alternative approach for equilibrium constant measurements is to use the method of frontal analysis. In this... 

One disadvantage of frontal analysis is that it requires a relatively large amount of analyte for study. However, frontal analysis does provide information on both the association constant for a solute and its total number of binding sites in a column. This feature makes frontal analysis the method of choice for high accuracy in equilibrium measurements, because the resulting association constants are essentially independent of the number of binding sites in the column. 

Figure 4.2 illustrates the best competitive adsorption isotherm model for benzyl alcohol and 2-phenylethanol [16]. The whole set of competitive adsorption data obtained using Frontal Analysis was fitted to obtain the Langmuir parameters column saturation capacity qs =146 g/1), equilibrium constant for benzyl alcohol bsA = 0.0143) and the equilibrium constant for 2-phenylethanol (bpE = 0.0254 1/g). The quality of the fit obtained with this simple model is in part explained by the small variation of the activity coefficients of the two solutes in the mobile phase when the solute concentrations increased from 0 to 50 g/1. The Langmuir competitive adsorption isotherm simplifies also in the case where activity coefficients are of constant value in both phases over the whole concentration range [17]. 

Equation 14.45 applies in linear chromatography. The correct HETP equation in frontal analysis imder constant pattern behavior, and with the same solid film linear driving force model is Eq. 14.36b. Comparison of these two equations shows that an error is made when the latter is used to replace the former, in the equilibrium-dispersive model. We should replace in Eq. 14.44 and 14.45 Atq by k — FAq)/AC. In the case of the Langmuir isotherm, this would give k = fc )/(l + bCo) = X. 

An alternative approach for equilibrium constant measurements in affinity chromatography is to use frontal analysis. This method is sometimes known as frontal affinity chromatography FAC). In this technique, a solution containing a known concentration of the analyte is continuously applied to an affinity column at a fixed flow rate (Fig. 2). As the analyte binds to the immobilized ligand, the ligand... 

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