Frontal analysis single-component

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. 

Figure 17 shows a section of two individual overlaid staircase chromatograms resulting from single component frontal analysis of (S) and (R)-2-phenylbutyric acid, respectively. At the first step up to 30.5 mM the enantiomers are clearly separated from each other, at the second step up to 61.0 mM they are still separated and even at the third step up, to 91.5 mM it is still a very small tendency for separation. This figure indicates that the chiral capacity is somewhat higher than 90 mM. 

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]. 

This isotherm model has been used successfully to accoimt for the adsorption behavior of numerous compounds, particularly (but not only) pairs of enantiomers on different chiral stationary phases. For example, Zhou et ah [28] foimd that the competitive isotherms of two homologous peptides, kallidin and bradyki-nine are well described by the bi-Langmuir model (see Figure 4.3). However, most examples of applications of the bi-Langmuir isotherm are found with enantiomers. lire N-benzoyl derivatives of several amino acids were separated on bovine serum albumin immobilized on silica [26]. Figure 4.25c compares the competitive isotherms measured by frontal analysis with the racemic (1 1) mixture of N-benzoyl-D and L-alanine, and with the single-component isotherms of these compounds determined by ECP [29]. Charton et al. foimd that the competitive adsorption isotherms of the enantiomers of ketoprofen on cellulose tris-(4-methyl benzoate) are well accounted for by a bi-Langmuir isotherm [30]. Fornstedt et al. obtained the same results for several jS-blockers (amino-alcohols) on immobilized Cel-7A, a protein [31,32]. 

We compare in Figure 10.12 the band profiles calculated for the (+) isomer of Troger s base using the forward-backward numerical method and an OCFE method. To avoid a circular argument, the isotherms were obtained by frontal analysis and the column efficiency was measiued imder linear conditions (from very small size injections) [59]. There is a significant difference between the band profiles, because the column efficiency is poor, 110 and 150 theoretical plates for the (-) and (+) enantiomers, respectively, under analytical conditions. As expected, the finite difference method introduces significant errors even in the case of a single component profile. 

In all these figures, we used the competitive Langmuir isotherm model to calculate the band profiles. However, the coefficients of the isotherms used for Figures 11.21 are the coefficients of the single-component isotherms determined by frontal analysis, while the coefficients of the isotherms used to calculate the profiles in Figure 11.22 are measured by the simple wave method (Chapter 4, Section 4.2.4). These latter coefficients are certainly empirical coefficients, and their use would not permit an accurate prediction of single-component bands. However, they permit the calculation of band profiles that are in much better agreement... 

Quinones et al. measured by frontal analysis the single-component, binary and ternary isotherms of benzyl alcohol (BA), 2-phenyl ethanol (PE) and 2-methyl benzyl alcohol (MBA) on Symmetry-Cis, using a binary mobile phase (MeOH HaO... 

This value is in agreement with the one derived from band profiles calculated with the equilibrium-dispersive model [9]. The time given by Eq. 16.20 provides useful information regarding the specifications for the experimental conditions under which staircase binary frontal analysis must be carried out to give correct results in the determination of competitive isotherms. The concentration of the intermediate plateau is needed to calculate the integral mass balances of the two components, a critical step in the application of the method (Chapter 4). This does not apply to single-pulse frontal analysis in which series of wide rectangular pulses are injected into the column which is washed of solute between successive pulses. 

This equation is identical to the one derived in single-component frontal analysis, if applied to the displacer [12]. 

Piqtkowski el al. measured the single-component and the competitive equilibrium isotherms of phenetole (ethoxy-benzene) and n-propyl benzoate on a 150 x 3.9 mm S3onmetry -Cig (endcapped) column (Waters), using a methanol/water (65 35, v/v) as the mobile phase [26]. The adsorption equilibrium data of the single-component systems were acquired by frontal analysis. For both compoimds. 

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. 

Frontal analysis Integration of step Single-component... 

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.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, for example, elution by characteristic points (ECP) or frontal analysis by characteristic points (FACP). Frontal chromatography as a tool for the determination of single-component adsorption isotherms will be discussed in the following section. 

The frontal chemical concentration method can also be used for the intermediate concentration trace amounts of heavy components [60] in the analysis of gaseous monomers (ethylene, propylene). The concentration was conducted on a short intermediate column containing diethanolamine and pure carbon dioxide was used as the carrier gas. The method permits three steps to be integrated into a single run preliminary separation, concentration of heavy trace components and analytical determination of the composition of the concentrate. The concentration of the heavy trace components to be determined was 10" %. Trace analytical methods based on selective retention of the main component are becoming more common in chromatography. 

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