Chromatographic retention-solvent strength

Use of Solvatochromic Dyes To Correlate Mobile Phase Solvent Strength to Chromatographic Retention in Supercritical Fluid Chromatography... 

In this work correlations between mobile phase solvent strength and chromatographic retention of a number of different solute families will be presented. The first solvent strength measurements on ternary mobile phases will also be presented. Finally, a retention mechanism for packed column SFC is proposed. 

Binary systems. A typical plot of the log of partition ratios (k ) vs. % modifier is presented in Figure 2. Like solvent strength, chromatographic retention is a nonlinear function of mobile phase composition. However, plots of log k vs solvent strength (i.e., Ej ) are linear, for at least some solutes, as shown for phenols... 

Ternary mixtures of additives, less polar modifiers (like methylene chloride), and non-polar supercritical fluids have not been extensively used as chromatographic mobile phases. A few measurements suggest that they do not produce the dramatic decrease in solute retention (compared to the additive in methanol) implied by the increases in solvent strength in Figure 8. 

As in analytical liquid chromatography (LC), analyte retention depends on sample concentration, solvent strength, and sorbent characteristics. An empirical approach to methods development initially involves screening the available sorbents. The first step is to determine which sorbents best retain the analyte. The second consideration is to evaluate the solvents needed to elute the compound and the compatibility of those sorbents to the chromatographic testing procedure. The third step is to test the blank sample matrix to evaluate the presence of possible interferents. Finally, recoveries of known quantities of analyte added to the sample matrix must be determined. 

Equations (16.12) and (16.13) are very important, since they easily can be used to predict the influence of any operational parameter on the steepness factor, h, and therefore on the analysis time, efiSciency, and resolution. However, they are based on the validity of Equation (16.10). It has been shown that some deviations occur for some compounds and chromatographic systems (6), especially when retention is not governed solely by hydrophobic interaction. This is, for example, the case when the solutes are strongly basic and the stationary-phase acidity is high. Nevertheless, it is always possible to modify the form of the mobile-phase variation with time in order to maintain the applicability of the linear-solvent-strength theory [Equation (16.1)]. As we have seen above, this type of gradient offers a considerable help in the fundamental understanding of the retention behavior of the solutes and in the optimization of a separation. 

General computational chemical analysis of liquid chromatographic retention is performed without solvents in the calculation. Generally, mixed solvents with and without pH-controlled ions are present as the eluent components in liquid chromatography. At present, these solvent systems cannot be handled by computational chemical calculations. The measurement of direct interactions, however, reveals the different strengths of molecular interactions between an analyte and the packing material surface. The difference in molecular interaction energy values can be used as a relative retention time. 

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