Parameter for predicting retention in planar chromatography

Abstract

A parameter, deslgnated x, for planar chromatography In either the normal-or reversed-phase mode applles to each of a serlos of weak solvents used as a blnary mlxture wlth a glven strong solvent or, atternatlvely, to a serles of strong solvents used wlth a glven weak solvent. A strong h e a r relatlonshlp between #her the average R, of solutes (sterdds or esters of dansyl amino aclds) or the R, of lndlvklual solutes and the value of x for each of the serles of solvents Is found for many of the systems studled. Thls relationship can be used to predlct the retention of a solute when a mobile phase of known x value Is used. The relatlonshlp between x and establlshed solvent strength parameters Is also dlscussed.Solvent strength is important in all forms of liquid chromatography, and parameters such as P' (1) and to (2) have been introduced to quantify it. Other parameters such as the Hildebrand parameter, 6 (3), the solvatochromic polarity parameter, E~( 3 0 ) (4), and the solvatochromic dipolaritypolarizability parameter, A* (5), originally introduced for other purposes, have been applied to chromatography usually by a multiparameter approach (6-8) but also by a single-parameter approach (9). Both to (10) and ET(30) (9) have been used to accurately predict retention as a function of concentration in systems with defined components. Reference 10 includes a summary of reports on the use of to to predict retention, and ref 9 includes a summary of the use of solvatochromic parameters in chromatography.The cluster center is an empirical parameter for planar chromatography that has been briefly discussed previously with respect to the separation of either steroids or a mixture of azo dyes by normal-phase thin layer chromatography (TLC) (11, 12). In the initial report, the value of the cluster center was shown to decrease uniformly with increasing eluotropic strength for each of 14 solvent systems. This parameter differs from those noted above in that it applies only to binary mixtures of solvents in which one component is held constant, and it is defined with respect to the solutes of a given class. In spite of these constraints, it appears to be a useful parameter because, under defined conditions, it allows R, to be predicted in untried solvents. The current report discusses this use of the parameter and compares cluster center values of solvents to the corresponding values for several of the established parameters.EXPERIMENTAL SECTION The steroids and dansyl amino acids were purchased from the Sigma Chemical Company (St. Louis, MO), and the solvents were purchased from the Aldrich Chemical Company (Milwaukee, WI). The reagents for preparing the esters of dansyl amino acids were a gift from Pierce (Rockford, IL). The esters were prepared according to the procedures in the 1989 Pierce Handbook, which are based on the following references: methyl esters (13), pnitrobenzyl esters (14), phenacyl esters (15), and p-bromophenacyl esters (16). A single spot was obtained for all derivatives, except serine and threonine, which ...