Pyrene was adsorbed on three different chemically modified silicas, RP-2, RP-8, and RP-18, commonly used in liquid chromatography separations. The mobility of pyrene on the surfaces and Its dependence of temperature was studied by measuring the time dependence of pyrene excimer formation with a fluorescence method. The rate constant for excimer formation Is 1.2 X 10s s~1 M~1 at room temperature on RP-18. The apparent activation energy for diffusion, Ed, of pyrene on RP-18 IS ~20 kJ/mol in the temperature range 290-325 K.For lower temperatures Ed Increases rapidly, which indicates a phase transition In the temperature range 273-290 K. The values for k^, and Ed at room temperature are consistent and show that the surface of RP-18 has liquidlike properties. We found that pyrene excimer formation is a dynamic process on RP-2 and RP-8 as well. In these cases the evaluation of the different rate constants Is more complex, mainly due to differences In the microenvironment for different pyrene molecules.
A theoretical framework for the effect of eluting salt concentration on the capacity factor of small ions in ion-exchange and ion chromatography is described. The model is based on the Gouy-Chapman theory for the electrical double layer complemented with the possibility for specific adsorption of the counterions as well as the analyte ions to the chromatographic surface. Because of the complex dependence of the capacity factor on parameters such as net charge density of the stationary phase, eluent salt concentration, and electrostatic potential of the surface, numerical evaluation of the model is needed in the general case. The calculations show, in agreement with the general experimental observation, that a log k'vs log (eluent concentration of 1:l salt) plot is linear with a slope value close to -1 and -2 for mono-and dicharged analytes, respectively. It is also shown that the linearity as well as the slope is insensitive to the type of analyte ion, eluent counterion, and type of stationary phase and its charge density. The theory therefore offers a physically consistent approach to the analysis of retention data without resorting to the unrealistic stoichiometric models which has mainly been used so far in the ionexchange chromatography of small ions.
A theory for system peaks and the indirect detection technique in ion pair chromatography is developed. The theory Is based on the electrostatic theory for Ion pair chromatography In combination with the mathematical treatment of multicomponent chromatography. Equations are derived for the direction as well as the magnitude of the analyte peaks. The developed theory Is used to explain the typical response pattern and response magnitude that are obtained In the indirect detection technique. The developed theory is found to agree with experimental data in the literature.
To understand the mechanism of Ion pair chromatography, a correct description of the adsorption isotherm of the amphiphilic modifier Is Important. The adsorption Isotherm of tetrabutylammonlum Ion onto a RP-18 stationary phase Is determined with different electrolytic counterions (H2P04', CL, Br") and for two different Ionic strengths. The electrostatic surface potential created by the adsorbed tetrabutylammonlum Ions Is determined by applying the concepts of the electrostatic theory for Ion pair chromatography. The different experimentally determined adsorption Isotherms are found to coincide with the same surface-potential-modified Langmuir isotherm. The results are therefore In accordance with the electrostatic theory for Ion pair chromatography.
ACKNOWLEDGMENTWe are most grateful to M. Almgren and B. Jonsson for valuable discussions during the preparation of this work and to A. Furángen for valuable discussions of the manuscript. The computer program for a numerical solution of the Poisson-Boltzmann equation was obtained from the
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