Movement of components in reversed-phase chromatography : I. Mobile phase space with multi-component eluents

Abstract

The retention volume of a peak that is neither retained nor excluded from the mobile phase space in the column is required for determination of retention (capacity) factor and related thermodynamic quantities. In liquid chromatography with multi-component eluents, however, the mobile phase space is theoretically indeterminate owing to interaction between the individual components and the stationary phase and therefore definition and measurement of the mobile phase necessitates that an appropriate convention be found. Here the following four conventions are examined: (i) “all mobile phase components are present in the solvation layer”; (ii) “no solvation layer exists”; (iii) “one given eluent component is not present in the solvation layer”; and (iv) “the most weakly bound solvent component is not present in the solvation layer”. Whereas all conventions appear to be acceptable for a thermodynamic description of the interaction between mobile phase components and the stationary phase in most cases, they yield different values of retention factors and associated thermodynamic properties for eluites. Furthermore, the use of some conventions can give rise to practical problems, e.g., experimental determination of void volume by use of the first convention is ill-defined in general and the second and third conventions can lead to negative values for the mobile phase space. The present work suggests that the fourth convention is free of these problems and provides retention factor values which are more appropriate for use in liquid chromatography than those obtained by the other conventions.