Characterisation of stationary phases in subcritical fluid chromatography by the solvation parameter model: II. Comparison tools

This study is an investigation of numerical and graphical tools for the comparison of the chromatographic stationary phases previously characterized with the solvation parameter model [C. West, E. Lesellier, J. Chromatogr. A, in press]. Numerous methods are presented and discussed: the coefficient ratio comparison, principal component analysis (PCA) and calculation of the distance. The coefficient ratio comparison allows to estimate the weight of a particular type of interaction relative to dispersive interactions, but is not practical when a lot of chromatographic systems need to be compared. The principal component analysis is mainly used to reduce the set of correlated variables, but is not useful for non correlated variables such as the solvation parameters. The distance calculation is an interesting tool to measure the differences between two systems but suffers from a confusion between two notions: the difference in the type of interactions involved in the chromatographic system and the intensity of these interactions. Finally, we chose to associate each stationary phase to a vector in a five-dimensional space, according to a method proposed by Ishihama and Asakawa [Y. Ishihama, N. Asakawa, J. Pharm. Sci., 88(12) (1999) 1305] for the comparison of lipophilicity scales. Thus, the angles between the different vectors are used to compare the selectivities of the stationary phases, and the lengths of the vectors are used to compare the relative intensities of the interactions. This method was applied to different alkyl phases characterized in subcritical fluid chromatography (SubFC). The angle values are well suited to the description of the differences in chromatographic behaviour. As expected, a small angle value is obtained between C8, C12 and C18, when a greater one is noticed between C4 and the longer chain length alkyl bonded phases, showing the different acidity and basicity of the C4 phase. Moreover, a satisfactory correlation is obtained between the length of the vectors and the carbon number of the alkyl chain. The differences between classical alkyl-bonded silica phases and polar-embedded alkyl-bonded phases or fluorinated phases are also conveniently evidenced. Finally, graphical tools are investigated and a new type of representation, based in part on a radar plot, is proposed. This plot allows the comparison of stationary phases without reducing the number of studied variables. The comparisons based on this method are consistent with the observed chromatographic behaviour of the phases compared.