Σpider diagram: A universal and versatile approach for system comparison and classification. Part 2: Stationary phase...

Many different sorts of bonded phase chemistries may be used in high-performance liquid chromatography (HPLC) in the reversed-phase mode: C8, C18 (type A or B), polar-embedded C18, phenyl, pentafluorophenyl, or cyanopropyl. To assess their retention and selectivity properties, chromatographic tests exist. The data obtained from these tests may be presented in three ways. First, simple classification diagrams may be plotted, when only two or three parameters are studied. Secondly, chemometric treatments such as principal component analyses (PCA) or hierarchical cluster analyses (HCA) may be computed, when at least 4 parameters are studied. These are sometimes uneasy to interpret. Thirdly, the "distance" between one column and a reference column may be estimated, through calculated ranking (F or CDF) or selectivity (s) factors. In this paper, another treatment type is applied to the data of Euerby (Tanaka test) and Snyder (Hydrophobic Subtraction Model), each of these tests having 6 parameters. This treatment produces a visual classification, called spider diagram. In the first part of this series, this type of classification was applied to the classification of solvents. A logical and easily comprehensible classification is obtained for the varied types of bonded phases, with a clear location, which can be related to the chromatographic properties. The comparison of these diagrams shows that the classification based on Snyder's hydrophobic subtraction model discriminates the stationary phases more effectively than the one based on the Tanaka test. Finally, on the basis of the parameter relevance and in order to favour comparison between these two tests and a third one called the carotenoid test, simplified classification maps are proposed. For Tanaka test, the selected parameters are the pentylbenzene retention factor (hydrophobicity), the benzylamine/phenol separation factor at pH 7.6 (polar surface activity) and the triphenylene/ortho-terphenyl (shape selectivity) separation factor. For Snyder test, the parameters selected are the ethylbenzene retention factor (hydrophobicity) the C term at pH 7.0 (polar surface activity) and the S* term (steric electivity). The location of some stationary phases onto the maps and their rankings are compared and shown to agree well between the three tests.