Description and comparison of chromatographic tests and chemometric methods for packed column classification

The main tests developed in last 20 years to investigate the chromatographic behaviour and the stationary phase properties are described in this paper. These properties are the hydrophobicity, depending on the surface area and the bonding density, the number of accessible residual silanol groups having sometimes different acidity, which can interact with neutral solutes by hydrogen bonds or with the ionic form of basic compounds and the shape or steric selectivity, depending on both the functionality of the silanising agent and the bonding density. Two types of tests are performed, either based on key solutes having well defined properties such as phenol, caffeine, amitriptyline, benzylamine, acenaphtene, o-terphenyl, triphenylene, p-ethylaniline, carotenoid pigments, or on retention models (solvation parameter, hydrophobic subtraction) obtained from the analyses of numerous and varied compounds. Thus, the chromatographic properties are either related to selectivities or retention factors calculated from key solutes, or they are described by interaction coefficients provided by multilinear regression from retention models. Three types of comparison methods are used based on these data. First, simple plots allow the study of differences between the columns as regards to one or two properties. Columns located in the same area of the plot display close properties. Second, chemometric methods such as principal component analysis (PCA) or hierarchical cluster analysis (HCA) can be performed to compare columns. In this case, all the studied properties are included in the comparison, done either by data projection to reduce the space in which the information is located (PCA) or by distance calculation and comparison for drawing a classification (HCA). Neighbouring columns are expected to provide identical chromatographic performances. These two chemometric methods can be used together, PCA before HCA. The third way is to calculate a discrimination factor from a reference column, through calculation methods based on the Pythagorean Theorem: the lower this factor, the closer the column properties. Following the presentation of the analytical conditions, the compounds and the data treatments used by the teams working in this field, the pertinence of the different selectivities, i.e. of the different probe solute couples or of the different interaction coefficients, are discussed as regards their discrimination capacity. The accuracy of chemometric treatments in the discrimination of stationary phases having different functionalities (octadecylsiloxane (ODS), cyano, fluorinated, phenyl, polar embedded group or "aqua" type) will be discussed, as well as their performances in the finer ODS discrimination. New two-dimensional plots, from data gained by different studies will be suggested, to improve the classification of stationary phases having different nature of bonded chains.