Review of the methods of stationary phase classification in gas chromatography
The article discusses the traditional methods of classification of gas chromatographic stationary phases. Rorschneider’s five-dimensional scheme is based on the alleged independence of five types of energy in five reference samples. The calculation of the scheme is based on a system of equations with five unknowns, which are characteristics of stationary phase polarity. However, there are no substances with only one type of intermolecular interaction. In addition, there are not so many independent types of intermolecular energy (less than five). This implies the fallacy and redundancy of the scheme, since the Rorschneider parameter space has a smaller dimension, and the independent characteristics of the reference substances summarise the contributions of different types of energy. The semi-empirical Abraham’s model of linear solvation energy relationships is more theoretically substantiated. Some of its parameters can be derived theoretically. However, it is even more complicated with regard to calculations than Rorschneider scheme, and the determination of all parameters by gas chromatography is impossible. Other methods use one-dimensional characterisation schemes, for example, various polarity parameters or hydrophobic-hydrophilic balance methods, where the stationary phases have one estimated parameter. This is not enough due to the complex nature of intermolecular interactions. In this regard, the parameters of the characteristic can coincide in stationary phases with different selectivity. Traditional methods have limited predictive power and cannot be equally applied to different experiments. However, due to the lack of alternatives, these empirical methods are widely used to assess stationary phase selectivity. The article proposes a three-parameter characterisation method, which is based on the representation of the energy of intermolecular interaction by the contributions of non-polar, polar forces, and hydrogen bonds. All parameters used in mathematical expressions have physical meaning, are portable, and do not require additional experiments. The characteristics of the stationary phases can be calculated both from experimental data on chromatographic retention, and a priori, using the structural formula of the stationary phase, if known.