Reversed-phase HPLC on the “monomeric” reversed phases: factors determining the retention of sorbates

Abstract

This brief review discusses options for the characterization of “monomeric” reverse phases aimed at understanding the types of interactions that determine the retention of sorbates in liquid chromatography. Traditional point (determined at one certain composition of the mobile phase) methods for determining the retention capacity, hydrophobicity (as methylene selectivity) relevant to the assessment of dispersion interactions are considered; their advantages and disadvantages are shown and the option of using substance separation maps based on the relative retention analysis method was proposed as an alternative. Attention was drawn to the fact that in real reverse-phase sorbents the density of grafting of alkyl chains is two times less dense compared to solid n-alkanes. In this case, sorbate molecules can penetrate into the grafted phase, and such penetration depends on the shape of the molecules. Consequently, conventional “monomer” reverse phases have a specific selectivity with respect to the retention of a number of substances with a special structure. It has been shown that modern methods of analysis do not pay enough attention to the difference in the mechanisms of retention of substances: absorption or adsorption, although different parameters are important for these mechanisms. Moreover, attention to this characteristic is not paid in the two most frequently used now very interesting and informative methods - the linear solvation energy relationships (LSERs) and the hydrophobic-subtraction model. The linear solvation energy relationships method does not differentiate between sorbates retained by different mechanisms (absorption or adsorption), and the resulting noticeable differences between calculated and experimental data are not surprising. At the same time, interpretation of the analysis results using the LSERs is complicated by the uncertainty of the contribution of sorbates of partial properties in the mobile and stationary phases to the solvation energy, since only their difference is calculated. However, comparison of different columns in the same mobile phases can be quite informative. The disadvantage of the second approach is not only the requirement to use several columns, but also the requirement of significant qualitative differences between the retention of sorbates on them. In addition, the possibility of decomposing all interactions into characteristic types is questionable, since the method does not use any of the orthogonal (independent of the calculation used) properties of sorbates.