Indexation of mixed mode adsorbents using low temperature nitrogen adsorption and Silica-C18 surface energy scale

Abstract

Mixed-mode chromatography is becoming increasingly popular in pharmaceutical and biopharmaceutical applications due to its unique selectivity and retention of a variety of compounds. Mixed-mode chromatography is a chromatographic method in which solutes interact with stationary phase through more than one interaction mode or mechanism. It is because of these complex interactions, however, that it is difficult to predict chromatographic behavior of analytes on mixed-mode columns. Characterization of different mixed-mode columns on the basis of surface energy characteristics has not been explored. In this study, different ratios of porous silica (Axia Luna 5 µm Silica – Phenomenex), and C18 (Axia Luna 5 µm C18 – Phenomenex), single mode adsorbents were blended in various ratios and analyzed by Low Temperature Nitrogen Adsorption (LTNA). A variety of commercially available mixed mode columns were also unpacked and the materials were analyzed by LTNA. Adsorption isotherms, surface area, and BET constants were obtained for all the blended materials in various ratios and the mixed-mode materials. A linear relationship between the BET C-constant and the blend ratio of silica has been observed. The results from the commercial mixed mode packing material showed similar surface energy characteristics to single mode C18 materials. This data suggests that the BET C-constant maybe a viable parameter that can be used to characterize mixed-mode columns on the basis of surface energy. Utilizing this surface energy scale in conjunction with the study of retention behaviors on mixed-mode material could lead to improvements in the understanding of method development on mixed-mode columns, allow the comparison of commercialized mixed-mode columns, and even predict retention behavior of an analyte in mixed-mode columns.