Usual, unusual and unbelievable retention behavior in achiral supercritical fluid chromatography: Review and discussion.

Retention rules are well known in liquid chromatography. For the mobile phase composition, retention decreases when adding organic solvent to water for reversed-phase or increasing water proportion for hydrophilic interaction liquid chromatography, and a decrease in temperature usually increases retention. For supercritical fluids, the fluid density, which is related to temperature and column back-pressure, is significant for neat CO and with low percentages of organic modifiers, i.e. with compressible mobile phases. The increase in the modifier percentage reduces the fluid compressibility, leading to retention behaviors close to those observed with liquid mobile phases, for instance for temperature changes. Moreover, adsorption of carbon dioxide or modifiers modify the solutes/stationary phase interactions, further complicating the understanding of the observed retention changes, either with low amount of modifier, or with specific modifiers. Besides, the polar and nonpolar stationary phases (SPs) do not always behave identically, depending on physico-chemical properties. Silica, amino or ethyl-pyridine polar phases display mostly identical behavior for classical differences of compounds of different polarity, but can provide different retention order for more subtle differences, such as the position of polar groups. Moreover, the nature of the silica, inorganic or hybrid, or the additional charges onto the silica surface can also lead to different results. Even if the C18-bonded phases are not as popular as polar SPs, the non-polar SPs provide very high separation performances for suited compounds, i.e. for non-polar compounds, which are perfectly solubilized by supercritical fluids. Recently, unusual retention behaviors were observed with some specific C18-bonded phases, which display polar interactions in addition to dispersion interactions. Whatever the SPs used, supercritical fluids appear to favor specific effects that are not observed with liquid mobile phases that are more uniform in terms of physico-chemical properties. The objective of this paper is to describe different separation behaviors observed in SFC, to improve the general understanding of the specificities of the association of supercritical fluids and varied SPs.