Investigation of Modifier and Additive Adsorption in Supercritical Fluid Chromatography

Abstract

The importance of chromatography as an analytical technique in industrial applications arises from the ability to separate a mixture into individual components as a result of different rates of distribution of the components between stationary and mobile phases. Supercritical fluid chromatography (SFC) utilizes a mobile phase in a supercritical phase, which is characterized by “gas-like” viscosity and “liquid-like” density. The supercritical phase is obtained when both temperature and pressure are above the critical values. In general, SFC is considered an environment-friendly approach due to utilizing carbon dioxide as the main eluent, in turn less consuming of organic solvents in comparable to high performance liquid chromatography (HPLC), hence interest in SFC is growing under the increased care given to policies related to environmental issues (especially the use of solvents) and as a result of continuous rationalization efforts in every industrial area. The applications of SFC using 100% CO2 as the mobile phase are limited due to the low polarity of CO2, which is very close to that of liquid pentane or liquid hexane; hence, it can only dissolve nonpolar compounds. Therefore, the majority of SFC experiments are carried out with alcohol modified - CO2, such as methanol or isopropanol, to improve the solubility of the analytes in the mobile phase, which would give better chromatographic peaks. Furthermore, for expanding the range of analytes polarity, which can be analyzed with SFC, a third component called an additive, which can be a base, acid, salt, or water, is highly recommended to be used in the mobile phase in a very small amount, typically less than 1% of the modifier. It is usually premixed with the modifier, and then both are transferred to the mobile phase, resulting in a better peak shape of the analytes. For instance, the basic analytes (e.g., nitrogen-containing molecules) are considered problematic, such as the dibenzylamine and benzylamine mixture, which was studied by Berger and his co-workers. They reported that the mixture does not elute with a good peak shape even using methanol-modified CO2 but in the presence of isopropylamine as an additive, a good improvement in the peak shapes was obtained. It is known in nonlinear chromatography that the separation process and the migration rate of sample components through a porous stationary phase are highly influenced by the adsorption isotherms of each component. For instance, in a ternary mobile phase (CO2 + modifier + additive) where they are at a sufficient concentration (multicomponent system), their adsorption on the stationary phase will be influenced by each other; hence, determining the adsorption isotherm will be a useful tool to describe the adsorption behavior of each component on the stationary phase. For determining the isotherms for a multicomponent system, the competitive form should be used, which is called ‘competitive isotherms’, whereas a single component isotherm is used to describe the adsorption for a single-component system.