CO2-modified solvents for chromatographic separation

Abstract

Developing alternatives to organic solvents and salts in chromatographic separation is highly desired. CO₂ is abundant, inexpensive, nontoxic, environmentally benign, and easily removed. In this work, the impact of CO₂-modified aqueous solvents on reversed phase chromatographic separation is studied. A robust custom CO₂ delivery system (1 bar CO₂) is able to provide CO₂-modified aqueous solvent with pH 3.9–6.5. A significant hydrophobicity switch of the stationary phase was observed by a reduction in the retention factor, k, when using CO₂-modified solvents for the diethylaminoethyl (tertiary amine) and polyethylenimine (primary, secondary and tertiary amine) functionalized columns. In particular, the polyethylenimine column can be used to perform separation with 100% water without any organic solvent added. Carboxymethyl based columns were not switched by CO₂-modified solvent, indicating the need for more significant pH change. The CO₂ triggered selectivity switch resulting from analyte ionization provides an important tool in controlling the selectivity of separation. Overall, CO₂-modified solvent is valuable in tuning the elution strength and selectivity of reversed phase chromatography.