Expansion of Solution-Seeded Supercritical CO₂ Characterized by Mass Spectrometry and Laser Spectroscopy

Solution-seeded supercritical CO₂ mixtures are at the core of modern supercritical fluid chromatography (SFC) setups, which are commonly coupled to mass spectrometry but barely combined with gas phase laser spectroscopy despite its numerous advantages. In this context, a case-study pulsed expansion of a high-pressure CO₂:ethanol:toluene ternary mixture was investigated by mass spectrometry and resonant two-photon ionization (R2PI) UV laser spectroscopy. Various toluene-containing species going from monomers to large ternary clusters were identified, and their longitudinal distribution in the pulse was determined. These observations suggested that the ternary mixture was not homogeneous before expansion but was likely composed of aggregates of solution dispersed in CO₂, contributing to efficiently form an aerosol under vacuum. The solute (toluene) solvated by either its solvent in the initial solution (ethanol) or CO₂ within clusters could be easily characterized by UV spectroscopy recorded on mass-selected ion or photoelectron signals. In contrast, the amount of toluene monomers, although sufficient for gas phase investigations, was negligible relative to that embedded in clusters. In order to extend this approach to low-volatile solutes, further developments are needed to reach a complete vaporization of the initial solution and fully achieve the spectroscopic characterization of its isolated constituents by gas phase laser spectroscopy.