Hyphenation of Supercritical Fluid Chromatography and NMR with In-Line Sample Concentration

By coupling supercritical fluid chromatography (SFC) and nuclear magnetic resonance (NMR) in-line, a powerful analytical method arises that enables chemically specific analysis of a broad range of complex mixtures. However, during chromatography, the compounds are diluted in the mobile phase, in this case supercritical CO₂ (scCO₂), often resulting in concentrations that are too low to be detected by NMR spectroscopy or at least requiring excessive signal averaging. We present a hyphenated SFC-NMR setup with an integrated approach for concentrating samples in-line, which are diluted in scCO₂ during chromatography. This in-line concentration is achieved by controlled in-line expansion of the scCO₂. As a proof of concept four isomers of vitamin E (tocopherol) were isolated by SFC, concentrated in-line by expanding CO₂ from 120 to 50 bar, and finally shuttled to the NMR spectrometer fitted with a dedicated probehead for spectroscopic characterization of microfluidic samples. The abundant isomers were readily detected, supporting the viability of SFC-NMR as a powerful analytical tool.