Solvent Exchange Module for LC-NMR Hyphenation Using Machine Vision-Controlled Droplet Evaporation

We report the use of pendant droplet evaporation for exchange of eluents for (1)H nuclear magnetic resonance ((1)H NMR) purposes. Analytes are fed and retained in 500 nL droplets, which are concentrated by evaporation and subsequently redissolved in deuterated solvent. Droplet size is monitored by machine vision (MV), and heating rates are adjusted concordingly to maintain a stable droplet volume. Evaporation control is independent of solvent properties, and the setup handles feed rates up to 7 μL min(-1). The interface is capable of exchanging up to 90% of solvent for deuterated solvent, with a good recovery and repeatability for tomato extracts (Solanum lycopersicum). The system was capable of handling both polar and nonpolar analytes in one run. Volatiles such as formate, acetate, and lactate and the thermosensitive compound epigallocatechin gallate were recovered without significant losses. Ethanol and propionate were recovered with significant losses due to the formation of a minimum boiling azeotrope. The current setup is ideally suited for on- and off-line hyphenation of liquid chromatography and NMR, as it is comprehensive, fully automated, and easy to operate.