Probing solvent fractionation in electrospray droplets with laser-induced fluorescence of a solvatochromic dye

Laser-induced fluorescence spectroscopy is used to profile solvent fractionation in an electrospray plume containing Nile Red, a solvatochromic dye (i.e., a dye for which spectral features are sensitive to solvent polarity). The results confirm gradual enrichment of the less volatile component in spray droplets as a result of solvent evaporation. Changes in solvent composition are evident in both axial and lateral profiles. The influences of capillary voltage, nebulizing gas, an electrolyte modifier, and flow rate on the extent of solvent fractionation are assessed. The results suggest that, in addition to volatility of the solvents, major factors affecting solvent fractionation include initial droplet size, droplet velocity, and the cooling effect of the nebulizing gas. Isolating contributions from these factors will require complementary data from other experiments.