Fluorometric measurement and modeling of droplet temperature changes in an electrospray plume.

The evolution of droplet temperatures in an electrospray plume was measured via ratiometric fluorescence. Under typical operating conditions, droplet temperatures decrease ∼30 K over the first 5.0 mm along the spray axis, followed by a slight (∼2-3 K) rewarming. Experimental axial profiles (Z-axis) were fit by use of diffusion-controlled and surface-controlled evaporation models. Both models fit the experimental data well for the cooling portion of the spray (Pearson correlation coefficient R ≥ 0.994), but the surface-controlled model required unrealistic droplet radius values to obtain a good fit. In lateral profiles at a given Z near the emitter tip, temperatures are lower (by 3.0-10 K) in the periphery than on the spray axis. This behavior is consistent with the expected enrichment of the spray periphery with smaller droplets. At longer axial distances, lateral profiles were relatively flat. Droplet temperature as a function of axial displacement fell more rapidly at lower liquid flow rates, possibly attributable to changes in droplet size and/or velocity with flow rate.

Infrared, Raman, and DFT vibrational spectroscopic studies of C60F36 and C60F48.

IR and Raman spectra of two fluorofullerenes, C60F48 and C60F36, are thoroughly studied. Assignment of the experimental spectra is provided on the basis of density functional theory (DFT) computations. Perfect correspondence between experimental and computed spectra enabled us to confirm that the major isomer of C60F48 has D3 symmetry. It was found that as-synthesized samples of C60F36 consist mainly of C3 and C1 isomers in ca. 2:1 ratio and 2-3% of T-symmetric structures. Extensive AM1 and DFT computations have shown that all three structures are the most stable isomers of C60F36. Previous structural assignment of the C3 isomer (Gakh, A. A.; Tuinman, A. A. Tetrahedron Lett. 2001, 42, 7137-7139) was confirmed by the vibrational data.

Profiling an electrospray plume using surface-enhanced Raman spectroscopy.

We report the use of silver nanoparticles to obtain surface-enhanced Raman spectra of Crystal Violet in an electrospray plume. Surface enhancement allowed detection at low concentrations with the high specificity afforded by vibrational spectroscopy. SERS spectra were used to obtain an axial concentration profile closely matching that obtained in previous fluorescence experiments. SERS can provide more analyte structural information than has been obtainable from fluorescence studies of the plume.