Spectroscopic characterization of aqueous microdroplets containing inorganic salts

Abstract

We have developed and studied methods to characterize the time-varying composition of liquid microdroplets, under controlled changes to environmental conditions, using Raman tweezers. This work has focussed on measurements of inorganic salts, such as nitrate and sulfate anions, which comprise a major fraction of the dissolved solutes in atmospheric aerosols. The experimental Raman intensities for the anions of inorganic salts in optically tweezed droplets were found to be in good agreement with theoretical estimates of photon scattering. The detection limit for sodium nitrate salt in a single particle was found to be ~4 pg. The mass of an inorganic salt in the droplet can be estimated from the Raman intensity of the anion bands using a calibration curve which is independent of droplet volume. The volume of the droplet, and concentration of solute, can be found directly from the spacing of morphology dependent resonances in the Raman band of water, or indirectly from the integrated-intensity of the Raman band for the solvent. The later strategy eliminates the uncertainty in the collection efficiency of Raman-scattered light related to varying particle sizes.

Aerosol Optical Tweezers Constrain the Morphology Evolution of Liquid-Liquid Phase-Separated Atmospheric Particles

“…This morphology analysis capability comes from the cavity-enhanced Raman spectrum induced from the tweezed droplet that acts as an optical resonating cavity, thus providing a direct real-time morphology measurement of levitated droplets. 32,53,54 Alternate techniques to study phase-separation have used droplets placed on optical microscopy slides 27,29,[55][56][57] and more recently droplets suspended in oil within microfluidic devices. 58 Electron microscopy studies [59][60][61] suggest that the micron-scale equilibrium morphologies observed in our tweezers can be extrapolated down to 50-nm-diameter particles (d p ) and thus are relevant for ambient atmospheric aerosol.…”

Selective Monitoring of Oxyanion Mixtures by a Flow System with Raman Detection

“…This work particularly applies the capability of flow Raman selective analysis on the simultaneous detection of oxyanions, which are common compounds found in many industries that work with biomolecules production, enzymatic fermentation, mining residues, water and waste treatment, etc. The static Raman detection of oxyanions such as sulphate, nitrate, perchlorate, carbonate, thiosulphate, and chlorate, either as solid salts or in solutions, has been extensively reported in the literature [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. However, the simultaneous Raman detection of several oxyanions in flow solutions showed few advances along the years.…”

Heterogeneous oxidation of nitrite anion by gas-phase ozone in an aqueous droplet levitated by laser tweezers (optical trap): is there any evidence for enhanced surface reaction?

“…The nitrite peak occurred at B1049 rel cm À1 , which concurs with literature, 21 and the nitrate peak occurred at B1333 rel cm À1 , which concurs with literature. [22][23][24][25][26]…”