AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
A New Approach for in Situ Picolitre Sampling of Aerosol Using Optically Trapped Droplets
MALCOLM KITTLE, Rachael E.H. Miles, Jason Murrell, Rebecca Hopkins, Jonathan P. Reid, University of Bristol
Abstract Number: 78 Working Group: Instrumentation and Methods
Abstract In the event of a Chemical Warfare Agent (CWA) release, it is anticipated that CWA would be present in the atmosphere as vapour and/or aerosol. A sensing capability must provide a fast, high confidence response with low false alarms to background aerosols commonly encountered in operational environments. In addition, the technique should allow detection of a broad range of hazards, and be able to sample and analyse both gas phase species and condensed phase particles (typically >100 nm in diameter).
A novel instrument has been designed that uses a single optically trapped droplet of sub-picolitre volume as an ultrasensitive sampler of gas phase or aerosol composition, with potential applicability to CWA detection. Vapour or aerosol present in the surrounding environment is sampled through mass transfer into the probe droplet. This uptake produces a change in the probe droplet size and composition, which can be characterised by monitoring the evolution of the probe droplet’s Raman spectrum. New spontaneous Raman bands emerge as chemicals are partitioned into the tweezed droplet, allowing composition of the sampled mass to be determined. Further, the radius and refractive index of the droplet confined in the optical trap can be determined from the whispering gallery modes in the simulated Raman spectrum via Mie theory.
As a proof-of-concept, a constant output atomiser is used to generate aerosol from sodium chloride and sodium citrate. The mass concentration and size distribution of aerosol particles flowing into the trapping cell are measured by a nanoparticle sizer. Raman spectroscopy is then used to characterise the change in size and composition of the probe droplet. Use of these combined techniques allows the relationship between aerosol mass concentration and mass uptake by the trapped droplet to be investigated, ultimately providing an understanding of aerosol accretion rates and instrument sensitivity of this novel approach.