10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Spore Aerosol Viability Dependence on Radiation Exposure
MATTHEW B. HART, Jozsef Czege, Cathy S. Scotto, Jana Kesavan, Vipin Rastogi, Frank Handler, Jay D. Eversole, Naval Research Laboratory
Abstract Number: 1445 Working Group: Bioaerosols
Abstract We report results from our study in which two types of Bacillus spores were prepared and aerosolized into a linear electrodynamic quadrupole (LEQ) trap for the purpose of exposing them to ultraviolet (UV) light sources to determine loss of viability as a function of exposure (time) based on subsequent collection, plating and counting. As part of a DTRA JSTO funded collaboration, Bacillus thuringiensis Al Hakam and B. anthracis Sterne samples were prepared as phase-bright spores, multiply washed and resuspended in pure water with 0.01% polysorbate 80. Spore samples were titered at the beginning of each experiment, and 70 µm diameter suspension microdroplets were generated using a piezo-electric transducer coupled to a quartz capillary nozzle.
The LEQ was aligned with its symmetry axis vertical, and the nozzle tip was positioned at the top of the trap pointing down along the central axis. Microdroplets were inductively charged as they were ejected into the LEQ. The LEQ was partitioned into upper and lower chambers. Typically, several hundred to a thousand droplets were initially produced and confined in the upper chamber. Drying occurred in seconds, and the spores in each microdroplet (with some fraction of surfactant residue) formed a single particle (spore cluster). Since droplets were generated with a fixed uniform size, the number of spores in each particle was proportional to the spore suspension concentration. The distribution of spores per particle (particle size distribution) is governed by Poisson statistical variability of the number of spores in a 70 µm diameter droplet of suspension fluid. For example, a concentration of 1.25 x 108 spores/ml yields a mean number of 22 spores per droplet, with a standard deviation of +/-4.7 (corresponding to ≈ 3.3 +/-0.2 µm diameter clusters when dried).
The LEQ consists of four 120 mm long, 1.5 mm diameter electrodes arranged on a square with 8.0 mm sides. An airtight enclosure for this system was fabricated with windows for optical exposure and interrogation. The LEQ was partitioned into upper and lower chambers by two concentric conductive rings perpendicular to the electrodes, and centered on the symmetry axis. These rings were held at a fixed potential with the same polarity as the particles, and functioned as an electrical valve holding the charged particles in the upper portion of the LEQ during exposure. After exposure, trapped particles could flow from the top chamber to the lower chamber by decreasing the ring potential. A second pair of rings was positioned at the bottom of the lower chamber to hold particles there for image-based counting prior to flowing them through a membrane filter for collection as they exit the enclosure.
The radiation exposure study uses both a solar simulator (Oriel model 94021A), and a low-pressure Hg discharge lamp (>94% intensity @ 254 nm). Both the spectrum and the intensity of these lamps are periodically monitored to ensure continuity for a fixed output. We are currently scoping exposure times for two or three sample concentrations as described above. This initial study will bracket “short” and “long” exposure times (with no-exposure controls with the same times) to select an appropriate exposure range to obtain meaningful decay rate data. We will present our latest results for both spore species generated as “single” spore particles and also as larger spore clusters, for both the 254 nm (UV-C) lamp and the solar simulator.