Design and fabrication of an aerosol collector for infrared spectroscopy analysis

Particulate matter (PM) pollution causes adverse health effects and millions of deaths each year. PM or aerosol, is difficult to characterize because of its wide range of particle sizes, constituents (various organic and inorganic compounds), concentration, morphology, state (liquid or solid), and time-dependent modification. Infrared (IR) spectroscopy is a non-destructive method which provides useful chemical information about the constituents of PM. Current methods for collecting samples use filters made of materials which interfere with the IR spectra and thus have lower detection capabilities. In order to improve the chemical resolution, collection on an IR-transparent substrate is desirable. However, as most current collector types modify or preferentially sample certain size ranges, chemical composition, morphology or state, a new collector design is required. Electrostatic precipitation (ESP), impaction and filtration are common methods for aerosol collection. Impactors preferentially sample certain size range and liquids and suffer from particle bounce-off effects. Filtration onto filters uses a high pressure-drop that can modify the aerosol chemical composition and has inherent size-dependence. ESP is a versatile method of aerosol collection and does not suffer from high pressure-drop, or from bounce-off effects and is highly tunable, allowing its use in various applications. However, most ESP designs in the public domain have been designed for different purposes and face limitations in direct application in quantitative chemical composition measurement using IR spectroscopy. In order to rapidly develop prototypes and test the feasibility of such a device we used numerical simulations along with 3D-printing. A device was fabricated and tested for mass loading response and surface deposition profile with image analysis and variable aperture IR-spectroscopy. After observing near-quantitative response of the collector, we developed an analytical model to evaluate the particle collection response over a wider range of geometries and operating variables, with a goal to achieve specific objectives with the collector. Low size dependence, low chemical interference, and high collection efficiency are required to obtain an aerosol sample identical to the aerosol in air. High spatial uniformity in the deposition pattern is important for reducing the optical artefacts or the spectrometer dependence, and a high collection mass flux reduces the sample collection time needed for making a confident claim. The analytical model embodies the physics of particle migration trajectories due to fluid dynamics and electrostatics in a two-stage ESP device. We evaluated it against numerical simulations, which can only be run for a limited number of configurations. The scalable model allowed translating the objectives into quantifiable variables and to relate the ESP design to the collection performance, and evaluate the trade-offs to arrive at a design that is optimized across multiple length scales. The proposed collector should provide high chemical- and time-resolution information of PM in laboratory or ambient sampling settings. It has applications in laboratory studies into volatile and labile aerosol species that have low life-time, some of which are linked to harmful oxidative-stress in the human body.