Development of Infrared Photothermal Deflection Spectroscopy (Mirage Effect) for Analysis of Condensed-Phase Aerosols Collected in a Micro-Orifice Uniform Deposit Impactor

Abstract

The potential of mid-infrared photothermal deflection spectrometry for aerosol analysis is demonstrated. Ammonium nitrate aerosols are deposited on a flat substrate using a micro-orifice uniform deposit impactor (MOUDI). Photothermal spectroscopy with optical beam deflection (mirage effect) is used to detect deposited aerosols. Photothermal deflection from aerosols is measured by using pulsed infrared laser light to heat up aerosols collected on the substrate. The deflection signal is obtained by measuring the position of a spot from a beam of light as it passes near the heated surface. The results indicate non-rotating impaction as the preferred MOUDI impaction method. Energy-dependent photothermal measurement shows a linear relationship between signal and laser intensity, and no loss of signal with time is observed. The detection limit from the signal-mass curve is 7.31 ng. For 30 minutes collection time and 30 L/min flow rate of the impactor, the limit of detection in terms of aerosol mass concentration is 0.65 microg m(-3).

Finite Element Analysis Modeling of Pulse-Laser Excited Photothermal Deflection (Mirage Effect) from Aerosols

“…Recent work in our laboratory has been directed towards the application of photothermal spectroscopy to characterize airborne particulate matter (aerosols). 1 Photothermal spectrometry is a novel optical technique that measures low absorbance by probing the refractive index change created in the sample or the surrounding medium when absorption of electromagnetic radiation and subsequent thermal relaxation occur. 2 Photothermal deflection with crossed pump-probe beam geometry ''mirage effect'' is a special form of photothermal deflection spectroscopy (PDS) that probes the heat diffusion within a sample in an indirect way.…”