Real-Time Detection of Chemical Warfare Agents Using Multi-Wavelength Photoacoustics

Abstract : We present a proof-of-concept study designed to investigate the utility of operating a conventional photoacoustic spectroscopy technique in a "multi-wavelength" mode applied to chemical vapor/aerosols for application of trace species detection and identification. The technique involves propagating three or more laser sources through a non-resonate, flow through photoacoustic cell. Each laser source is modulated at a different frequency, chosen at some convenient acoustic frequency. A portion of each laser's power is absorbed by a particular test gas/aerosol that is passing through the PA cell, resulting in a acoustic signal that is found to be proportional to the absorption cross section of the gas/vapor at the particular laser wavelength. A superposition of frequency component (equal to the number of laser wavelengths used), combines with the ambient acoustic noise spectrum and is recorded by an electret microphone housed in the photoacoustic cell. The signal is deconvolved using phase sensitive detection where each component (one corresponding to a particular modulation frequency for a particular laser) is amplified and recorded as function of species concentration. Ratios of the resultant absorption information are used to produce an identifiable metric that remains constant for all concentrations. For the study presented here, we used 3 laser wavelengths all lying in the spectrally rich long-wave infrared (LWIR), i.e., 8.72, 9.27, and 10.35 um. Test nerve agents simulants include (but not limited to), diethyl phosphonate (DEMP), dimethyl methylphosphonate (DMMP), and diisopropyl phosphonate (DIMP). Measured photoacoustic absorption results compare well with Fourier Transform Infrared (FTIR) analysis that is conducted in situ with the photoacoustic portion of the measurement.