Pushing the limits of vibrational spectroscopy-based detection: the potential of cavity ring-down spectroscopy for trace threat vapor detection

Abstract

Traditional vapor sensing based on vibrational spectroscopy methods employs Fourier-transform infrared (FTIR) spectroscopy, which can produce reliable identification of ppm-level chemical vapors; however, chemical warfare agent poses a threat down to ppb and lower levels. Novel cavity ring-down spectroscopy-based sensors offer a possible path toward combining the high amount of spectral fingerprint data available from traditional IR methods, and the sensitivity of higher-sensitivity technologies, such as nanomaterial arrays and ion-mobility spectroscopy (IMS) which lack high amounts of data which can be used to uniquely identify molecules and are still plagued by high false-alarm rates and low selectivity. Cavity ring-down expands the effective path length of a gas analyzer by orders of magnitude by reflecting the IR light back and forth across an analyzer cavity. By characterizing the loss of light with each bounce by understanding the reflectivity of the mirrors employed at each end of the cavity, a characteristic "ring-down" time of the reflected light through a gas medium can be measured. Then, as analyte is introduced into the cavity, the ring-down times at each wavelength are shifted as a function of the absorbance of the analyte. Comparison of the measured vs. expected ringdown times can be interpreted to produce an IR absorption spectrum of the analyte. The effective pathlength of on order of kilometers allows for extremely high sensitivity beyond the capability of modern FTIR-based analyzers; however, this sensitivity comes at the expense of easier-to-saturate detectors as well. Additionally, cavity ring-down systems have already demonstrated the ability to measure absorption of aerosols both solid and liquid, filling a major gap left by FTIR vapor analyzers and opening the possibility of bioaerosol detection. We present a comparison of the two technologies and where they complement as well as fill in gaps in detection capabilities and offer a path forward for future generations of CB detection.