Trace species detection in the near infrared using Fourier transform broadband cavity enhanced absorption spectroscopy: Initial studies on potential breath analytes

Cavity enhanced absorption measurements have been made of several species that absorb light between 1.5 and 1.7 m using both a supercontinuum source and superluminescent light emitting diodes. A system based upon an optical enhancement cavity of relatively high finesse, consisting of mirrors of reflectivity ∼99.98%, and a Fourier transform spectrometer, is demonstrated. Spectra are recorded of isoprene, butadiene, acetone and methane, highlighting problems with spectral interference and unambiguous concentration determinations. Initial results are presented of acetone within a breath-like matrix indicating ppm precision at <∼10 ppm acetone levels. Instrument sensitivities are sufficiently enhanced to enable the detection of atmospheric levels of methane. Higher detection sensitivities are achieved using the supercontinuum source, with a minimum detectable absorption coefficient of ∼4 × 10^-9 cm^-1 reported within a 4 min acquisition time. Finally, two superluminescent light emitting diodes are coupled together to increase the wavelength coverage, and measurements are made simultaneously on acetylene, CO₂, and butadiene. The absorption cross-sections for acetone and isoprene have been measured with an instrumental resolution of 4 cm^-1 and are found to be 1.3 ± 0.1 × 10^-21 cm² at a wavelength of 1671.9 nm and 3.6 ± 0.2 × 10^-21 cm² at 1624.7 nm, respectively.