Trace detection of volatile organic compounds by diode laser cavity ring-down spectroscopy

Abstract

The use of continuous wave cavity ring-down spectroscopy (cw CRDS) with near infra-red diode lasers is demonstrated for quantitative detection of trace levels of unsaturated volatile organic compounds (VOCs) at wavelengths that avoid overlapping absorptions by more abundant atmospheric constituents such as H₂O and CO₂. The current detection limit, with due allowance for pressure broadening by 1 atmosphere of air, is 6 parts per billion by volume (ppbv) for ethyne at an air wavelength of 1519.670 nm, and is sufficient for direct atmospheric detection of this molecule in many urban environments. Detection limits for alkenes are inferior, and, without incorporating the consequences of pressure broadening, include 78 ppbv for ethene and 900 ppbv for 1,3-butadiene. While the CRDS detection method offers several advantages over established gas chromatographic techniques for monitoring of small VOCs such as ethyne, it appears to be less well suited to study of larger organic compounds. Methods are discussed for improving the instrument to reach the sensitivities required to monitor the various alkenes and other C–H containing molecules in the troposphere.