Combining preconcentration of air samples with cavity ring-down spectroscopy for detection of trace volatile organic compounds in the atmosphere

Quantitative detection of small volatile organic compounds in ambient air is demonstrated using a combination of continuous wave cavity ring-down spectroscopy (cw-CRDS) and the preconcentration of air samples with an adsorbent trap. The trap consists of a zeolite molecular sieve, selected for efficient trapping of the test compounds ethene (ethylene) and ethyne (acetylene). Upon heating of the trap, these organic compounds desorb into a small-volume ring-down cavity, and absolute concentrations are measured by CRDS at 6150.30 cm(-1) (ethene) and 6512.99 cm(-1) (ethyne) without the need for calibration. The efficiency of the trapping and desorption was tested using commercial standard gas mixtures and shown to be 100% in the case of ethene, whereas some ethyne is retained under the current operating conditions. Samples of indoor and outdoor air were analyzed for ethene content, and measurements were made of mixing ratios as low as 6 ppbv. Removal of water vapor and CO(2) from the air samples prior to trapping was unnecessary, and the selectivity of the trapping, desorption, and spectroscopic detection steps eliminates the need for gas chromatographic separation prior to analysis. With anticipated improvements to the design, measurements of these and other trace atmospheric constituents should be possible on time scales of a few minutes.