Combining Preconcentration of Air Samples with Cavity Ring-Down Spectroscopy for Detection of Trace Volatile Organic Compounds in the Atmosphere

Abstract

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.

Development of a sensitive gas sensor by trapping the analytes on nanomaterials and in situ cataluminescence detection

“…Improvement of sensitivity can be achieved by increasing the sensitivity of the sensing element itself, or by preconcentrating the sample before detection [5][6][7]. Preconcentration techniques such as solvent extraction and solid-phase extraction (SPE) have been widely employed for sensitivity enhancement in the development of analytical methodologies [8].…”

Cavity Enhanced Spectroscopy: Applications Theory and Instrumentation

“…Many of the initial measurements focused on small compounds. Field measurements have all been demonstrated for ethyne, (52,56 -58) ethene, (58,59) ethane, (60) and formaldehyde. (61) Measurements have been demonstrated for larger molecules; however, the rovibrational spectra of larger hydrocarbons become rapidly diffuse as the size of the compounds increase beyond two carbon atoms.…”

Cavity Ringdown Laser Absorption Spectroscopy

“…The same group combined a preconcentration technique typically used in gas chromatography (GC) with CW-CRDS and demonstrated the capability of measuring ethane in urban air at a mixture ratio level of 6 ppb. (172) Winstead et al measured absorption cross sections of the first C−H overtone of several VOCs, such as benzene, toluene, chlorobenzene, and dichlorobenzene using an NIR external cavity diode lasers cavity ringdown spectroscopy (ECDL-CRDS) system. (173,174) More recently, Cias reported measurement of absorption cross sections of the first C−H overtone of 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene), and 2,3-dimethyl-1,3-butadiene using CW-CRDS at 1650 nm.…”