Novel, real-time measurements of VOCs using a Cavity Ring-Down Spectrometer (CRDS)

Measurements of volatile organic compounds (VOCs) are important in a wide variety of scientific disciplines, including air quality, biogeochemistry, hydrology, plant and animal physiology, human health, and petrochemistry. Generally, these measurements are performed with expensive laboratory-based mass spectrometers, slow gas chromatographs, non-speciated flame- or photo-ionization detectors, or insensitive Fourier transform infrared spectrometers. Laser-based spectrometers based upon cavity enhanced techniques like cavity ring-down spectroscopy (CRDS) would in principle provide significant advantages over these methods in sensitivity, speed of response, simplicity, stability, and portability, as has been demonstrated in the last decade for the quantification of simple molecules like carbon dioxide, methane, or ammonia.  However, the majority of these instruments are based upon narrowly tunable lasers.  These lasers cannot tune across the broad spectral features, characteristic of VOCs.  In this paper we present a novel CRDS instrument based upon a broadband laser source that can in principle span more than hundred nanometers.  We show early measurements of ethylene oxide and BTEX (benzene, toluene, ethylbenzene, and xylene) at parts per billion and even parts per trillion levels.  Though our initial work has focused on measurements of hazardous air pollutants, there is future potential for speciated detection of other VOCs as well as greenhouse gases. These systems can be deployed on a bench-top and/or vehicle, and with per-second measurement intervals, are ideal for correlation with weather data for accurate source attribution. The analyzers are being developed to meet indoor and outdoor air quality requirements and may be deployed near sources (stack or fenceline monitoring) or far from sources (community monitoring).