We describe the application of cavity ring-down spectroscopy (CRDS) to the detection of trace levels of ethylene in ambient air in a cold storage room of a fruit packing facility over a several month period. We compare these results with those obtained using gas chromatography (GC), the current gold standard for trace ethylene measurements in post-harvest applications. The CRDS instrument provided real-time feedback to the facility, to optimize the types of fruit stored together, and the amount of room ventilation needed to maintain sub-10 ppb ethylene levels for kiwi fruit storage. Our CRDS instrument achieved a detection limit of two parts-per-billion volume (ppbv) in 4.4 minutes of measurement time.
The spatial profile of methyl radical concentration in a hot-filament reactor has been measured using cavity ring-down spectroscopy (CRDS) at a wavelength of 213.9 nm for which the CH3 absorption cross section has been shown to be nearly independent of temperature. Methyl radicals are generated with a 25 mm long tungsten filament heated to 2400 K in a slowly flowing mixture of 0.6% CH4 in H2 (20 Torr total pressure). CRDS is employed to measure CH3 absorbance as a function of a distance perpendicular to the axis of the filament. The CH3 absorbance profiles do not change when the direction of the process gas flow through the reactor is reversed, which indicates cylindrical symmetry of the CH3 distribution about the filament. Consequently, the radial CH3concentration in the reactor is determined by Abel inversion of the CH3 absorbance profile. The CH3concentration peaks ∼4 mm from the filament (1.04×1014molecules/cm3). Methyl radicals decay rapidly as a function of a distance from the filament and disappear about 2 cm from the filament within our present detection sensitivity (3×1012molecules/cm3).
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