Abstract
We report on the development of a photoacoustic sensor for the detection of formaldehyde (CH2O) using a thermoelectrically cooled distributed-feedback quantum cascade laser operating in pulsed mode at 5.6 μm. A resonant photoacoustic cell, equipped with four electret microphones, is excited in its first longitudinal mode at 1,380 Hz. The absorption line at 1,778.9 cm−1 is selected for CH2O detection. A detection limit of 150 parts per billion in volume in nitrogen is achieved using a 10 seconds time constant and 4 mW laser power. Measurements in ambient air will require water vapour filters.
Highlights
Formaldehyde (CH2O) is a carcinogenic pollutant emitted as an intermediate product in the oxidation of most biogenic and anthropogenic hydrocarbons
The quantum cascade laser (QCL) used in this paper works in single mode emission at a wavelength around 5.6 m, where the formaldehyde C=O stretching mode ( 2 fundamental band) is located [17]
Our CH2O sensor results compact (~ 0.05 m3 including a thermoelectric cooled/chemical trap water removal system) and exhibits parameters comparable with that obtained by Horstjann et al [20] via a quartz-enhanced PA fork system coupled with a liquid nitrogen cooled (78 K) inter-cascade laser (ICL) (D = 2.2 × 10−8 Wcm−1Hz−1/2 for signal-to-noise ratio (SNR) = 1) and lower than that reported by Angelmahr et al
Summary
Formaldehyde (CH2O) is a carcinogenic pollutant emitted as an intermediate product in the oxidation of most biogenic and anthropogenic hydrocarbons. The detection limits of the wet chemical techniques are in the ppbv range; they suffer from interference from the environmental conditions (temperature, humidity), are expensive and have to be performed by highly specialized personnel These methods exhibit slow response times, typically on the order of minutes, related with the chromatographic separation and this prevents application requiring real–time and continuous gas monitoring. Direct absorption and cavity enhancement spectroscopies (i.e. cavity ring down spectroscopy) take advantage of long optical path in multi-pass cell and high finesse optical cavities, respectively These techniques allow high sensitivities (up to sub-ppbv), they need sophisticated and/or cumbersome equipments, not suitable in applications which require compact and transportable sensors [11,12]. The sensor meets the international environmental regulations in terms of minimum detectable CH2O concentration
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