Impact of humidity on quartz - enhanced photoacoustic spectroscopy based detection of HCN

The architecture and operation of a trace hydrogen cyanide (HCN) gas sensor based on quartz-enhanced photo-acoustic spectroscopy and using a λ = 1.53 µm telecommu-nication diode laser are described. The influence of humidity content in the analyzed gas on the sensor performance is investigated. A kinetic model describing the vibrational to translational (V–T) energy transfer following the laser excitation of a HCN molecule is developed. Based on this model and the experimental data, the V–T relaxation time of HCN was found to be (1.91 ± 0.07)10 −3 s Torr in collisions with N 2 molecules and (2.1 ± 0.2)10 −6 s Torr in collisions with H 2 O molecules. The noise-equivalent concentration of HCN in air at normal indoor conditions was determined to be at the 155-ppbv level with a 1-s sensor time constant. 1 Introduction Hydrogen cyanide (HCN) is a highly toxic chemical species. It is used in a variety of industrial syntheses including the production of adiponitrile (for nylon), methyl methacrylate, pharmaceuticals, and other specialty chemicals. Manufacturing activities releasing hydrogen cyanide include electroplating, metal mining, metallurgy, and metal-cleaning processes. Besides, HCN is considered as a warfare chemical agent. The National Institute for Occupational Safety and Health (NIOSH) has established a recommended exposure limit (REL) for hydrogen cyanide of 4.7 ppmv (5 mg/m 3) as a short-term exposure limit (STEL) [1]. Therefore , the development of a HCN sensor capable of detecting sub-ppmv concentration levels and operating in an industrial environment is an important step toward ensuring industrial safety. Hydrogen cyanide is also formed during the incomplete combustion of nitrogen-containing polymers, such as certain plastics, polyurethanes, wool, and paper. Therefore, HCN detection is important in early fire detection on board of an aircraft or spacecraft. In this work we report the design and performance evaluation of a HCN sensor based on the quartz-enhanced photo-acoustic spectroscopy (QEPAS) technology [2]. The QEPAS approach has been previously used for the quantification of trace concentrations of a number of small molecules, as reported in [2]. The sensitivity to a chemical species when normalized to the absorption line strength and the excitation laser power is primarily determined by the rate of vibrational to translational (V–T) energy transfer, reaching its maximum when this rate is higher than the laser modulation frequency. This rate may depend on the major composition of the gas carrying the target trace component. In particular, the presence of H 2 O …