Highly-Sensitive Infrared Spectroscopy Technique For Trace Gas Detection

Cavity ring-down spectroscopy technique is a highly sensitive absorption technique widely used for trace gas detection.The accurate measurement of absorption spectroscopy is of great important to study the internal structure and properties of molecules.Detection of trace gas in mid-infrared spectral region is beneficial due to more abundant and stronger spectral lines in this region.In this paper,the cavity ring down spectroscopy techniques based on the pulse and continuous quantum cascade lasers are developed,the trace gas is detected in the mid infrared region.The main causes of line broadening are discussed and different line profiles are introduced,including the Lorentz,Gauss and Voigt.The absorption spectra of air and methane in high purity nitrogen were measured.The wavelength of the laser is calibrated by the absorption line in the HITRAN database and the line-width of the quantum cascade laser is measured.The effect of the spectral line-width of laser on the measurement of small molecules with narrow absorption line-width is analyzed and the correction curve is developed to correct the measurement results.CRDS is combined with an external cavity,widely tunable pulsed quantum cascade laser operating at 3.8 ? m,thus multiple broadband absorbers can be simultaneously measured with high sensitivity and selectivity.The absorption spectra of ethanol,ether and acetone in this region are first measured and the compared with PNNL database.The experimentally achieved detection limits(,or three times of standard deviation)for ethanol,ether and acetone are 157 ppb,60 ppb and 280 ppb,respectively.By employing a 5.2?m external-cavity tunable quantum cascade laser,a continuous-wave cavity ring-down spectroscopy(CRDS)experimental setup is established and applied to detect trace moisture in high-purity nitrogen gas.In the experiment,the CRDS signal is averaged to improve the detection sensitivity,and the optimal averaging number is determined by Allan variance calculation to be 602.and the minimum detectable moisture concentration of 24.8ppb.The infrared spectrum of NO and interfering gases are analyzed by HITRAN database.Multi-parameter fitting program is developed to analysis the absorptionspectra and the detection limit of NO in high purity nitrogen is less than 0.45 ppb.The system is optimized and the best test pressure is selected for the determination of NO from human breath on line,while the detection limit of NO in exhaled human breath is less than 0.41 ppb.A new method based on cavity ring down spectroscopy is present to measure the gas tightness.Besides,a novel pressure gauge calibration method based on cavity ring-down spectroscopy(CRDS)and pressure induced line-broadening effect is presented and experimentally demonstrated at room temperature by a CRDS setup.The proposed method is applied to calibrate a small-scale pressure gauge with sufficient accuracy and the pressure measurement limit of 0.18 KPa is achieved.