The Measurement Of Water Isotope Ratio In Atmosphere By Infrared Laser Spectroscopy

Infrared laser spectroscopy is a kind of detection technology based on fundamental frequency strong absorption properties of gas molecules, due to its high resolution, high sensitivity and fast time response characteristics; it has been widely used in the field of environmental monitoring, industrial and agricultural production and medical diagnosis. In this paper, the measurement system of gas molecular isotope ratio was set up based on infrared spectroscopy technology and combined with the current application needs. In addition, we have made a deep research on the measurement method of the isotopic of water vapor in atmosphere.First of all, this thesis introduces the principle of gas molecular isotope ratio measurement and compares the advantages and disadvantages of each measurement method. Secondly, the performance of the components was introduced. Particularly, the properties of the quantum cascade laser and the lead salt semiconductor laser were compared. Then a real-time data acquisition program was implemented for isotopic concentration retrieval by LABVIEW. This program has a great advantage in its fitting subroutines, for example, Gauss Fitting procedure, Lorenz Fitting procedure and Psdvoigtl and Psdvoigt2 Fitting procedure. The purpose is to find the best fitting effect quickly. Finally, the detailed process of measuring the isotopic ratio of water vapor in the air was introduced and the measurement results were presented. The measurement system consists of the mid-infrared continuous-wave quantum cascade laser and multipass absorption cell, which the number of reflection is 238 with the optical path of 76 meters. There are nine absorption lines in the band 1387.9cm-1-1389.2cm-1, selecting seven lines and testing three sections. In measurement, absorption lines have very good characteristics except for the H217O. Besides, the line shape of the spectral lines is tested at the same position with different pressure. Then the variation tendency of the line width and the possible error sources are analyzed. The measurement results of isotope ratio are presented, as follows:HDO/2[H216O]=164.7×10-6,H217O/H216O=256.16×10-6,H218O/H216O =2006.62×10-6. These measured results were thoroughly compared to the Marine Bureau of standard value and L.Joly experimental results. The results show that the performance of the measurement system meets the requirement of the accuracy of isotope ratio.