Stable Gaseous Isotope Measurement Method Based on Highly Sensitive Laser Absorption Spectroscopy and Its Applications (Invited)

Stable isotope technology has been widely used in geochemistry, geophysics, agriculture, biology, clinical medicine and ecological environment science. It is very important for the study of atmosphere, soil, water quality and health to realize the accurate measurement of gas isotope abundance. At present, isotope abundance is measured and analyzed mostly by Isotope Ratio Mass Spectrometers. It is a relatively large, expensive and complex pre-processing, which is not conducive to real-time measurement of isotope abundance in the field. With the development of laser technology, the output wavelength can cover more and more wide bands, and the performance is more better. Many kinds of gases can be detected by laser absorption spectroscopy, and the detection sensitivity is further improved. This makes it possible to measure isotopic gases, which have attracted more and more attention and become one of the hot spots of laser spectroscopy. Compared with the traditional stable isotope analysis instruments, the isotope analysis technology based on laser absorption spectroscopy, as a relatively new isotope abundance measurement method, has the advantages of high selectivity, high accuracy, small volume, real- time online monitoring concentration and abundance, etc. So, the gas isotope analysis system based on laser absorption spectroscopy technology has important application value in environmental monitoring, ecosystem research, pollution tracing and tracing because of its miniaturization, fast response and real-time online measurement. Taking Tunable Diode Laser Absorption Spectroscopy (TDLAS), Integrated Cavity Absorption Spectroscopy (ICOS)and Cavity Ring- Down Absorption spectroscopy (CRDS)as examples, the basic principle, spectrum line selection, system structure and some test results are described in this paper. Under the control of the PID system of temperature and pressure, the temperature control error is 0.003 degrees C, and the pressure control error is 0.03mbar, which can meet the requirements of temperature and pressure control during the measurement of isotope gas. The C-13 measurement accuracy of atmospheric CO2 is 0.3%, the (CH4)-C-13 measurement accuracy of coalbed methane is 1.25%, the O-18, O-17 and D measurement accuracy of glacial H2O are 0.3%, 0.2% and 0.5%, respectively. And also, the" gold standard" for (CO2)-C-13 of detecting helicobacter pylori infection is also introduced. The feasibility and reliability of laser absorption spectroscopy in isotope measurement are verified. And it is also fully demonstrated that the measurement method based on laser absorption spectroscopy has a very good technical advantage. TDLAS, ICOS and CRDS have become the best choice for the high-sensitivity detection of trace gases and isotope gases. The three technologies have their characteristics:TDLAS technology has the simplest structure, but its cavity volume is large and requires the largest gas samples. The system structure of ICOS technology is relatively simple, but the technology is based on absorption intensity measurement technology, limited by laser intensity noise and frequency drift, which is also a major problem in applying ICOS technology in high sensitivity detection. Therefore, the signal-to-noise ratio and measurement accuracy of ICOS technology is slightly lower than that of CRDS technology. The structure of CRDS technology is relatively complex, and the conditions are relatively harsh, but it can achieve the absorption optical path that the above two technologies cannot reach, so it has the best sensitivity and measurement limits. Based on the above advantages and practical feasibility, laser absorption spectral isotope analysis system will be more and more applied to various fields. And the related system research still has a long way to go. At the same time, laser absorption spectroscopy is also faced with many shortcomings, such as:in these measurements, it is also necessary to consider the error sources such as the concentration dependence of the instrument and the time drift. A certain correction frequency should be set for periodic correction to eliminate the influence of the instrument drift, and the measurement results with accuracy and accuracy meeting the research needs should be obtained through appropriate correction strategies.