基于高灵敏激光吸收光谱技术的稳定气态同位素测量及其应用(特邀)

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

稳定同位素测量技术已经在地球化学、地球物理、农业、生物、临床医学和生态科学等领域得到了众多应用。相对于传统的稳定同位素分析方法,基于激光吸收光谱技术的同位素分析技术,作为一种较新的同位素丰度测量方法,具有选择性好、精度高、体积小、无需样品预处理、可以实时原位同时测量气体浓度及同位素丰度等众多优点,得到了极大的关注和使用。本文主要以目前同位素测量的可调谐半导体激光吸收光谱技术、积分腔吸收光谱技术、腔衰荡吸收光谱技术三种激光吸收光谱方法为例,阐述了其基本原理、谱线选择、温压影响因素及其控制、系统组成结构以及部分应用测试结果。通过对测量系统的压力与温度的稳定控制的前提下,选取了合适的同位素测量谱线对,实现了大气CO_(2)气体的13C测量精度为0.3‰,煤层气CH4气体的13CH4测量精度为1.25‰,冰川水H_(2)O中18O、17O和2H的测量精度分别为0.3‰、0.2‰和0.5‰,以及呼吸气体中的13CO_(2)判识“金标准”。通过分析验证了激光吸收光谱技术在同位素测量方面的可行性和可靠性,也充分说明了基于激光吸收光谱技术的测量方法具有非常好的技术优势,将是光谱研究领域关注的重点内容,并在后续的科学研究中占据举足轻重的作用。

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℃,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 13C measurement accuracy of atmospheric CO_(2)is 0.3‰,the13CH4measurement accuracy of coalbed methane is 1.25‰,the18O,17O and D measurement accuracy of glacial H_(2)O are 0.3‰,0.2‰and 0.5‰,respectively.And also,the“gold standard”for13CO_(2)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.