High-precision Trace Gases Measurement Based On Mid-Infrared Laser Absorption Spectroscopy

Trace gases play an important role on the earth,which carry a lot of important information and are closely related to human life.Emissions of trace greenhouse gases are heating the planet,which is a terrifying threat in this century.Nitrous oxide(N2O)is the third most important individual contributor to the combined forcing,and carbon monoxide(CO)is also considered to play a non-negligible role in global warming,therefore,the observation of N2O and CO in the atmospheric is an important process to realize the carbon peaking and carbon neutrality goals.However,the concentrations of N2O and CO are both in the order of ppb(part per billion),and the compatibility goal is strict,so there are still challenges especially in China.In addition,the Helicobacter pylori infection which is associated with trace carbon dioxide(CO2)isotopes in human exhalation,is another hot topic that threatens human survival.As reported,there are about 340,000 gastric cancer patients per year caused by Helicobacter pylori infection in our country.Therefore,the analysis of CO2 isotopes in human breath can provide an effective,convenient,and fast method for the screening of Helicobacter pylori infection in a large-scale population.With the rapid development of laser technology,laser based direct absorption spectroscopy has been widely used in trace gas detections as a particularly sensitive and non-destructive measurement method with a fast response time.In this paper,the applications of laser direct absorption spectroscopy in trace gas detection are studied.Since the fundamental vibrational bands of N2O,CO,and CO2 are all in the midinfrared region,mid-infrared laser absorption spectroscopy is undoubtedly a very suitable method for the measurement of trace N2O and CO in atmospheric and trace CO2 isotopes in exhaled breath.A breath analysis system has been experimentally developed based on single path absorption spectroscopy.A single path absorption cell with a volume of fewer than 5 ml was used in the spectroscopic system,which is very suitable for breath analysis with small gas samples.Benefiting from the fundamental vibrational transition of CO2 isotopes at 4.3 μm,the 12CO2,13CO2,18O12C16O,and 17O12C16O in exhaled breath have been simultaneously detected with high sensitivity,and the δ values of isotopic abundances were analyzed.In order to reduce the interference from temperature,a highprecision temperature control system was designed with a standard deviation of 0.0013℃,which reduces the isotopic abundance measurement error caused by temperature to less than 0.03‰.To reduce the unnecessary measurement errors,the standard deviation of the pressure in the absorption cell was also optimized to 0.0035 Torr.After calibration,the precisions of the system for δ12C,δ18O,and δ17O within 1s response time are 0.12‰,0.18‰,and 0.47‰,respectively,with a detection error less than 0.3‰,and.To realize the simultaneous high-precision measurement of ppb-level N2O and CO in the atmosphere,the theory of the multipass absorption cell was first studied.On the basis of the original optical matrix transmission theory,a space light transmission theory has been proposed.Using the method of solving the space straight line and spherical equations to calculate the light spot positions on the spherical mirrors,and then the transmission direction of the reflected light was calculated with the vector method.Based on the proposed theory,an improved White cell with an effective optical path of 76 m was designed.Combined with a single interband cascaded laser near 4.5μm,the simultaneous high-precision measurement of trace N2O and CO in the atmosphere has been achieved.A new method of perturbing the laser with Gaussian RF white noise was proposed to suppress the interference caused by the multipass cell.The output laser wavelength is calibrated in real time using the correlation method.The resulted 1σ precision of the system is 0.065 ppb for N2O and 0.133 ppb for CO.The precision and daily drift meet the compatibility goal.Finally,the reliability of the system has been verified in actual atmospheric measurements.