Development of a high precision measurement system for atmospheric oxygen concentration

本课题研制了一套高精度大气氧浓度测量系统。该系统主要由探测器、派热克斯玻璃双进气道、高精度压力调节器和计算机组成。选择Nier-McKinney型质谱仪作为系统的检测器。25 mm内径的熔融石英毛细管。双进样口与检测器的连接长度为1.35m。空气样品可以通过毛细管在大气压下引入到入口。我们证实，空气成分的分馏没有发生在毛细管。发现富氧现象发生在气流线温度较低的部位。为此，将该系统安装在有空调的实验室中，并将热容量很小的毛细管用双层保温管包裹起来。在质谱计的离子源中，由于CO2生成CO而产生的对N2的等压干扰是影响高精度测量的一个严重问题。通过实验建立了CO_2和O_2浓度的经验公式，用于校正空气样品的干扰，并自制了O_2测量用标准气体。天然空气通过水蒸气捕集器收集并储存在高压气缸中。在不同的季节多次收集大量空气，以制备不同O2浓度的标准气体。为了确认这些标准品的O2浓度的稳定性，我们在这些标准品之间进行了超过1年的相互校准。结果表明，所有标准气体在± 20 μ g/m2范围内稳定，O_2浓度无漂移，四次分析的标准误差为± 5.6 μ g/m2。

A high precision measurement system of atmospheric oxygen concentration has been developed by the research project. The system consisted mainly of a detector, double inlets made of pyrex glass, a high precision pressure adjuster and a computer. A mass spectrometer of the Nier-McKinney type was selected for the detector of the system. The fused-silica-capillaries of 25mm I.d. and 1.35m length were used for connection between the double inlets and the detector. Air samples could be introduced into the inlet at an atmospheric pressure by the capillary. We confirmed that fractionation of air components did not occurred in the capillary. We found that oxygen enrichment occurred at a part of lower temperature of airflow line. Therefore, the system was settled in our air-conditioned laboratory and the capillary with a very small heat capacity was covered by double insulation tubes. Isobaric interference for N2 due to the formation of CO from CO2 in the ion source of the mass spectrometer was a serious problem for the high precision measurements. An empirical formula for CO2 and O2 concentrations was established by an experiment for correction of the interference of air samples.Standard gases for O2 measurements were prepared by ourselves. Natural air was collected and stored in a high-pressure cylinder through water vapor traps. Such a collection of large amount of air was made several times in different seasons to prepare standard gases of different O2 concentrations. To confirm stability of O2 concentration of these standards, we made mutual calibration between these standards for more than 1 years. As a result, all the standard gases were stable within +-20 per meg and any drift of O2 concentration could not observable.The standard error of +-5.6 per meg for a sample analyzed in quadruplicate could be attained for the measurement system.