Development of a high precision measurement system for atmospheric oxygen concentration

本课题研制了一套高精度的大气氧浓度测量系统。该系统主要由一个探测器、由耐热玻璃制成的双入口、高精度压力调节器和一台计算机组成。系统的检测器选用了尼尔-麦金尼型质谱仪。双入口与检测器之间的连接采用直径为25mm，长度为1.35m的熔融二氧化硅毛细管。空气样品可以在大气压力下通过毛细管进入进口。我们证实，空气成分的分馏没有发生在毛细管。结果表明，氧富集发生在气流管路较低温度处。因此，系统被安置在我们有空调的实验室，热容很小的毛细管被双保温管覆盖。质谱仪离子源中CO2生成CO对N2的等压干扰是高精度测量的一个严重问题。通过实验建立了CO2和O2浓度的经验公式，对空气样品的干扰进行了校正。测量氧气的标准气体是我们自己配制的。自然空气被收集并通过水蒸汽疏水器储存在高压钢瓶中。在不同的季节多次收集大量的空气，以制备不同O2浓度的标准气体。为了确认这些标准的O2浓度的稳定性，我们在这些标准之间进行了1年多的相互校准。因此，所有标准气体在+-20 / meg范围内稳定，O2浓度的任何漂移都无法观察到。该测量系统的四份分析样品的标准误差为+-5.6 / meg。

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.