Method for measuring changes in the atmospheric O2/N2 ratio by a gas chromatograph equipped with a thermal conductivity detector
Method for measuring changes in the atmospheric O2/N2 ratio by a gas chromatograph equipped with a thermal conductivity detector
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用装有热导检测器的气相色谱仪测量大气中O2/N2比值变化的方法
DOI：
10.1029/2000jd900057
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发表时间：
2000
影响因子：
--
通讯作者：
Y. Tohjima
中科院分区：
文献类型：
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作者：
Y. Tohjima
我们提出了一种方法，用于测量大气中的O2/N2比的变化的基础上，从气相色谱仪（GC）配备了热导检测器（TCD）的数据。在该方法中，空气样品中的O2和N2在填充有分子筛5A的柱上与H2载气分离。由于分离出的O2中含有与O2的滞留时间相同的Ar，因此实际上测定（O2+Ar）/N2比。基于大气Ar/N2比几乎恒定的事实，测量的（O2+Ar）/N2比的变化可以容易地以非常小的误差转换为O2/N2比的变化。为实现高精度测量，对色谱柱头、TCD和进样环出口处的压力进行了稳定。此外，通过重复样品和参考气体的交替分析，在统计上提高了精度。参比品和样品分析重复循环的标准偏差约为18/mcg（相当于空气中3.8 ppm O2）。这意味着，对于7个循环的交替分析，标准误差约为7/mcg（空气中1.5 ppm O2），这需要约70分钟。该方法的响应可能具有2%的非线性。在压力下将环境空气样品收集在玻璃烧瓶中，玻璃烧瓶配备有两端用氟橡胶O形环密封的两个旋塞阀。在O2/N2测量期间烧瓶中的压力耗尽不会引起O2/N2比的任何可检测的变化，但是发现烧瓶中的O2/N2比在储存期间逐渐降低。我们还介绍了1997年7月至1999年3月在波照间岛（北纬24°03′，东经123°49′）采集的空气样品的初步结果。观测到的O2/N2比值明显表现出季节性变化，春季和夏季增加，秋季和冬季减少。
We present a method for measuring changes in the atmospheric O2/N2 ratio based on data from a gas chromatograph (GC) equipped with a thermal conductivity detector (TCD). In this method, O2 and N2 in an air sample are separated on a column filled with molecular sieve 5A with H2 carrier gas. Since the separated O2 includes Ar, which has a retention time similar to that of O2, the (O2+Ar)/N2 ratio is actually measured. The change in the measured (O2+Ar)/N2 ratio can be easily converted to that in the O2/N2 ratio with a very small error based on the fact that the atmospheric Ar/N2 ratio is almost constant. The improvements to achieve the high-precision measurement include stabilization of the pressure at the GC column head and at the outlets of the TCD and the sample loop. Additionally, the precision is improved statistically by repeating alternate analyses of sample and a reference gas. The standard deviation of the replicate cycles of reference and sample analyses is about 18 per meg (corresponding to 3.8 parts per million (ppm) O2 in air). This means that the standard error is about 7 per meg (1.5 ppm O2 in air) for seven cycles of alternate analyses, which takes about 70 min. The response of this method is likely to have a 2% nonlinearity. Ambient air samples are collected under pressure in glass flasks equipped with two stopcocks sealed by Viton O-rings at both ends. Pressure depletion in the flask during the O2/N2 measurement does not cause any detectable change in the O2/N2 ratio, but the O2/N2 ratio in the flask was found to gradually decrease during the storage period. We also present preliminary results from air samples collected at Hateruma Island (latitude 24°03′N, longitude 123°49′) from July 1997 through March 1999. The observed O2/N2 ratios clearly show a seasonal variation, increasing in spring and summer and decreasing in autumn and winter.