Employing a mass spectrometry method, a high precision measurement system was developed for analysis of the atmospheric O
2/N
2 ratio. Sample air and reference air were introduced into the mass spectrometer through thermally-insulated thin fused silica capillaries from an inlet system. Interference by CO generated in the ion source of the mass spectrometer from CO
2 in the sample air, and the O
2/N
2 ratio biased due to pressure imbalance between the sample air and the reference air during their introduction into the mass spectrometer were experimentally corrected. Deterioration of sampled air during storage in flasks, as well as air sampling procedures, was also examined. The precision of our measurement system was estimated to be 5.4 per meg for the O
2/N
2 ratio, which corresponds to 1.1 ppmv of the atmospheric O
2 concentration. Our standard air with 6 different O
2/N
2 ratios were prepared by drying the atmosphere and then stored in 47 L high-pressure cylinders; their O
2/N
2 ratios were confirmed to be stable within 20.0 per meg over the last 2 years. This system has been used for actual measurements of the atmospheric O
2/N
2 ratio since May 1999. Preliminary results of the measurements made in the suburbs of Sendai, Japan showed clear evidence for the seasonal cycle and the secular trend of the atmospheric O
2/N
2 ratio, which are opposite in phase with those of the CO
2 concentration.
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