Certified reference materials of pure oxygen were produced by National Metrology Institute of Japan mainly for the metrological traceability source of JCSS oxygen in nitrogen standard gases. The property value of the certified reference material is the amount of substance of oxygen which was determined by the subtraction method described in ISO/IEC 6142:2001. The expected impurity components in the certified reference material were CO, CO
2, CH
4, N
2O, H
2O, Ar and N
2. CO, CO
2, CH
4 and N
2O in the certified reference material were determined by FT-IR with a long-path gas cell. Ar and N
2 were determined by GC-TCD with an oxygen absorber, and H
2O was determined by a capacitance hygrometer. The stability of the amount of substance of impurities was monitored. The impurities except for CO
2 were not detected and were regarded as being stable. The amount of substance of CO
2 seemed to increase. The uncertainty for long-term stability of the property value was determined from the expected increase of the CO
2 amount of substance on the expiration date. The typical certified value and its expanded uncertainty (
k = 2) are 0.9999993 mol mol
−1 and 0.9 μmol mol
−1, respectively. The dominant uncertainty for the certified value was the uncertainty for the impurity analysis and long-term stability. The stability was also monitored after the certification. The increase in the CO
2 amount of substance is smaller than the uncertainty for the long-term stability, and drift of the amount of substance of oxygen is smaller than the expanded uncertainty of the certified value.
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