| Accurate conclusion should be based on accurate experimental data. As main research object, we measured argon isotopic composition of 51 natural gas samples which determined by ZHT1301 and MAT 271 mass spectrometry ten years ago.In this work, we establish a high-vacuum and lower background purification and separation line with an oil diffusion pump (O.D.P) and a sputter ion pump (I.P.). The pressure range in the purification line was about 1 X 10"6Pa. It's higher about 4 orders of magnitude than old one. The high vacuum purification and separation system can partially alleviate atmospheric contaminant on samples. With this high-vacuum system, the contaminant of original gases in background is enough lower and can be ignored. At the same time, using self-making cold trap which temperature can be adjustable, He+Ne, Ar+Kr+Xe and Ar, Kr+Xe were rough separated. "Transferring sample under tranquil state by different intervals" we established can avoid fractionation of isotopes which have different mass number.By comparison between the new data and old determinations, it is found that the old measurement on argon isotopic composition was contaminated by the high background and the method of sample transferred with the isotopic fractionation, and old data about 73% can be used to calculate the source rock age. We suggest that the conclusion based on the old determinations were still scientific.
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