A mass specrometer for the quantitative analysis of micro-size gas samples, especially rare gases, has been constructed at Osaka University. The mass spectrometer is 90°-sector type and its mean ion radius is 200mm. Its main parts are made from stainless steel and gaskets of aluminium of 0.5mm thikness are used for vacuum shielding. The shape of wedges and ditchs being on flanges for vacuum shielding are quite similar to the Hintenberger's. And also the main valve and the gas inlet needle leak were designed according to Hintenberger's principle. Theevacuating system consists of a 4″ oil diffusion pump(Apiezon oil C)followed by dry-ice and liquid nitrogen cold traps in series. The analysing tube, including the ion source part, the ion collector part and the main valve part, can be baked out up to about 400°C. At the present state, by about 20 hours' evacuation and baking(at about 150°C)followed by about 20 hours' baking of ion source the ulutimate pressure of 5-20×10-9mmHg can be easily obtained. On this condition, the main residual peaks are those of M/e 18 and 28, and their peak heights are about ten times of detector noise level(1-2×10-5 amp.). M/e 17, 32 and 44 peaks are detectable but other peaks up to M/e 200 are covered with the noise level. Under these coditions, the static operation of 30-60min. duration can be carried on and the increasing characteristics of back ground peaks were investigated. As the preliminary experiment the isotopic abundance ratios of some argon samples of 10-4-10-5ccNTP(radiogenic argon extracted from a few gram of muscovite, electromagnetically enriched argon 36 and their mixture gas)were determined under static operation. For example, the measured Ar36/Ar40 ratio of one mixture sample was 0.3583±0.0002. (This value is not corrected for any kinds of expected mass discriminations.)
