|Budget Amount *help
¥11,500,000 (Direct Cost : ¥11,500,000)
Fiscal Year 1993 : ¥3,200,000 (Direct Cost : ¥3,200,000)
Fiscal Year 1992 : ¥8,300,000 (Direct Cost : ¥8,300,000)
We are constructing a vacuum system equipped with a quadrupole-type mass spectrometer (MSQ400, ULVAC Co., Japan) for analyzing isotopic composition of nano-mole carbon in a static mode.
We analyze carbon isotopes as carbondioxide and measure mass 42,44,45 and 46. Peaks are electrically scanned and 10 sets of data are obtained in 160 seconds. Carbondioxide tends to decompose partly into CO and partly into C to form possibly metal-carbide during the analysis due to interaction between gas and hot filament. In order to minimize this effect, we replace first a tungsten filament with a yttria (Y_2O_3) -coated iridium filament which has lower work function. This lengthen the "half life" of CO_2 in the mass spectrometer by a factor of about 5. Secondly, we lower the emission current from 1.0 mA to 0.1 mA.In this condition, the "half life" of CO_2 becomes longer than 120 sec. The ion source we are using is an "open-type" (Prosser et al., 1990) However, the observed "half life" of CO_2 is much longer than that for other "open sources" (-35 sec, Carr et al., 1986) and only a factor of three shorter than that for the "closed source" (-450 sec, Prosser et al., 1990). We found more than 80% of the gas existed as CO_2 even at the end of the analysis.
At present, the^<13>C/^<12>C ratios observed for repeated analyzes of a standard gas (about 1 nano-mole CO_2 ) are reproducible within <plus-minus>0.5%. However, there seems to be some pressure dependence in the discrimination factor for ^<12>C and ^<13>C.The lower limit of the sample size for carbon isotope analysis with this system is about 0.1 nano-mole.