Construction of a mass spectrometric system for isotope analysis of argon from micro-gram-size samples

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A new VG5400 was installed without VG engineer. Residual gas mass spectrum was obtained on June 20,1996. After that, it was improved and modified. Helium isotopic analyzes of geothermal gas samples began on December 27,1996. All noble gases (He, Ne, Ar, Kr and Xe) of rock samples, mostly mantle derived xenoliths, were measured from January 18,1997. Fairly good stability and blank level have been attained through the analyzes of samples with low noble gas concentrations. However, sensitivity is not satisfactory if it is compared with that of the mass spectrometer, MS-II,with the highest sensitivity in my laboratory. The reason is probably due to an insufficient alignment of filament in the ion source. It will be improved in the near future.Summary of performed specifications1) Mass spectrometerVacuum : <5x10^<-10>TorrAbsolute sensitivity : 5x10^<-4>A/Torr (for Ar)Detection limit : 5x10^<-15>cm^3STP (for Ar)Precision of isotope ratio analysis : 1% (for ^<40>Ar/^<36>Ar with 5x10^<-10>cm^3STP^<40>Ar)Blank : 5x10^<-12>cm^3STP (for ^<40>Ar)2) Noble gas extraction and purification systemVacuum : <5x10^<-10>TorrBlank of Ta-heater : <3x10^<-9>cm^3STP (for ^<40>Ar)Blank of laser extraction : <10^<-11>cm^3STP (for ^<40>Ar)3) Total systemDynamic range of measurable absolute volume of noble gases : 10^<-7>-10^<-16>cm^3STPAvailability of different methods of noble gas extraction : Ta-furnace : mg-g sample size Laser extraction : mg-mug (-ng) Crush and melt