|Budget Amount *help
¥2,300,000 (Direct Cost: ¥2,300,000)
Fiscal Year 1997: ¥1,000,000 (Direct Cost: ¥1,000,000)
Fiscal Year 1996: ¥1,300,000 (Direct Cost: ¥1,300,000)
Stability of 1ppbv standard gases containing methyl chloride, methyl bromide, methyl iodide, trichloroethylene, tetrachloroethylene, carbon tetrachloride, were tested in pressurized cylinders with five types of material ; widely-used Al, widely-used Mn/Fe, 1s (surface roughness<1um) -Al, 1s-Mn/Fe, widely-used Ni-Fe. Significand reduction of methyl bromide and methyl iodide was found in the three canisters other than 1s-Al and 1s-Mn/Fe after a 3-month experiment, suggesting that these two compoounds are unstable on the rough surface. Carbon tetrachloide was significantly reduced in 1s-Mn/Fe cylinder, probably due to its high reactivity with Fe.
Stability of 100 pptv standard gases in 1s-Al and 1s-Mn/Fe was also tested using eight cylinders for each. For such low concentration of standards, any of 1s-Al and 1s-Mn/Fe cylinders showed some decrease of methyl bromide and methyl iodide after two-month storage. Furthermore, the reduction rate was not always same even for the same type of cylinders : they ranged from <10% to 99%.
These findings suggest that working standard gases containing unstable components (such as methyl iodide) at low concentration should be calibrated with dynamic dilution of 1 ppm standard gases which have been proved to be stable for more than 1 year. Since the reduction rates were found to be linearly related with the time after the preparation of standards, correction factors for each standard gas can be speculated from periodic calibration (-once a month).