A study on the stability of low level standard gas (halocarbons) in pressurized cylinders

采用5种材料，对含氯甲烷、甲基溴、碘化甲基、三氯乙烯、四氯乙烯、四氯化碳等1ppbv标准气体的稳定性进行了测试；广泛使用的Al，广泛使用的Mn/Fe， 1s（表面粗糙度<1um） -Al, 1s-Mn/Fe，广泛使用的Ni-Fe。经过3个月的实验，除1s-Al和1s-Mn/Fe外，其余3个罐子中甲基溴和甲基碘化物均有明显减少，说明这两种化合物在粗糙表面上不稳定。四氯化碳在1s-Mn/Fe圆筒中被显著还原，可能是由于其与铁的高反应活性。100 pptv标准气体在1s-Al和1s-Mn/Fe中的稳定性也分别用8个钢瓶进行了测试。在这种低浓度的标准下，任何一个1s-Al和1s-Mn/Fe钢瓶在储存两个月后，甲基溴和甲基碘化物都有所下降。此外，即使对于同一类型的钢瓶，还原率也不总是相同的：它们的范围从<10%到99%。这些发现表明，含有低浓度不稳定成分（如碘化甲酯）的工作标准气体应采用已被证明稳定超过1年的1ppm标准气体的动态稀释进行校准。由于发现还原率与标准制备后的时间呈线性相关，因此可以通过定期校准（每月-一次）推测每种标准气体的校正因子。

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).