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

用五种材料在加压钢瓶中测试了1 ppbv标准气体的稳定性，其中包括氯甲烷、甲基溴、碘甲烷、三氯乙烯、四氯乙烯、四氯化碳；常用的铝，广泛使用的Mn/Fe，1S(表面粗糙度和lt；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，其余三种样品中的甲基溴和甲基碘都有明显的还原现象，表明这两种化合物在粗糙表面上是不稳定的。在1S-Mn/Fe钢瓶中，四氯化碳被显著地还原，这可能是由于它与Fe的高反应性。还用8个钢瓶分别测试了100个PPTV标准气体在1S-Al和1S-Mn/Fe中的稳定性。对于如此低浓度的标准，1S-Al和1S-Mn/Fe钢瓶在储存两个月后，甲基溴和甲基碘的含量都有一定的下降。此外，即使对于同一类型的钢瓶，其还原速率也不总是相同的：它们的范围从-lt；10%到99%。这些发现表明，在低浓度下含有不稳定成分(如甲基碘)的工作标准气体应该用1ppm的动态稀释进行校准，这些标准气体已经被证明稳定了一年以上。由于减量率与标准制备后的时间线性相关，因此可以从定期校准(-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).