Gas sampling method by the substitution of helium gas

Abstract

The direct sampling method with use of a vacuum bottle has been widely used for sampling harmful substances in atmosphere and chemicals under work environment. This method is useful for the instantaneous sampling, but has some disadvantages: it takes rather long time to evacuate the bottle whose vacuum is difficult to be kept for long time. To solve this problem, we investigated an inert gas substitution sampling method based on the utilization of the difference of gas specific gravity : the sample bottle was preliminaily filled with helium gas and at sampling we opened two silicon rubber stoppers at both the necks of the bottle, keeping it perpendicularly. With this procedure we can sponteneously substitute sample gas for helium gas (Helium gas is 7.24 times smaller in the specific gravity than the air). The experimental result on a sampling bottle of 200 ml in volume and 4.5 mm in opening diameter showed that 3 min were good enough to substitute helium completely in practice and no leak of the helium gas in the bottle was observed even after allowing it to stand for 10 d. The present method is simpler and more reliable than the vacuum bottle method, hence more practical for sampling hydrocarbons or inorganic gases in the atmosphere.