Purity is commonly expressed as a two digit number. For example, Helium with a purity of 99.9996% would be described as HiQ Helium 5.6. Often it may not just be a question of the gas purity, but the degree and type of impurity that makes the real difference. Two gases of nominally identical purity may not necessarily have the same effect. For example, your instrument may require a 5.0 grade gas with a total hydrocarbon level < 0.5 ppm. If you use a grade 5.0 product with hydrocarbon level <1 ppm, the increased noise could affect your analysis. For some gases, purity is described by the application, e.g. Helium ECD or Carbon Dioxide SFC/SFE. In other cases, purity information can be stated as freedom from a specific, particularly unwelcome impurity, e.g. CO-free Nitrogen or C_nH_m-free synthetic air.