Negative Ion Mobility and Effects of Impurities in O₂

Abstract

In this study, we have measured negative ion mobility in high-purity O₂ (99.9999%) and ultrahigh-purity O₂ purified with a gas purifier to remove impurities in cylinder O₂ (99.99995%) for the investigation on the effects of impurities. It is considered that negative ion mobility is strong affected by impurities such as N₂, CO₂ and H₂O at high pressure including atmospheric pressure. However, mobility measurement at high pressure has carried out only a few researchers including our laboratory. Then, we have performed the measurement at atmospheric pressure. An interesting result was obtained from ultrahigh-purity O₂ that is the reduced ion mobility increased to 2.39±0.03 cm²/V·s from the constant value of 2.31±0.03 cm²/V·s obtained in high-purity O₂ at 2.83×10^-2 ≤ E/N ≤ 5.31 Td. The difference of these values 2.31 and 2.39 cm²/V·s was attributed to the impurities that existed in the O₂. The impurity is H₂O contained in the cylinder O₂ which led to a decrease in negative ion mobility. Other impurities such as N₂ and CO₂ are the main constituents of the atmosphere attached on the surfaces of the chamber and electrodes. They led to an increase of ion mobility at high E/N values exceeding 1.77 Td.