Measurement of negative ion mobility with a trace moisture analyzer in O$_{2}$

We have been investigated the effects of impurities on the negative ion mobility in O$_{2}$ at high pressures including atmospheric pressure using a high-pressure ion drift tube with a positive point plate gap that acts as a negative ion detector [1]. When a small admixture of impurities such as N$_{2}$ and CO$_{2}$ from atmosphere are existed in O$_{2}$, negative ion mobility is increased at $E$/$N$ \textgreater 1.77 Td due to formations of NO$_{2}^{-}$, NO$_{3}^{-}$, CO$_{3}^{-}$, CO$_{4}^{-}$ as impurity ions. In addition, existence of H$_{2}$O in O$_{2}$ leads to decrease negative ion mobility because O$_{2}^{-}$$\cdot$(H$_{2}$O)$_{n}$ forms by ion-molecule between O$_{2}^{-}$ or O$_{4}^{-}$ and H$_{2}$O [2]. In this work, we describe the experimental results on the measurement of negative ion mobility in ultrahigh-purity O$_{2}$ with a trace moisture analyzer (HALO-H$_{2}$O). The ion drift tube is mounted in the stainless steel chamber with stainless steel pipes to connect the trace moisture analyzer and O$_{2}$ bottle of 99.99995{\%} purity with a gas defecator (MICROTORR: MC200-203). Mobility measurements were carried out after gas flowing, baked and pumped the chamber and gas lines at least two months due to remove impurities. During the measurements, O$_{2}$ were flowed through the chamber at 0.5 L/min. As the results, a constant mobility 2.39 cm$^{2}$/V$\cdot$s was observed at H$_{2}$O concentration between 30 and 100 ppb. This value is good agreement with the polarization limit of mobility for O$_{4}^{-}$.\\[4pt] [1] Y. Okuyama et al, J. Phys. D: Appl. Phys., 45, 195202 (2012). [2] Y. Okuyama et al, 65th Annual Gaseous Electronic Conference, 57, 8, MW2.00006 (2012).