Water Clusters in Gas Phases Studied by Liquid Ionization Mass Spectrometry

Abstract

Studies of water clusters under atmospheric pressure are very important. In previous methods, however, clusters existing in vacuum at low temperature have been measured. Therefore, the size distributions of observed cluster ions are likely different from those existing at normal conditions. Liquid ionization (LPI) mass spectrometry we have developed provides information about hydrogen-bonded clusters in a gas phase and also at the liquid surface at ambient temperature under atmospheric pressure. Thus, LPI-MS was applied to study water clusters in gas phases. The results indicated that the sizes of water clusters in vapor increased with the temperature of liquid water and those in the air increased with the relative humidity, while they decreased with increasing the temperature of the gas phase. In general, the size distribution of water clusters shows one smooth envelope, except for a few magic numbered clusters. Adiabatic condensation is negligible to occur in this method and thus, LPI mass spectra give useful information about clusters in gas phases under the normal conditions.