Cluster Size Distributions of Water at Liquid Surface and in Gas Phase Using Liquid-ionization Tandem Mass Spectrometry

Water has several special properties, such as extremely high melting and boiling points. Although the existence of water clusters has been suggested, the cluster size distributions of water have not yet been known. Water clusters under atmospheric pressure at an ambient temperature were measured by liquid-ionization (LPI) tandem mass spectrometry. All ions observed as LPI mass spectra were expressed as (H2O)nH+. The number of molecules (n) in a cluster observed at the liquid surface ranged from 2 to around 30, and the average number (N) of water molecules was around 15 – 17. (N = Σn In/ΣIn, In: peak intensity). Water clusters also existed in the gas phase, but their sizes were smaller than those at the liquid surface, and became smaller with the distance (d) apart from the liquid surface. The size distributions of water clusters should be related to the absolute abundance of water in the unit space, and such abundance of water was diluted with Ar gas flow. When the flow rate of water (liquid) was increased over 10 μL h−1, mass spectra observed by the second mass spectrometer (Q3: 3rd quadrupole) showed much larger cluster ions than those observed by the first mass spectrometer (Q1: 1st quadrupole). The ion-molecule reactions between ions and sample vapor occurred mainly in the collision chamber (Q2) may cause the formation of large cluster ions. Because water clusters are formed by hydrogen bonding, the results and the fact that steam and fog are visible, indicate that large clusters (aggregate) of water may exist in gas and liquid phases. The existence of these clusters may cause special properties of water.