Clarifying the Identity of the / 36 Ion in Water Microdroplet Mass Spectra.

Several lines of evidence are presented that when ultrapure water is sprayed into air and the resulting water microdroplets are mass analyzed a peak at / 36 appears, which is identified as the water dimer cation. The water dimer cation, (HO), and the hydrated ammonium cation, NH·HO, have essentially indistinguishable mass differences in a low-resolution mass spectrum. By comparing the behavior of (HO) and NH·HO with temperature and salt concentration, we can rule out the formation of NH·HO in the spraying of ultrapure water into laboratory air.