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

Abstract

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 m/z 36 appears, which is identified as the water dimer cation. The water dimer cation, (H₂O)₂^+•, and the hydrated ammonium cation, NH₄⁺·H₂O, have essentially indistinguishable mass differences in a low-resolution mass spectrum. By comparing the behavior of (H₂O)₂^+• and NH₄⁺·H₂O with temperature and salt concentration, we can rule out the formation of NH₄⁺·H₂O in the spraying of ultrapure water into laboratory air.