Experimental study on the interference of water vapor on the chemical ionization of oh by sulfur hexafluoride ion

The interference of water vapor on the chemical ionization (CI) of hydroxyl radicals (OH) by sulfur hexafluoride ion (SF₆ -) was investigated using a flow tube system coupled to a high-pressure CI mass spectrometer. Water vapor, which is required to study heterogeneous reactions of OH under real tropospheric conditions, transforms the reagent ion SF6 - into SF₄O- and F-(HF)n, resulting in a substantial loss in CI sensitivity. Therefore, under humid conditions, peaks corresponding to OH are drastically diminished, while those corresponding to OH-water complex ions ([OH(H₂O)n]-) are enhanced. [OH(H₂O)₃]- was observed as the major OH species. The obsercation of [OH(H₂O)n]- by isolating humid conditions to the CI region and preliminary ab initio calculations suggested that [OH(H₂O)n]- ions were produced from reactions between OH ions (OH-) and water molecules. An additional helium buffer flow introduced into the CI region reduced loss of the reagent ion and resulted in a partial recovery of OH peak intensities under humid conditions.