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Water radical cations, (H2O)n+•, have attracted considerable attention owing to their potential practical applications in analytical chemistry, structural chemistry, radiotherapy, and radiochemistry. Recently, atmospheric pressure chemical ionization has emerged as a versatile method for direct mass spectrometric analysis. Usually, H3O+ is the major proton donor during ionization and only the pseudo molecular ion peaks, instead of molecular ions are detected. In this work, (H2O)2+• ions with high oxidizability and reactivity were generated using hydrothermally grown TiO2 nanoneedle arrays in combination with a linear ion trap mass spectrometer under low operating voltage and applied to the direct mass spectrometric analysis of a mixture of volatile aromatic hydrocarbons. (H2O)2+• ions were generated with a high absolute ion current of up to 1.07 × 105 counts/s at atmospheric pressure. Using the generated (H2O)2+• as the primary ion permitted the tandem mass spectrometric analysis of a mixed vapor sample of aromatic hydrocarbons. [ABSTRACT FROM AUTHOR]