Real-time detection of gaseous isotopic water molecules via photoinduced associative ionization mass spectrometry: Direct identification realized by distinctive mass spectrum patterns

The real-time detection of gaseous H₂O and its typical isotopic molecules, e.g., H₂¹⁸O, D₂O, HDO, and HTO, is highly desirable in many fundamental scientific studies and practical monitoring, such as mechanistic studies of H₂O-involved chemical reactions and radiation risk warning of abnormal HTO emissions. However, ionization methods for mass spectrometry (MS) to directly measure isotopic water molecules are limited, and the discrimination of those with similar molecular weights (MWs) usually requires the use of high-resolution mass spectrometers. In this study, we present a highly efficient ionization method for water vapor based on a photoinduced associative ionization (PAI) reaction, where one excited-state CH₂Cl₂∗ reacts with two H₂O molecules to transfer a proton from one H₂O molecule to another (i.e., the analyte). Benefit to the distinctive ionization pathway, different featured ions of H₂O, D₂O, and H₂¹⁸O were observed. Surprisingly, each isotopic water molecule has its own unique PAI mass spectrum pattern according to the same ionization principle, which offers a convenient method for distinguishing isotopic water molecules with similar MWs, such as D₂O, H₂¹⁸O, and HTO (20 Da), without the need for high-resolution mass spectrometers. The measured detection sensitivities of a laboratory-built PAI time-of-flight mass spectrometer towards D₂O and H₂¹⁸O were 1465.1 ± 37.0 and 1324.4 ± 86.4 counts ppbv^-1, respectively, in a detection time of 10 s. The corresponding 3σ LODs were 0.10 and 0.11 ppbv, respectively. This study provides a novel ionization method for the direct detection and discrimination of isotopic water molecules, which has potential to combine with portable MS for on-site measurements.