Pressure-Tunable Oxygen Attachment Dissociation in a Deuterium Lamp-Based Photoionization Source for Isomer...

Single-photon ionization (SPI) is a powerful analytical technique for on-site analysis, whose performance critically depends on the characteristics of the employed light source, notably light intensity and wavelength. In this study, the performance of two commercial vacuum ultraviolet lamps─deuterium (D₂) lamps and krypton lamps─was compared in the SPI source. The results demonstrated that the signal intensity of benzene in N₂ obtained using the D₂ lamp was approximately 17-fold greater than that achieved with the latter. More importantly, a pronounced in-source dissociation phenomenon induced by the D₂ lamp was discovered for the first time when switching the carrier gas from N₂ to air. By comparing the intensity of the fragment ion after introducing high concentrations of ozone, the process of in situ ozone-induced dissociation was ruled out. Kinetic analysis confirmed that this dissociation mechanism is driven by ground-state atomic oxygen, O(³P), generated from the photodissociation of O₂ by photons emitted from the D₂ lamp within the 135-170 nm range. By leveraging this in-source oxygen attachment dissociation (OAD), some isomers such as quinoline and isoquinoline, which are difficult to distinguish by collision-induced dissociation techniques, were successfully differentiated. Furthermore, the degree of OAD was strongly dependent on the pressure within the ionization source. For benzene and isoquinoline, product ions were nearly absent below 100 Pa, whereas the degree of dissociation increased to 73.8% and 65.5% at 900 Pa, respectively. Collectively, the D₂ lamp-based SPI source offers enhanced sensitivity in N₂, while providing a convenient approach for isomer differentiation through pressure-tunable OAD processes in air.