Pressure-Tunable Photoionization Tandem Ion Mobility Spectrometry for Investigating Proton Transfer Reactions of Binary Systems

Investigating and manipulating the ion-molecule reactions within the ionization source of ion mobility spectrometry (IMS) or mass spectrometry can contribute to developing advanced ionization sources and novel analytical techniques. In this study, a pressure-tunable photoionization tandem ion mobility spectrometer (PI-tandem-IMS) was developed to investigate the ionization suppression caused by unusual proton transfer reactions in dual-analyte systems in which high proton affinity (PA) ions are deprotonated by low PA molecules. The proton transfer reaction mechanisms in the toluene-acetone and toluene-ethanol systems were studied. The experimental results showed the linear correlation between the ln(<i>S</i>_X2H⁺·<i>K</i>_0(T⁺₎/<i>S</i>_T⁺·<i>K</i>_0(X2H⁺₎ + 1) and the square of the reactant concentration <i>c</i>_X², as well as the cubic power of the pressure <i>p</i>³. Based on this, the generation of the proton-bound dimers in the toluene-acetone and toluene-ethanol systems was assigned as a termolecular process. The reaction rate coefficients <i>k</i> of the toluene-acetone and toluene-ethanol systems were calculated at different temperatures, and the Arrhenius plot showed that rate coefficients were both negatively correlated with temperature, implying that elevated temperatures suppress the proton transfer reaction. At 313.15 K, the calculated <i>k</i> values for the toluene-acetone and the toluene-ethanol systems were 2.2 × 10^-26 cm⁶/s and 5.2 × 10^-28 cm⁶/s, respectively, suggesting a higher inhibitory effect of acetone on toluene ionization than that of ethanol. Besides, the suppressive effect of reducing the pressure or increasing the reaction region electric field strength on proton transfer reactions was shown, which demonstrated the PI-tandem IMS was a good tool for understanding ion-molecule reactions in the ionization source.