Solvent composition dependent signal reduction of molecular ions generated from aromatic compounds in (+) atmospheric pressure photoionization mass spectrometry

Rationale: The ionization process is essential for successful mass spectrometric (MS) analysis because of its influence on selectivity and sensitivity. In particular, certain solvents reduce the ionization of the analyte, thereby reducing the overall sensitivity in atmospheric pressure photoionization (APPI). Since the sensitivity varies greatly depending on the solvents, a fundamental understanding of the mechanism is required. Methods: Standard solutions were analyzed using a (+)-APPI Q Exactive ion trap mass spectrometer (Thermo Scientific). Each solution was infused directly into the APPI source at a flow rate of 100 μL/min and the APPI source temperature was 300°C. Other operating mass spectrometric parameters were maintained under the same conditions. Quantum mechanical calculations were carried out using the Gaussian 09 suite program. Results: Density functional theory was used to calculate the reaction enthalpies (∆H) of the reactions between toluene and other solvents. The experimental and theoretical results showed good agreement. The abundances of analyte ions were well correlated with the calculated ∆H values. Therefore, the results strongly support the suggested signal reduction mechanism. In addition, linear correlations between the abundance of toluene and analyte molecular ions were observed, which also supports the suggested mechanism. Conclusions: A solvent composition dependent signal reduction mechanism was suggested and evaluated for the (+)-APPI-MS analysis of polyaromatic hydrocarbons (PAHs) generating mainly molecular ions. Overall, the evidence provided in this work suggests that reactions between solvent cluster(s) and toluene molecular ions are responsible for the observed reductions in signal.