Development of a Nanoscaled Ion Source for High-Sensitivity Photoionization Mass Spectrometry

Atmospheric Pressure Photoionization (APPI) has emerged as a versatile ionization method in mass spectrometry, able to ionize compounds of comparably low ionization potential (typically <10.6 eV), irrespective of their functionalities. Aromatic analytes are particularly well suited for photoionization because their delocalized π-electron systems result in comparatively low ionization potentials, enabling both direct single-photon ionization and ionization by charge transfer from dopant ions. While highly effective for a broad range of analytes, APPI faces issues of high sample consumption and elevated background noise, which can limit its effectiveness in trace analysis. Prior advances in miniaturizing flow rates, as seen with the transition from conventional Electrospray to nano-Electrospray, have shown that reducing flow rates not only conserves sample but can also enhances sensitivity by increasing the signal-to-noise ratio. Applying similar miniaturization strategies to APPI holds promise for overcoming current limitations and improving its analytical performance. This study investigates the impact of lowered analyte concentrations and reduced flow rates on the sensitivity of APPI for ultratrace analysis. We introduce a prototype ion source for APPI applications and systematically explore the effects of flow rates below 1 μL min⁻¹ on APPI performance, evaluating the signal-to-noise ratios and detection limits achieved. Our findings indicate that reduced flow rates significantly improve sensitivity, demonstrating the potential to detect ultratrace levels of environmental pollutants with higher efficiency and lower background interference.