Photoionization mass spectrometry generates gas-phase ions by absorption of photons and then separates those ions according to mass-to-charge ratio. When photon energy exceeds an analyte’s ionization energy, direct photoionization can form a molecular radical cation; wavelength or energy selection can therefore control ionization selectivity and fragmentation. In atmospheric-pressure photoionization (APPI), vacuum-ultraviolet photons ionize the analyte directly or first ionize a dopant, after which charge exchange or proton-transfer reactions generate analyte ions ^[1]. Source geometry, photon energy, dopant ionization energy, solvent composition, and gas-phase ion–molecule reactions determine the observed ion population ^[2]. The method differs from electron ionization, in which electrons initiate ion formation, and from chemical ionization lacking a primary photonic step. It also excludes optical spectroscopy that detects absorbed or emitted light without mass analysis. Essential features are a photon source, a region where neutral species or dopants absorb the radiation, ion extraction, and a mass analyzer that records the resulting ions ^[3].