CI ionizes an analyte through ion-molecule reactions between an ionized reagent gas and the analyte molecule, akin to an acid-base reaction. In the ionization chamber the partial pressure of the reagent gas is maintained higher than that of the analyte, ensuring frequent collisions. The reagent gas is first ionized by collisions with electron or alpha particles, creating charged species that then ionize the analyte. CI leverages differences in proton affinities, with the analyte typically having higher affinity in cation mode. Unlike in proton-transfer reaction MS (PTR-MS), which uses hydronium ions for ionization, CI offers customization of reagent ions based on the proton affinity of the analyte.( ) For example, nitrate CI-MS is used for acidic analytes, while water-CI-MS suits basic analytes. Various CI-MS methods, such as those using ethanol, acetate, or iodide, have enabled the detection of both cationic and anionic species in the atmosphere, including amines, acids, and unsaturated organics.(71; 72)