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In this study, we describe the characterization and field deployment of a Chemical Ionization Mass Spectrometry (CIMS) using a recently developed focusing ion-molecule reactor (FIMR) and ammonium-water cluster (NH₄⁺·H₂O) as the reagent ion (denoted as NH₄⁺ CIMS). We show that NH₄⁺·H₂O is a highly versatile reagent ion for measurements of a wide range of oxygenated organic compounds. The major product ion is the cluster with NH₄⁺ produced via ligand-switching reactions. Other product ions (e.g., protonated ion, cluster ion with NH₄⁺·H₂O, with H₃O⁺, and with H₃O⁺·H₂O) are also produced, but with minor fractions for most of the oxygenated compounds studied here. The instrument sensitivities (counts per second per ppbv, cps ppbv^-1) and product distributions are strongly dependent on the instrument operating conditions, including the ratio of ammonia (NH₃) and H₂O flows and the drift voltages, which should be carefully selected to ensure NH₄⁺·H₂O as the predominant reagent ion and to optimize sensitivities. For monofunctional analytes, the NH₄⁺·H₂O chemistry exhibits high sensitivity (i.e., > 1000 cps ppbv^-1) towards ketones, moderate sensitivity (i.e., between 100 and 1000 cps ppbv^-1) towards aldehdyes, alcohols, organic acids, and monoterpenes, low sensitivity (i.e., between 10 and 100 cps ppbv^-1) towards isoprene and C1 and C2 organics, and negligible sensitivity (i.e., < 10 cps ppbv^-1) towards reduced aromatics. The instrumental sensitivities of analytes depend on the binding energy of the analyte-NH₄⁺ cluster, which can be estimated using voltage scanning. This offers the possibility to constrain the sensitivity of …