Proton-transfer-reaction mass spectrometry (PTR-MS) is seeing an increased use of NH4+ ions for the detection of amines and labile oxygenated organic compounds. NH4+ ions are usually generated from ammonia or ammonium-containing chemicals. We herein present a simple method for generating NH4+ reagent ions in the hollow cathode glow discharge ion source that nowadays most proton-transfer-reaction mass spectrometry (PTR-MS) instruments are equipped with. We show that NH4+ ions can be generated in high purity and yield by simply introducing a mixture of water vapor and nitrogen in the ion source. We also show that rapid switching (∼10 s loss of data) between the H3O+ and NH4+ operation modes is possible. Our new method can be easily implemented in most PTR-MS instruments that are currently in use, thus opening the possibility to easily operate the PTR-MS analyzer in the NH4+ chemical ionization mode.
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