A novel method for producing NH4+ reagent ions in the hollow cathode glow discharge ion source of PTR-MS instruments

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.