Proton transfer reaction mass spectrometry (PTR-MS) and selected ion flow tube mass spectrometry (SIFT-MS) are well-established, field-deployable chemical ionization techniques known for their portability.
These methods enable the rapid detection of numerous gases with high temporal resolution, capturing multiple measurements per second at extremely low concentrations—down to pmol mol⁻¹, equivalent to a single drop of water in an Olympic-sized swimming pool.
Both techniques employ various reagent ions to ionize target compounds, enhancing their detectability. One of the most commonly used reagent ions is the hydronium ion (H₃O⁺), which ionizes compounds with proton affinities higher than that of water. This selectivity prevents interference from major atmospheric components such as nitrogen, oxygen, carbon dioxide, and argon, eliminating the need for sample pretreatment.
PTR-MS and SIFT-MS rely on H₃O⁺ to detect compounds as protonated molecular ions, with minimal fragmentation due to the low excess energy of the proton transfer reaction. This allows for the identification of complex mixtures without requiring traditional chromatographic separation techniques.
These approaches are particularly valuable in time-sensitive applications, offering real-time measurements. Since their introduction in the mid-1990s, PTR-MS and SIFT-MS have been widely utilized across various fields, including air quality monitoring, breath analysis, biological research, drug detection, environmental studies, food and flavor science, and security screening.
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