Rapid Switching of Reagent Ions in Dopant-Assisted Positive Photoionization Ion Mobility Spectrometry for Highly Selective and Sensitive Ammonia Detection in Atmospheric Environments

Ammonia (NH₃), a critical precursor in the atmospheric formation of fine particulate matter (particularly PM_2.5), poses significant environmental challenges by contributing to eutrophication and acidification as its concentration increases. Consequently, the real-time and accurate monitoring of atmospheric NH₃ is of great environmental and regulatory importance. In this study, an innovative technique for the rapid switching of reagent ions between (C₄H₈O)₂H⁺ and (H₂O)_nH⁺ in dopant-assisted positive photoionization ion mobility spectrometry (DAPP-IMS) within 2 s by simply controlling the on-off of water dopant is proposed. This facilitates NH₃ detection under two distinct modes: high selectivity and high sensitivity. In the high-selectivity mode, 2-butanone is employed as the dopant, generating (C₄H₈O)₂H⁺ as the predominant reagent ion. Under these conditions, the method achieves a limit of detection (LOD) of 5 ppb, enabling selective quantification of NH₃ in complex matrices. To achieve high-sensitivity detection, we introduce a synergistic 2-butanone/water dopant system. This dual strategy enables the efficient conversion of (C₄H₈O)₂H⁺ to (H₂O)_nH⁺ at ambient temperature. The resulting reagent ion, possessing a lower proton affinity, emerged as the predominant species. As a result, the NH₃ LOD is significantly lowered to 0.47 ppb, an improvement in sensitivity by approximately an order of magnitude. It exhibits superior stability, robust resistance to humidity interference, and enhanced peak-to-peak resolution, with a remarkably low relative standard deviation (RSD) of just 1.24%. The DAPP-IMS method thus offers robust, highly selective, and sensitive long-term online monitoring of NH₃ concentrations in diverse environments, including vehicle emissions and humid outdoor atmospheres. The rapid reagent ion switching mechanism not only suppresses interference from complex matrices but also significantly enhances detection stability and sensitivity.