关键词: 一氯胺, 饮用水, 喷雾进样, 质子转移反应质谱仪, 煮沸

Abstract: Objective　This research was undertaken to address the urgent need for real-time, accurate monitoring of disinfection byproducts in municipal water systems. The primary objective was to develop and validate a novel analytical methodology based on spray inlet-proton transfer reaction-mass spectrometry (SI-PTR-MS) for the rapid detection and quantification of monochloramine in tap water. Although monochloramine is an effective secondary disinfectant, it may pose potential health risks at elevated concentrations, and at the same time, conventional detection methods lack the speed and online capability necessary for dynamic monitoring. The secondary objectives included investigating the spatial distribution pattern of monochloramine within a local water distribution network to assess the impact of pipeline transport, and scientifically evaluating the efficacy of boiling, a ubiquitous household practice, in removing this disinfectant and co-occurring volatile organic compounds (VOCs). This study aimed to promote technological advancement in water quality analysis and provide practical insights for public health protection. Methods　The methodological framework was centered around a self-constructed, integrated analytical platform. The core instrument was a laboratory-developed SI-PTR-MS system, consisting of a pneumatic spray inlet module for efficient sample introduction and a high-sensitivity proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) for detection. The spray inlet adopted a controlled nebulization process to rapidly transfer aqueous monochloramine into the gas phase, while simultaneously delivering the analyte to the reaction chamber with a constant airflow of 50 mL/min. To conduct unambiguous identification and to overcome potential isobaric interferences inherent in direct mass spectrometry, a hyphenated gas chromatography-PTR-MS (GC-PTR-MS) system was employed for confirmatory analysis based on chromatographic retention times. Tap water samples were systematically collected from multiple points within the Science Island area of Hefei, China. The sampling points included three primary buildings: Anhui Institute of Optics and Fine Mechanics (Building 1), Hefei Cancer Hospital of the Chinese Academy of Sciences (Building C), and the Sanhe Graduate Apartments. To examine micro-scale variations, sub-samples were taken from three different rooms (C305, C306, C307) within the same floor of Building C of Hefei Institutes of Physical Science, Chinese Academy of Sciences. A controlled boiling experiment was further conducted on the sample from room C307 to quantify removal efficiencies of monochloramine. Key operational parameters were optimized: the GC was operated isothermally at 70 °C; the PTR-MS scanned a mass range of m/z 20–150 with the drift tube pressure stabilized at 140 Pa. According to the established protocols, authentic monochloramine standards were synthesized in-house for calibration. Results and Discussion　The SI-PTR-MS system successfully detected monochloramine as a distinct peak at m/z 52, corresponding to the protonated species NH2 35ClH⁺. Confirmatory analysis via GC-PTR-MS demonstrated a precise match in retention time between the standard and the analyte in tap water, thus providing definitive qualitative verification. The method exhibited a favorable signal-to-noise ratio of 21.94 for real samples, indicating its robust performance in a complex matrix. Quantitative analysis revealed that the monochloramine concentrations across the three main buildings were 0.01 mg/ L, 0.007 mg/L, and 0.009 mg/L, respectively. All values were substantially below the World Health Organization (WHO) - recommended and Chinese national safety guidelines of 3 mg/L, indicating that the tap water at the sampling point generally meets safety standards. A key finding was the significant spatial heterogeneity in monochloramine levels. While the interbuilding differences were modest, the intra-building variation was pronounced. Within Building C of Hefei Cancer Hospital, the monochloramine concentrations varied by a factor of over four, ranging from 0.0016 mg/L in room C306 to 0.007 mg/L in room C307. This variability is primarily attributed to the hydraulic conditions and physicochemical dynamics within the building's plumbing network, such as water age, interactions with pipe material, and temperature gradients, which affect the natural decay of monochloramine. This underscores that the water quality at the tap is not uniform and is highly influenced by the "last mile" of water distribution. The study also detected a series of other VOCs, including acetaldehyde (m/z 45) and a cluster of ions tentatively identified as dichloromethylamine and its fragments (m/z 64, 65, 100, 102). Their co-variation with monochloramine in some locations suggests that they may originate from common sources or have related formation pathways. The boiling experiment yielded clear and significant results that there was an 84% reduction in monochloramine concentration, and the associated VOCs were also effectively removed, with efficiencies ranging from 67% to 93%. This thermal degradation of monochloramine is attributed to the accelerated decomposition at elevated temperatures, following known reaction pathways. This result provides strong, empirical support for the traditional practice of boiling water, which is regarded as a highly effective point-of-use treatment strategy for reducing exposure to these chemical species. Conclusions　This study successfully developed and demonstrated the application of a novel SI-PTR-MS method for rapid, sensitive, and online detection of monochloramine in tap water. The combination with GC-PTR-MS provided a reliable confirmatory mechanism, establishing a robust analytical workflow. The research conclusively documented significant spatial variability of monochloramine within a water distribution system, and highlighted that infrastructure and local conditions are critical determinants of final water quality at consumers' tap, even with uniform treatment of source water. In addition, the study scientifically validated that boiling is a simple, accessible, and highly effective household method for significantly reducing monochloramine and a range of other volatile organic contaminants, thereby directly enhancing the safety of drinking water. These findings have important implications for public health policies, suggesting the need to pay more attention to monitoring at the point of consumption and to public education for promoting boiling as a reliable risk mitigation measure. And these findings also show that the SI-PTR-MS technology itself presents a valuable tool for water utilities and researchers to conduct high-frequency, real-time surveillance of the dynamic changes of disinfection by-products.

Key words: monochloramine, drinking water, spray inlet, proton transfer reaction mass spectrometer, boiling