Optimizing Vocus proton-transfer-reaction mass spectrometry for detecting trace atmospheric amines

Abstract

Amines, as important alkaline gases in the atmosphere besides ammonia, profoundly influence air quality, climate and human health through various atmospheric processes. Besides well-studied alkylamines, emerging amines (e.g., monoethanolamine (MEA), piperazine (PZ), 2-amino-2-methyl-1-propanol (AMP)) from the rapidly expanding amine-based carbon capture facilities may be emitted into the atmosphere, posing new demands for their measurement. Vocus Proton-Transfer-Reaction Mass Spectrometry (Vocus-PTR) is an effective technique for detecting volatile organic compounds (VOCs), but its capability to measure a broader variety of trace-level amines remains to be explored. In this study, we optimized the focusing ion-molecule reactor condition of Vocus-PTR to improve the chemical ionization efficiency and ion transmission efficiency of amine detection. The optimized Vocus-PTR achieved good performance for detecting both atmospheric amines and VOCs, with limits of detection (LODs) for amines (0.16~0.55 pptv) lower than those for VOCs (9.34~38.79 pptv). The observed shift in the reduced electric field strength (from ~147 to 107~117 Td) indicates an increased abundance of water-cluster reagent ions in the reactor, enhancing sensitivity and selectivity for amine detection. The optimized instrument was deployed in urban Beijing. C2-6 alkylamines, C1-6 amides and several emerging amines were identified and quantified, showing characteristic diurnal variations.