A new method for the determination of imidazole-like brown carbons using gas chromatography-mass spectrometry

Imidazole-like compounds were important components of brown carbon, which played a significant role in radiative forcing in the atmosphere. In this study, a new detection method of imidazole-like compounds in the atmosphere was developed using gas chromatography-mass spectrometry (GC-MS) due to its high resolution and sensitivity. Seven imidazole-like compounds, including imidazole (IM), 4 (5)-methylimidazole (4MEI), 2-ethylimidazole (2EI), 2,4-dimethylimidazole (2,4-DMI), 2-methylimidazole-4-carbaldehyde (2MI4C), 2,2-biimidazole (BI) and 2-phenylimidazole (2PI) were performed using the GC-MS method. Acetonitrile, pyridine, anhydrous ethanol and isobutyl chloroformate were used to conduct the derivatization of imidazole-like compounds before GC-MS analysis. Optimal amounts of 90 μL acetonitrile, 60 μL pyridine, 200 μL anhydrous ethanol and 70 μL isobutyl chloroformate ensured the derivatization of the 7 imidazole-like compounds. The pH value influenced imidazole-like compounds extraction from the aqueous phase. The result showed that the pH value for the extraction of 7 imidazole-like compounds was adjusted to be 8.0. Limits of detection and quantification for the 7 imidazole-like compounds were 0.0553–0.8914 and 0.2370–1.9373 μg/mL, respectively, using this method. The spiked recoveries for the 7 imidazole-like compounds ranged from 58.84% to 160.99%. The method was further applied to measure imidazole-like compounds from aqueous reactions in the experiment study and atmospheric aerosols in the field study. IM, BI, 4MEI and 2,4-DMI were observed in the aqueous reaction of carbonyls with reduced nitrogen. Furthermore, for the first time in the atmospheric particles quantitative IM and 4MEI, China's northern coast. The developed method would help us better understand the source and evolution of imidazole-like brown carbon aerosols in the atmosphere.