Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MSIN K8-88, Richland, Washington 99352, USA.
Anal Chem. 2010 Mar 1;82(5):2048-58. doi: 10.1021/ac902801f.
Characterization of the chemical composition and chemical transformations of secondary organic aerosol (SOA) is both a major challenge and the area of greatest uncertainty in current aerosol research. This study presents the first application of desorption electrospray ionization combined with high-resolution mass spectrometry (DESI-MS) for detailed chemical characterization and studies of chemical aging of organic aerosol (OA) samples collected on Teflon substrates. DESI-MS offers unique advantages both for detailed characterization of chemically labile components in OA that cannot be detected using traditional electrospray ionization mass spectrometry (ESI-MS) and for studying chemical aging of OA. DESI-MS enables rapid characterization of OA samples collected on substrates by eliminating the sample preparation stage. In addition, it enables detection and structural characterization of chemically labile molecules in OA samples by minimizing the residence time of analyte in the solvent. In this study, DESI-MS and tandem mass spectrometry experiments (MS/MS) were used to examine chemical aging of SOA produced by the ozonolysis of limonene (LSOA) in the presence of gaseous ammonia. Exposure of LSOA to ammonia resulted in measurable changes in the optical properties of the sample observed using ultraviolet (UV)-visible spectroscopy. High-resolution DESI-MS analysis demonstrated that chemical aging results in formation of highly conjugated nitrogen-containing species that are most likely responsible for light-absorbing properties of the aged LSOA. Detailed analysis of the experimental data allowed us to identify several key aging reactions, including the transformation of carbonyls to imines, intramolecular dimerization of imines with other carbonyl compounds in SOA, and intermolecular cyclization of imines. This study presents an important step toward understanding the formation of light-absorbing OA (brown carbon) in the atmosphere.
对二次有机气溶胶 (SOA) 的化学成分和化学转化进行特征描述，既是当前气溶胶研究的主要挑战，也是最大的不确定性领域。本研究首次应用解吸电喷雾电离结合高分辨质谱 (DESI-MS) 对聚四氟乙烯 (Teflon) 基底上采集的有机气溶胶 (OA) 样品进行详细的化学特征描述和化学老化研究。DESI-MS 为 OA 中化学不稳定成分的详细特征描述提供了独特的优势，这些成分使用传统的电喷雾电离质谱 (ESI-MS) 无法检测到，同时也为 OA 的化学老化研究提供了独特的优势。DESI-MS 通过消除样品制备阶段，使基底上采集的 OA 样品能够快速得到特征描述。此外，通过将分析物在溶剂中的停留时间最小化，它还可以检测和结构描述 OA 样品中的化学不稳定分子。在本研究中，使用 DESI-MS 和串联质谱实验 (MS/MS) 来研究β-蒎烯臭氧氧化 (LSOA) 过程中存在气态氨时 SOA 的化学老化。LSOA 暴露于氨会导致使用紫外-可见光谱观察到的样品的光学性质发生可测量的变化。高分辨 DESI-MS 分析表明，化学老化导致形成高度共轭的含氮物种，这些物种很可能是老化 LSOA 光吸收特性的原因。对实验数据的详细分析使我们能够识别出几个关键的老化反应，包括羰基向亚胺的转化、SOA 中亚胺与其他羰基化合物的分子内二聚化以及亚胺的分子间环化。本研究为理解大气中光吸收性 OA（棕色碳）的形成迈出了重要的一步。
