Department of Chemistry, University of California-Irvine , Irvine, California 92697-2025, United States.
Department of Chemistry, University of Redlands , Redlands, California 92373, United States.
Anal Chem. 2018 Feb 6;90(3):2055-2062. doi: 10.1021/acs.analchem.7b04164. Epub 2018 Jan 11.
Real-time in situ mass spectrometry analysis of airborne particles is important in several applications, including exposure studies in ambient air, industrial settings, and assessing impacts on visibility and climate. However, obtaining molecular and 3D structural information is more challenging, especially for heterogeneous solid or semisolid particles. We report a study of extractive electrospray ionization mass spectrometry (EESI-MS) for the analysis of solid particles with an organic coating. The goal is to elucidate how much of the overall particle content is sampled, and determine the sensitivity of this technique to the surface layers. It is shown that, for NaNO particles coated with glutaric acid (GA), very little of the solid NaNO core is sampled compared to the GA coating, whereas for GA particles coated with malonic acid (MA), significant signals from both the MA coating and the GA core are observed. However, conventional ESI-MS of the same samples collected on a Teflon filter (and then extracted) detects much more core material compared to EESI-MS in both cases. These results show that, for the experimental conditions used here, EESI-MS does not sample the entire particle but, instead, is more sensitive to surface layers. Separate experiments on single-component particles of NaNO, GA, or citric acid show that there must be a kinetics limitation to dissolution that is important in determining EESI-MS sensitivity. We propose a new mechanism of EESI solvent droplet interaction with solid particles that is consistent with the experimental observations. In conjunction with previous EESI-MS studies of organic particles, these results suggest that EESI does not necessarily sample the entire particle when solid, and that not only solubility but also surface energies and the kinetics of dissolution play an important role.
实时原位质谱分析空气中的颗粒在多个应用中非常重要，包括环境空气中的暴露研究、工业环境以及评估对能见度和气候的影响。然而，获取分子和 3D 结构信息更具挑战性，特别是对于异质的固体或半固体颗粒。我们报告了一种用于分析具有有机涂层的固体颗粒的萃取电喷雾电离质谱（EESI-MS）的研究。目的是阐明总体颗粒含量中有多少被采样，并确定该技术对表面层的灵敏度。结果表明，与 GA 涂层相比，涂有戊二酸（GA）的 NaNO 颗粒中只有很少一部分固体 NaNO 核被采样，而涂有丙二酸（MA）的 GA 颗粒中则观察到 MA 涂层和 GA 核的显著信号。然而，在同样的实验条件下，对于在特氟龙滤纸上收集的相同样品进行的常规 ESI-MS（然后进行提取）检测到的核心物质比两种 EESI-MS 方法都要多。这些结果表明，对于这里使用的实验条件，EESI-MS 不会采样整个颗粒，而是对表面层更敏感。对 NaNO、GA 或柠檬酸的单一组分颗粒的单独实验表明，在确定 EESI-MS 灵敏度方面，溶解必须存在动力学限制。我们提出了一种新的 EESI 溶剂液滴与固体颗粒相互作用的机制，与实验观察结果一致。结合以前对有机颗粒的 EESI-MS 研究，这些结果表明，当颗粒为固体时，EESI 不一定会采样整个颗粒，不仅是溶解度，还有表面能和溶解动力学都起着重要作用。
