Development and Application of Experimental Methodology for Characterizing the Chemical Composition and Reactivity of Air-Water Interfaces

在化学系化学测量和成像计划的支持下，加州理工学院的比彻姆教授正在开发和应用新的实验方法，以灵敏地检测和表征优先占据气体和液体之间离散界面的各种分子物种。尽管水约占地球表面的70%，但人们对空气-水界面上发生的复杂反应知之甚少。这一范围只有几个分子的边界代表了自然界中最常见的化学环境之一，存在于海洋和湖泊表面、大气气溶胶，甚至人类呼吸系统中。比彻姆研究小组开发的工具，包括场诱导液滴电离和声波液滴喷射，与质谱学相结合，提供了对复杂界面过程的前所未有的深入了解，从大气中人为有机物的氧化形成二次有机气溶胶，到吸入臭氧时肺表面活性物质层中关键蛋白质的氧化。这些调查的进行为新一代科学家提供了培训，为他们提供了新发现和解决科学问题的工具。学生们参与到各个层面，向学校小组做报告，在实验室接待高中生实习生，培训研究生和博士后研究员。场诱导液滴电离质谱仪(FIDI-MS)技术，即应用脉冲电场拉伸微升液滴形成相反的正负电喷雾，用于许多这些研究。FIDI-MS已被证明能够对在空气-水界面发生化学反应的复杂分子物种进行选择性检测和时间监测。为了补充对液滴的研究，在比彻姆教授的实验室中演示的第二种方法使用聚焦的声脉冲将皮升液滴从表面射出，以进行随后的质谱分析，以检查平面空气-水界面的组成。后一个实验是在朗缪尔槽中进行的，在那里，表面压力的变化被认为是影响界面层结构和反应性的重要参数。研究人员利用这些技术对两亲性表面活性分子的化学反应活性进行了基础研究，从嵌入脂层的螺旋肽到大气气溶胶中存在的有机物，气相物种包括臭氧、羟基自由基、氮和硫的氧化物以及酸和碱。理论研究包括开发详细的定量模型来分析实验动力学数据，结合最先进的分子动力学和量子化学计算，为这些研究提供了进一步的见解。这项工作的广泛影响包括培训新一代研究科学家，他们将为我们理解从环境科学到人类健康的各种问题作出贡献。

With support from the Chemical Measurement and Imaging Program in the Division of Chemistry, Professor Beauchamp at the California Institute of Technology is developing and applying new experimental methodologies for the sensitive detection and characterization of a wide range of molecular species that preferentially occupy the discrete interface between gases and liquids. Although water occupies about 70% of the earth's surface, relatively little is known about the complex reactions that take place at the air-water interface. This boundary, only a few molecules in extent, represents one of the most common chemical environments in nature, present on the surface of oceans and lakes, on atmospheric aerosols, and even in the human respiratory system. Tools developed by the Beauchamp research group, including field induced droplet ionization and acoustic droplet ejection, are coupled with mass spectrometry to provide unprecedented insights into complex interfacial processes ranging from the oxidation of anthropogenic organics to form secondary organic aerosols in the atmosphere to the oxidation of key proteins in the pulmonary surfactant layer when ozone is inhaled. The conduct of these investigations provide training for a new generation of scientists, providing them with tools for new discoveries and the solution of scientific problems. Students are engaged at every level, with presentations to school groups, hosting high school student interns in the laboratory, and training graduate students and postdoctoral fellows.The technique of field-induced droplet ionization mass spectrometry (FIDI-MS), in which a pulsed electric field is applied to stretch microliter droplets to form opposing dual positive and negative electrosprays, is employed for many of these investigations. FIDI-MS has proven highly capable for selective detection and temporal monitoring of complex molecular species undergoing chemical reactions at the air-water interface. To complement studies of droplets, a second approach, demonstrated in Professor Beauchamp's laboratory, examines the composition of planar air-water interfaces using a focused acoustic pulse to eject picoliter droplets from the surface for subsequent mass spectrometric analysis. The latter experiment is performed in a Langmuir trough, where variation in surface pressure is expected to be an important parameter affecting both the structure and reactivity of interfacial layers. The investigator employs these techniques to conduct fundamental studies of the chemical reactivity of amphiphilic surface active molecules, ranging from helical peptides embedded in lipid layers to organics present in atmospheric aerosols, with gas phase species including ozone, hydroxyl radicals, oxides of nitrogen and sulfur, and acids and bases. Theoretical studies involving the development of detailed quantitative models for analyzing experimental kinetic data combined with state of the art molecular dynamics and quantum chemical calculations provides further insights in these investigations. Broad impacts of this work include the training of a new generation of research scientists who will be making contributions to our understanding of problems ranging from environmental science to human health.