Familiarize Students with Direct MS Analysis Methods: Localization of Components in Citrus Peel by Induced Electrospray Ionization

Abstract

A laboratory experiment was designed for senior undergraduate/graduate students to introduce direct current-induced electrospray ionization (DC-iESI), a technique which allows in situ mass spectrometric (MS) analysis of components in microregions of citrus peels. The citrus peels exhibit distinct microregions, such as oil glands and the flavedo, each containing unique components. Students were guided to compare traditional ESI (flow injection) and DC-iESI for the analysis of components in oil glands and the flavedo. As the flow injection ESI experiment has a requirement for a minimum amount of sample, students had to extract components from bulk citrus peels for MS analysis, which failed to show any differences in the components in these two typical regions. In the DC-iESI experiment, students used nanoelectrospray emitters to freehand collect liquids directly from oil glands and the flavedo, allowing MS analysis of components in these two different microregions. By comparison of these two methods, students have the opportunity to realize the limitations of traditional ESI analysis and learn about using DC-iESI for in situ MS analysis. We believe that this training course can help students enhance their understanding of in situ analysis and broaden their appreciation of MS in modern applications.