Imaging of Dicamba Using Laser Ablation Electrospray Ionization Mass Spectrometry in Cabbage Leaves

Abstract

The distribution of herbicide active ingredients in plant tissue, after deposition, is valuable information when assessing the influence of formulation on efficacy. Typical techniques for assessing herbicide movement require time-consuming methods that involve collecting and isolating cuticles for Franz cells or the application, homogenization, and extraction of plant tissue. Franz cells speak only to cuticular penetration, while homogenization loses spatial resolution of analyte distributions. A key advancement in recent years is the use of mass spectrometry for imaging samples, which provides a visualization of the spatial distribution of analytes within samples. Here, laser ablation electrospray ionization mass spectrometry imaging was used to image the distribution of herbicides within plant material after application from droplets. In this technique, an herbicide is applied to plant tissue, such as a whole leaf, and allowed to spread and penetrate the surface. The exposed area is excised and placed into the ionization device, without the need for sample preparation such as cross-sectioning or homogenization and extraction. A laser is used to ablate material from specific locations, and ions are generated through entrainment in an electrospray plume and directed into the inlet of a mass spectrometer. Spectra are collected, and mass spectrometry images are extracted using characteristic ions of a target. After the application of three dicamba solutions over 2 and 24 h, it was determined that no significant penetration of dicamba occurred for any of the three solutions over 2 h. Over 24 h, two treatments containing 0.5% of polysorbate variants produced detectable penetration of dicamba, while the treatment containing only dicamba produced no measureable penetration of dicamba.