Resolving agrochemical penetration in wheat leaves with secondary ion mass spectrometry imaging and depth profiling

The foliar application of an agrochemical is first encountered by the hydrophobic cuticle and then the hydrophilic epidermis of the leaf. Understanding the movement and distribution of pesticides through these layers is vital in the research and development process of agrochemicals. Currently, no technique provides conclusive answers on active ingredient penetration through the leaf surface layers without chemical or physical treatment of the leaves. The advancements in modern time-of-flight secondary ion mass spectrometry (ToF-SIMS) technologies provide the necessary depth, spatial, and mass resolution to probe this complex area of interest with precision and relatively simple sample preparation. In this work, we describe a systematic ToF-SIMS workflow to comprehensively understand the foliar movement of an applied agrochemical through the surface waxes and in the bulk of the leaf. A combination of depth profiling experiments along with cross-section imaging of agrochemical-applied wheat leaves was used to better understand the mobility of a fungicide, azoxystrobin, in young wheat leaves. Extensive testing was done to optimise the analytical conditions and sample preparation strategies best suited for the experiments described in this work. A gas cluster ion beam composed of water clusters was selected for analysis. Freeze-dried leaves were used for depth profile experiments, while tape-sectioned leaf cross-sections were used for imaging. Parallel leaf sections to those taken for ToF-SIMS were also imaged with matrix-assisted laser desorption ionisation mass spectrometry imaging (MALDI-MSI). When applied, azoxystrobin moved rapidly across the leaf cross-section, being observed in the leaves as early as 24 h after application and persisting for 1 week. When comparing the intensity in the mesophyll region to that of the vascular bundle, more azoxystrobin content was observed in the mesophyll at both time points, with greater disparity observed at 1 week. The results obtained were corroborated with the MALDI study done on similar samples and with pre-existing literature.