Imaging the 3D trace metal and metalloid distribution in mature wheat and rye grains via laser ablation-ICP-mass spectrometry and micro-X-ray fluorescence spectrometry

Toxic trace metals and metalloids in human nutrient sources pose a severe health risk, and the processes governing metal accumulation should hence be well understood. In this work, the spatial distribution of toxic trace metals/metalloids and micronutrients (Cr, Mn, Ni, Cu, Zn, As, Cd, Hg and Pb) in mature wheat (Triticum aestivum L.) and rye (Secale cereale L.) grains at typical exposure levels was visualized and quantified via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) by serial sectioning. The bulk concentrations of these elements were also determined by pneumatic nebulisation-ICP-MS. Furthermore, longitudinal sections were scanned using m-X-ray fluorescence spectrometry to confirm the major element distribution. Serial sectioning in this study was realized via a polishing strategy. Although the methodology is time-consuming and laborious, it enables to access 3D information for samples which cannot be sectioned using a microtome on a depth scale that would otherwise be inaccessible by a laser probe. In the elemental images, strong local enrichment patterns for Mn and Zn are apparent in the aleurone layer/seed coat, vascular tissue of the crease, and embryonic tissue, whereas Cr, As, Cd and Pb have been mainly accumulated in the grain endosperm as a result of different transport and storage dynamics.