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

摘要

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 μ-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.