Improved Micro-X-ray Fluorescence Confocal Imaging of Two-Dimensional Distribution of Arsenic Concentration in Cucumber Hypocotyls Using Synchrotron Radiation.

Imre Szalóki,Laszlo Vincze,Ferenc Fodor,Anita Gerényi,Viktória Czech,Gábor Radócz

doi:10.1021/acs.analchem.1c00579

Abstract

An optimized micro-X-ray fluorescence confocal imaging (μXRF-CI) analytical method has been developed to determine the 2D distribution of elemental composition in small (1–3 mm) biological objects at a 10–20 μm spatial resolution. Plants take up chemical elements from soil, and the vascular system transports them toward shoots. In order to obtain biochemical information related to this biological process, 2D distributions of chemical elements in roots and in hypocotyls of cucumber plants were analyzed by synchrotron radiation based on micro-X-ray fluorescence computer tomography and μXRF-CI techniques. The experiments were carried out at HASYLAB Beamline L of the DORIS-III storage ring in Hamburg, a facility that provided optimal physical conditions for developing and performing these unique analyses: high flux monochromatic synchrotron beam, X-ray optical elements, precision moving stages, and silicon drift detectors. New methodological improvements and experimental studies were carried out for applicability of lyophilized samples and cryo-cooling. Experimental parameters were optimized to maximize the excitation yield of arsenic Kα radiation and improvement of the spatial resolution of the μXRF-CI analytical method.

Highlights

In several agricultural areas of the world, the arsenic concentration in groundwater is above the recommended health limit for drinking water that is 10 μg/L in the European Union.[1]
Plants are capable of uptaking chemical elements, accumulating them in tissues, and transporting them in the xylem sap from roots to the leaves
To obtain information about the ion-transport mechanisms in cucumber plants, optimized μXRF-CT and μXRF-CI microanalytical techniques were developed to determine a quantitative map of the chemical elements in the hypocotyl part of cucumber plants

Summary

■ INTRODUCTION

In several agricultural areas of the world, the arsenic concentration in groundwater is above the recommended health limit for drinking water that is 10 μg/L in the European Union.[1]. The calculation of the intensity of arsenic fluorescence radiation (As-Kα) can be approximated by the FPM model of XRF analysis.[32] FPM models consider the fundamental atomic parameters and energy distribution of flux density of the exciting SR beam I(E), which in these experiments was quasimonochromatic at energy E, produced by a Ni/C double multilayer monochromator system that consisted of 100 layers.[33] The FWHMp(E) functions of primary (p) polycapillary half-lenses were determined experimentally by Falkenberg et al.[34] They found that FWHMp(E)/E varied between 1.65 and 2.19% in an energy interval of 8−21 keV and the absolute values of FWHMp(E) were in a range of 150−460 eV. In order to draw relevant biological conclusions, a higher spatial resolution of the quantitative distribution of chemical elements is required Achieving this goal, optimization of measuring conditions was performed to improve the geometrical resolution of element maps: (i) tuning the excitation energy of the SR beam, (ii) setting the sampling frequency, (iii) setting the acquisition time, and (iv) varying the size of voxels. The result of the calculated line-scan is plotted at different step sizes as 1, 5, 10, 20, and 50 μm, demonstrating that the similarity between the calculated and original distribution of arsenic quantity increasingly becomes better with refinement of the step size

■ RESULTS

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES