X-Ray Fluorescence Measurements of Elements in High Matrix Solution Using Solid Substrate Having Nanoporus Structure

X-ray fluorescence (XRF) analysis can determine directly and non-destructively the elemental composition of various materials such as solid samples and water. For multi-elemental analysis of low volume (sub-milliliter levels) solution, XRF analysis combining with a droplet method is effective. In general, a dried droplet on a filter paper is analyzed in the droplet method. For measurements on high matrix solution, however, the sensitivity is reduced, resulting in large spread of values. In this study, to improve the sensitivity and repeatability of determination values of elements in high matrix solution by an XRF analysis with a droplet method, a silicon substrate having nanoporous structure was used instead of conventional filter paper. Two methods ((i) anodic etching using a hydrofluoric acid, (ii) metal-assisted chemical etching using gold and silver particles) were applied to silicon for introducing nanoporous structure on the surface of silicon. Different nanoporous structures were formed on the surface reflecting each etching condition. In the case of anodic etching, the silicon surface had spongy nanoporous structure. A dried residue containing 1 mg of the element was prepared on the etched silicon to investigate the effect of the nanoporous structure on the thickness and area of the dried residue. The size, shape and thickness of the dried residue were affected by the surface morphology of the silicon prepared by various etching conditions. Among them, the dried residue formed on anodic etched porous silicon was most circular and uniform. As compared with a flat silicon, the sensitivity and repeatability of energy dispersive XRF analysis by droplet method were improved by using a substrate having nanoporous structure. Especially, the intensity of Mg Kα increased by 11 times, and relative standard deviation ( n = 5) of X-ray intensity of Mg Kα decreased by 1/5.