Grazing exit micro X-ray fluorescence analysis of a hazardous metal attached to a plant leaf surface using an X-ray...

If human beings or animals repeatedly ingest plant leaves contaminated with minute quantities of hazardous metals (Pb, As, Hg, Cd, etc.), the metals will gradually accumulate in their bodies. When the quantities of the metals in the bodies reach toxic levels, they may cause serious symptoms of poisoning. Therefore, it is significant to detect and analyze the minute quantities of hazardous metals that attach to plant leaves in terms of epidemiology and disease prevention. We developed grazing exit micro X-ray fluorescence analysis (GE-micro-XRF), which was expected to analyze the localized surface of an aqueous plant leaf with a much faster and simpler sample treatment than with conventional analytical methods, to detect Pb attached to a surface of a leaf of Camellia hiemalis. A micro X-ray beam was produced by using a polycapillary X-ray lens. GE-v-XRF is a grazing exit X-ray analysis (GE-XA) method in which X-rays emitted from only the near-surface region of a specimen are selectively detected under a grazing exit angle condition (extremely low exit angle near 0 degrees). In any GE-XA method, X-rays emitted from inside the specimen must be absorbed inside the specimen and attenuated when X-rays pass through the specimen. However, we deduced that X-rays emitted from inside aqueous organic material such as a plant leaf are scarcely absorbed because X-ray absorption in any aqueous organic material is much smaller than that in most metallic and semiconductor materials, which was analyzed with GE-XA methods. Therefore, we have developed a novel GE-micro-XRF method in which a chip of a silicon wafer is placed between the analyzed leaf and an X-ray detector as an absorber of the X-rays emitted from inside the leaf. As a result of GE-XRF analysis of a leaf dipped in Pb standard solution using the X-ray absorber, we have for the first time selectively detected X-rays emitted from the near-surface region of an aqueous plant leaf. Therefore, we have detected X-rays emitted from Pb with much higher peak/background ratios (P/B ratios) as compared to those of conventional XRF analysis. In the analysis, we also found a difference in element distributions between the leaf surface and its interior. Therefore, we observed and analyzed a cross section of the leaf with a SEM-EDX to confirm the validity of this result. The result of the analysis of the cross section has been in excellent agreement with that of the XRF analysis.