X-RAY-FLUORESCENCE ANALYSIS OF TREE RINGS

X-ray fluorescence analysis was applied to the determination of the concentration of trace elements in the growth rings of trees. Some of these mineral elements can affect the growth and development of the plant. The results of the analytical measurements can provide information concerning the geochemical and atmospheric environment to which the tree has been exposed. It has been demonstrated that conventional x-ray fluorescence equipment is adequate to perform such measurements but, more important, the high intensity available from synchrotron radiation is an ideal source to probe the spatial variation in the low-level concentrations which can be transposed to an absolute temporal scale knowing the history of the tree. The ability to aperture or focus the synchrotron radiation to submillimeter size allows several points within individual rings to be analyzed, permitting a study of differences within a single growing season. Concentrations of mineral elements were determined in two types of tree samples, a cross-section of the trunk of a small sassafras tree and an incremental core from a red oak. Two experimental arrangements were employed, a special energy-dispersive x-ray analyzer with a primary beam size of 0.5 mm and a beam line at the National Synchrotron Light Source where the radiation was focused and apertured to 50-mu-m x 1 mm. Concentrations as low as 1 ppm were determined for nine elements. Major year-to-year variations were observed, implying that cross-ring elemental migration is slow (possibly negligible). The x-ray technique is fast and non-destructive, permitting the sample to be retained for additional studies.