Microfluorescence and Microtomography Analyses of Heterogeneous Earth and Environmental Materials

Abstract

Analytical techniques with high sensitivity and high spatial resolution are crucial for understanding the chemical properties of complex earth materials and environmental samples, and these so-called 'microprobes' have become workhorses of the geochemical community as well as important tools for environmental scientists. These microanalytical instruments are based on various forms of sample excitation and detection. They are complementary in terms of spatial resolution, element sensitivity, energy deposition and non-destructiveness. Several techniques fall in the class of methods employing charged particle excitation of X-ray fluorescence, including electron microprobe analysis (EMPA) and particle-induced X-ray emission (PIXE). EMPA is capable of {micro}m-sized spots with minimum detection limits near 100 mg kg{sup -1}. PIXE is well suited for analyses of relatively light elements with 10 mg kg{sup -1} sensitivity and {micro}m-sized spots. The relatively large energy deposited by the charged particle beam can complicate the analysis of volatile elements or induce valence state changes of redox sensitive elements. Sensitivity of these technologies is a relatively smooth function of atomic number. Other techniques are based on sample sputtering followed by mass spectrometry of the vaporized products, including secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Beam sizes are inmore » the few to tens of {micro}m range. Elemental sensitivities for SIMS are highly variable depending on ion yield, and quantification can be difficult because of matrix effects in the ion production process. SIMS and LA-ICP-MS have very high sensitivities for some elements and low sensitivity for others. These and other microanalytical techniques used in earth science research are described in Potts et al. (1995). The subject of this chapter is synchrotron X-ray fluorescence (SXRF) microprobe analysis (Horowitz and Howell 1972) and microtomography.« less