SREL Reprint #2662

Microfluorescence and Microtomography Analyses of Heterogeneous Earth and Environmental Materials

Stephen R. Sutton1,2 Paul M. Bertsch3, Matthew Newville2, Mark Rivers1,2,
Antonio Lanzirotti2, and Peter Eng2

1Department of Geophysical Sciences, University of Chicago, Chicago, Illinois, 60637, USA
2Consortium for Advanced Radiation Sources, University of Chicago, Chicago, Illinois, 60637, USA
3Savannah River Ecology Laboratory, University of Georgia, Aiken, South Carolina, 29802, USA

Introduction:
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 µm-sized spots with minimum detection limits near 100 mg kg-1. PIXE is well suited for analyses of relatively light elements with 10 mg kg-1 sensitivity and µm-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 in the few to tens of µm 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. Unlike other microanalytical techniques, SXRF uses photons for excitation. Spot sizes are in the µm range with sensitivities in the sub-mg kg-1 range. Like EMPA and PIXE, SXRF sensitivities are smooth functions of atomic number (Smith arid Rivers 1995). Quantification is comparatively straightforward for SXRF because the physics of photon interactions with matter is well understood. Another advantage is that a vacuum sample chamber is unnecessary allowing more versatility in sample state, including analyses of liquids and hydrated solids.

SREL Reprint #2662

Sutton, S. R., P. M. Bertsch, M. Newville, M. Rivers, A. Lanzirotti, and P. Eng. 2003. Microfluorescence and microtomography analyses of heterogeneous earth and environmental materials. pp. 429-483. In: P. A. Fenter, M. L. Rivers, N. C. Sturchio, and S. R. Sutton (Eds.). Applications of Synchrotron Radiation in Low-Temperature Geochemistry and Environmental Science. Mineralogical Society of America.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).