A Sampling Method for Semi-Quantitative and Quantitative Electron Microprobe Analysis of Glass Surfaces
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A Sampling Method for Semi-Quantitative and Quantitative Electron Microprobe Analysis of Glass Surfaces Jamie L. Weaver, Joelle Reiser, Owen K. Neill, John S. McCloy, Nathalie A. Wall. Washington State University, Chemistry Department, Pullman, WA 99164, USA ABSTRACT The determination of the long-term stability and corrosion of vitrified nuclear waste is an important aspect of research for the U.S. Department of Energy (DOE). It is necessary to understand the rate and mechanisms of Nuclear Waste Glass (NWG) corrosion to determine whether or not the glassy matrix will be able to retain radionuclides for the required repository performance time period. Glass corrosion and the rate of glass corrosions is determined by both chemical and microscopy. Electron Microprobe Analysis (EPMA) is a common and powerful method utilized in the examination of the chemographic difference between corroded and uncorroded NGWs. In this work, two forms of quantitative and semi-quantitative EPMA methods are defined by optimizing the instruments counting statistics against a standard glass and NIST minerals that have compositions similar to the glasses under examination. Data collected on both the planar and cross-sectioned surfaces of an unaltered simulated NWG by Standard based Wavelength Dispersive Spectroscopy (WDS) was found to be comparable to the theoretical composition of the glass. Conventional standardless Energy Dispersive Spectroscopy (EDS) data collected on the same surfaces was not comparable. However, standard-based EDS analysis is shown to be able to discriminate between unaltered and corroded glass surfaces. INTRODUCTION Energy Dispersive Spectroscopy-Electron Probe Micro Analysis (EDS-EPMA) is a common method used to analyze corroded glass surfaces. In the past EDS-EPMA has been used to identify the presence of a hydrated and silica-rich alteration layer and the formation of solid composites on the glass surface [1-3]. EDS allows the user to obtain clear and detailed images of the corroded surface morphology over large area (Pm2), and to characterize the element(s) involved in the glass corrosion process. Glass studies often couple EPMA with Induction Coupled Plasma – Optical Emission Spectrometry (ICP-OES) to determine glass corrosion rate, and to attempt definition of glass corrosion mechanisms [4-6]. Most of these studies utilize “standardless” EDS, a method that suffers from both accuracy and precision limitations, especially in comparison to wavelength-dispersive X-ray spectroscopy (WDS) [7]. This form of EDS is sufficient to detect the gross chemical variations caused by the alteration processes, such as complete loss of Na in the altered layers, but cannot accurately quantify the amount of loss. ICP-OES data is used to fill in this information. However, in situations when it is not possible to obtain or analyze a sample of the corroding solution, analysis of a corroded surface by EPMA becomes more important, and more rigorous methods are required. One such method is “standard” based EDS, which is semi-quantitative.
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