Chapter 5: Ion microprobe analysis: Basic principles, state-of-the-art instruments and recent applications with emphasis on the geosciences
Bärbel W. Sinha and Peter Hoppe
An ion microprobe is an instrument that uses a finely focused primary ion beam to erode, or ‘sputter’ a solid sample and to collect secondary ions ejected during that process into a mass spectrometer generating a spatially resolved mass spectrum. The underlying technique, Secondary Ion Mass Spectrometry (SIMS), has become a standard tool for the in-situ study of trace-element concentrations and isotope ratios in the fields of geochemistry, geochronology, biogeochemistry and cosmochemistry. An overview of the most recent developments in SIMS is given by Chabala et al. (1995), Ireland (1995), MacRae (1995), Becker (2005) Betti (2005) Deloule & Wiedenbeck (2005), Deloule (2006) and McPhail (2006). Secondary ion mass spectrometry offers parts per million (ppm) or better detection limits for almost all elements, imaging capabilities, periodic table coverage (H–U), and isotope analyses of major and trace elements. The following three examples illustrate the unique power of the SIMS technique in measuring and imaging isotope ratios and trace element distributions.
Firstly, the lateral distribution of elements of interest and isotope ratios can be measured. Figure 1 demonstrates the lateral resolution of SIMS imaging with the Cameca NanoSIMS 50. A spatial resolution of 50 nm is possible, even for biological samples. Scans of a cell culture were taken at appropriate mass number to recognize bacterial cells (CN–, major molecular ion image) on a nucleopore polycarbonate filter, to identify photosynthetic active cells by their incorporation of 13C-labelled bicarbonate (13C/12C ratio, isotope ratio image), and to recognize species with the help of a halogen marker (19F–, trace element ion image) that binds to the ribosome of the cell.
Secondly, 3-dimensional chemical maps of samples can be produced (see Figure 2 for example): the distribution of 32S in a sea-salt aerosol particle consisting of a 4 µm long gypsum needle and a 1 µm halite crystal. Trace quantities of S are visible on the surface of the halite particle.
Thirdly, in SIMS depth profiling, the composition of a sample can be studied as a function of depth. Secondary ion mass spectrometry is the only tool that can acquire depth profiles in solid samples with a depth resolution of several nm (typically 5–20 nm). A sample depth profile is shown in Figure 3. The S isotopic composition of a 9 µm wide NaCl aerosol, which showed traces of reaction with sulphuric acid, was studied as a function of depth. A straight mixing line between the isotopic composition of the anthropogenic H2SO4 (g), which reacted with the particle surface, and unaltered sea-salt sulphate trapped inside the NaCl crystal is found.
The aim of this chapter is to highlight successful applications of SIMS to a variety of topics in the geosciences and to provide a brief introduction into the fundamentals of SIMS. It is not intended as a comprehensive review of the theoretical background of ion-microprobe analysis. The physics of ion production and instrumental aspects are complex and beyond the requirements of the interested but non-expert reader and can be found elsewhere (e.g. Benninghoven et al. 1987). Likewise, it gives a short overview of approaches to quantify SIMS data and instrumentation currently in use for applications in geosciences without going into technical details. Sample-preparation techniques and selected applications of SIMS in the geosciences are presented to introduce ion microprobe analysis as a powerful tool to solve a variety of problems in many different fields of earth sciences and to encourage the reader to read on.
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