In Situ Analysis of Radiogenic Isotopes with Emphasis on Ion Microprobe Techniques and Applications*
Richard A. Stern, 1999. "In Situ Analysis of Radiogenic Isotopes with Emphasis on Ion Microprobe Techniques and Applications", Application of Radiogenic Isotopes to Ore Deposit Research and Exploration
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For the scientist interested in conducting in situ micrometer-scale radiogenic isotope ratio measurements, the two principal analytical tools available are the ion microprobe and various forms of the laser microprobe. The ion microprobe utilizes the technique of secondary ion mass spectrometry (SIMS), whereby a high energy oxygen or cesium ion (primary) beam excavates and ionizes (secondary) particles of the target mineral, which are then electrostatically extracted and mass analyzed using a double-focusing mass spectrometer. The laser microprobes employ a laser as a sampling device coupled to either a plasma-source or gas-source mass spectrometer depending on the isotopes being considered. For example, laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) utilizes a laser to extract particles of a mineral, which are subsequently transported in argon gas to an ICP chamber for ionization and then mass analysis in a quadrupole- or double-focusing mass spectrometer.
These microprobe techniques allow isotope ratio measurements to be done directly on sectioned raw minerals or other solids, either from prepared mineral separates or in rock samples directly, which distinguishes them from conventional bulk chemical methods. Nevertheless, the reader should be aware that it is possible, using saws, drills, and other tools, to isolate small mineral samples (e.g., <100 μm to hundreds of microns in dimension) from solid matrices mechanically, and subsequently, to conduct bulk chemical radiogenic isotope analyses. Such small sample analytical techniques may be entirely appropriate in cases of low sample complexity or large relative size of the sample domains. However, the sample context is destroyed in the process of extraction, and there exists the possibility of mixing of sample domains. In situ techniques provide unambiguous sampling of complex targets and are substantially faster, typically requiring only several minutes to an hour to acquire data from a raw mineral. The high throughput gives a feeling of real-time problem solving. To varying extents the microprobe techniques more easily allow nonexperts to participate directly in the acquisition of data, although all require considerable expertise and experience with the particular methods and instrumentation. Other operational differences between