Instrumentation

Secondary ionization mass spectrometry (SIMS) is a technique for the isotopic analysis of solids. SIMS instruments combine a sputter source to remove material from a solid target with a mass spectrometer to analyze ions generated during the sputtering process. The sputter source is designed to excavate and partially ionize atoms from near-surface sample volumes through collision of energetic primary ions in a beam with lateral dimensions of sub-micrometers to several 10’s of micrometers at the impact location. Depth resolution of sputtering is typically an order of magnitude less than the lateral resolution. Although sputtering is destructive, the amount of material consumed in SIMS analysis is typically negligible compared to material losses during sample preparation, e.g. through grinding and polishing. The resulting mass spectrum comprises atoms and polyatomic fragments. Mass spectrometry of the ionized secondary ions using a large radius magnet permits separation of isobaric interferences in the mass spectrum and analysis of the species of interest at low instrumental backgrounds. The advantage of SIMS is the capability of in-situ analysis of solids where sampling needs to be performed at small size, high sensitivity and low detection limits. Quantification using well characterized reference materials allows precise and accurate analysis of elemental and isotopic ratios with applications for stable isotope geochemistry, geochronology, and trace element analysis. In addition to spot analyses, SIMS can be used to generate depth profiles, as well as two and three dimensional maps to resolve elemental and isotopic heterogeneities in the samples.

The IMS 1280-HR is a second-generation large magnet-radius SIMS instrument capable of analyzing positive or negative secondary ions at high sensitivity and spatial resolution. Ion sources are a duoplasmatron (producing positive and negative oxygen ions) and a Cs₂CO₃ source (thermally generating positive Cs ions). A normal-incidence electron gun is available for charge compensation during negative ion analysis of insulators. Samples and mounting materials cannot exceed 25.4 mm in diameter and 5 mm in height, and they must be ultrahigh vacuum compatible, have flat surfaces, and for insulators be coated with a conductive layer (mostly Au or C). New high precision sample holders permit reliable analysis within 8 mm from the center of the mount; targeting outside this “bulls-eye” area is not recommended. An eight sample airlock system allows for rapid change of samples. The sample surface is imaged by a reflected light microscope. Automated analysis is possible at stage position reproducibility of 2 µm. The large magnet radius (585 mm) enables high transmission with insignificant loss up to a mass resolution m/Δm of 6,000, and high mass resolution (m/Δm up to 30,000) over a total mass range of ca. 300 amu (i.e., from H to UO₂). Multicollection using five moveable trolleys equipped with interchangeable electron multipliers (EM) and Faraday cups (FC) is possible from Li to U. Analysis spots are typically ~20 µm (Köhler mode for routine geochronology) and ~10 µm (stable isotope analysis using a focused Cs⁺ beam); smaller ion beams are tunable for special applications such as scanning ion imaging. The IMS 1280-HR also acts as an ion microscope which can image secondary ions emission from the sample surface by means of a channel plate, and a resistive anode encoder. Like its smaller (e.g., IMS 3F analogs, the IMS 1280-HR is also capable of depth profiling at 10’s of nm resolution. Routine geochronologic applications have a sensitivity (useful yield = ions detected over atoms removed) of 1% for Pb⁺ and reproducibility for Pb/U of 1-2% (1 standard deviation) that is equivalent to SHRIMP instrumentation. Uncertainties for routine stable isotope (e.g, C, O, S) analysis using FC detectors are ~0.2‰ (~0.6 ‰ with EMs).

• Correlative Raman-scanning electron microscope: ZEISS WITec RISE EVO MA15 (with GATAN ChromaCL2 detector)
   
• Microscope for polarized-light microscopy: ZEISS Axio Imager
   
• Laser microscope: KEYENCE VK-X200
   
• Depth profilometer: Bruker DektakXT
   
• Surface coater: QUORUM Q150T ES