THE Wisc-SIMS NATIONAL FACILITY FOR STABLE ISOTOPE GEOCHEMISTRY
The University of Wisconsin SIMS lab (Wisc-SIMS) was installed in 2005 and became a National Facility for Stable Isotope Geochemistry in 2008 with support from NSF, Division of Earth Sciences, Instrumentation and Facilities Program. A primary goal of Wisc-SIMS is to provide experienced and inexperienced SIMS users with the highest possible analytical precision and accuracy for in situ analysis of stable isotope ratios at the scale of 1 to 10 micrometers. We can also provide advice and assistance on a range of related scientific and technical issues. The unique ion microprobe capabilities permit analyses in thin section or polished mount to be correlated with textures and information from optics, imaging, or other forms of instrumental analysis. In combination, such data can potentially provide fundamental new levels of understanding for samples of Geological, Planetary, or Biological interest. The advantages are especially great for samples that are zoned, precious, or very small, which can not be analyzed by other means. In 2006-2007, 42 scientists from 26 universities worked with Wisc-SIMS and published their results.
The Wisc-SIMS lab houses the first CAMECA IMS-1280 (delivered 2005). The IMS-1280 is a large radius multi-collector ion microprobe incorporating many improvements over earlier instruments, several of which are designed to enhance precision and accuracy of isotope ratio analysis, including: better focusing of the primary and secondary beam, continuous monitoring of primary beam current during analysis, better control of external magnetic fields in the sample chamber and mass-spectrometer, NMR control of analyzer magnet, oil-free rough pumping for cleaner vacuum, all digital electronics, and new PC computer control. The detector assembly includes a total of 10 electron multiplier and Faraday Cup detectors with five moveable trolleys for simultaneous analysis of a wide range of isotope systems. Both alkali metal (Cs+) and duoplasmatron (O-, O+) sources are available. Spot sizes as small as 250 nm are possible (with Cs), but more generally, a spot of 10μm diameter by 1 μm deep is used to increase sample size (~1ng/analysis) and optimize precision. Precision for δ18O and δ17O in well-polished silicates is typically better than ±0.2 or ±0.3‰, respectively (1 SD, spot-to-spot) with 10μm diameter spots (Kita et al. 2007; Page et al. 2007).
As of January 2008, the lab has experience with analysis of Li, C, O, Mg, Si, and S isotope ratios. Best results come from well-prepared samples that are vacuum stable; have a smooth, flat, low-relief surface; and are 25 mm in diameter and <12 mm thick. Surface relief is minimized by careful polishing and is measured at nm-scale by white light profilometer. Other materials can be analyzed, as can isotope ratios for trace elements. Accurate analysis requires well characterized, homogeneous standards with similar chemistry and crystal structure to samples. For oxygen isotope ratios, Wisc-SIMS has many silicate, carbonate, and oxide standards. Other standards exist or are being developed. Potential users should enquire about standard availability.
Use of associated instrumentation at nominal cost can be arranged in support of projects at Wisc-SIMS including: an Hitachi S3400N Variable Pressure Scanning Electron Microscope with capability for imaging uncoated samples, HKL Electron Back Scatter Diffraction (EBSD), Gatan color-filtered CL, Thermo EDS, and BSE detectors; a CAMECA SX-51 electron microprobe with 5 crystal spectrometers, EDS, CL, and BSE; and a ZYGO NewView 6300 white light optical profilometer.
© The Department of Geoscience, University of Wisconsin-Madison
Page created: February 14, 2008; updated July 14, 2009. For comments and corrections please email.
