High precision SIMS oxygen isotope analysis and the effect of sample topography

Abstract

We have developed highly precise and accurate in situ SIMS stable isotope analytical protocols using the IMS-1280 at the University of Wisconsin, through careful tuning of the instrument, stable electronics, and improved protocols for sample preparation, standardization and automated analysis. Multicollection Faraday Cup (FC) oxygen two and three isotope analyses routinely achieve spot-to-spot reproducibility of 0.3‰ ( δ 18O and δ 17O, 2SD) from 10–15 μm single spots. Accuracy can be even better for multiple analyses of a homogeneous sample. Furthermore, reproducibility at the ≤ 1‰ level is achieved by using multicollection FC–Electron Multiplier (EM) analyses for primary ion beam spots of 1 to 3 μm in diameter. These results present a trade-off vs. conventional laser fluorination techniques; sample sizes are 10 6 to 10 9 times smaller, at the expense of a factor of 2 to 10 in analytical precision. SIMS is now a powerful tool for high precision and accuracy, and high spatial resolution stable isotope studies and provides the potential for fundamental new advances in stable isotope geochemistry. Analytical artifacts from sample geometry and topography (X–Y effects) are examined in detail. Several epoxy mounts containing mineral standards were prepared and the amounts of polishing relief were measured using an optical profilometer. No significant X–Y effect is identified within 7 mm from the center of the mount when the grains are polished flat with minimal relief (≤ 1 μm). However, significantly large topographic effects are found from standard grains with relatively large polishing relief (10–40 μm). The measured values of δ 18O vary depending on the amount of relief, inclination of surface, and geometry of analytical spots on the standard grains, resulting in elevated δ 18O value by as much as ~ 4‰ and degraded external precision as poor as ± 3‰ (2SD). These analytical artifacts may be caused by deformation of the local electrostatic field applied on the surface of the sample, which deviates the trajectory of secondary ions of individual isotopes. The results clearly indicate that polishing relief for highly accurate SIMS stable isotope analyses should be less than a few µm, which can be readily evaluated by using an optical surface profilometer.