Molar mass measurement of a 28Si-enriched silicon crystal with high precision secondary ion mass spectrometry (SIMS)

Several highly-enriched Si-28 crystals were produced to enable a better determination of the Avogadro constant through removing the uncertainty associated with the abundance determination of the minor isotopes Si-29 and Si-30. Previous determinations have used inductively coupled plasma mass spectrometry (ICP-MS), which typically requires rather large amounts of material and complex chemical pre-treatment. A secondary ion mass spectrometry (SIMS) protocol for ultra-high precision and accuracy Si isotope measurements of a Si-28-enriched crystal (AVO28) was developed in this study using a reverse-geometry ion microprobe SHRIMP RG. An approximate to 8 nA primary beam intensity was used to sputter the Si-28-enriched crystal. Secondary ions were measured on a combination of Faraday cup, for Si-28(+), and electron multiplier for the low abundance Si-29(+) and Si-30(+) signals respectively. The secondary ion intensities and isotope ratios were determined following baseline correction, gain correction, tailing correction and instrumental mass fractionation correction. An internal precision of +/- 0.000016 (2 s(m)) and external precision (reproducibility) of +/- 0.000043 (2 s) were achieved for Si-29/Si-28 of spot-to-spot analyses on natural silicon (WASO 04). The Si isotope ratios of AVO28 are n(Si-29)/n(Si-28) = 0.00004107(63) mol mol(-1) and n(Si-30)/n(Si-28) = 0.00000107(4) mol mol(-1), 2 s, respectively. As a result, an average molar mass of the AVO28 material was determined to be 27.97696972 g mol(-1) with an associated relative standard uncertainty of 1.3 x 10(-8) (k = 1). This newly developed SIMS method avoids the complex chemical pre-treatment and the molar mass was in good agreement (similar to 10(-8)) with solution multi-collection-ICP-MS results.