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

Several highly-enriched ²⁸Si 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 ²⁹Si and ³⁰Si. 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 ²⁸Si-enriched crystal (AVO28) was developed in this study using a reverse-geometry ion microprobe SHRIMP RG. An ≈8 nA primary beam intensity was used to sputter the ²⁸Si-enriched crystal. Secondary ions were measured on a combination of Faraday cup, for ²⁸Si⁺, and electron multiplier for the low abundance ²⁹Si⁺ and ³⁰Si⁺ 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 ²⁹Si/²⁸Si of spot-to-spot analyses on natural silicon (WASO 04). The Si isotope ratios of AVO28 are n(²⁹Si)/n(²⁸Si) = 0.00004107(63) mol mol⁻¹ and n(³⁰Si)/n(²⁸Si) = 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 × 10⁻⁸ (k = 1). This newly developed SIMS method avoids the complex chemical pre-treatment and the molar mass was in good agreement (∼10⁻⁸) with solution multi-collection-ICP-MS results.