An improved protocol for LA-MC-ICP-MS isotope ratio measurements of natural silicon at 213 nm: comparison of mass bias correction factor dependence (solution vs. solid single crystal) and solid sample homogeneity

Abstract

Nanosecond scanning laser ablation MC-ICP-MS (213 nm) was applied to the measurement of the intensity ratios of ultrapure single crystalline silicon (WASO04), which is used in the XRCD-method and general silicon isotope ratio measurements as a well characterized reference material. Parallel measurements in the same sequence with WASO04 samples (w(Si) = 4 μg g⁻¹) dissolved in TMAH (w(TMAH) = 0.0006 g g⁻¹) were conducted for the comparison of matrix and experimental related impact parameters of the derived calibration factors (K) for the correction of intensity ratios. Uncertainties associated with K factors determined via solid laser ablation multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) were in the range of u_rel(K(²⁹Si/²⁸Si)) = 0.58%, u_rel(K(³⁰Si/²⁸Si)) = 0.60%, and u_rel(K(³⁰Si/²⁹Si)) = 0.47%, and exhibit a scattering contribution of up to 50%, whereas K factors derived by Si samples in solution under the same conditions show a more stable course. Main influences on isotope fractionation were derived from the applied laser parameters. Matrix influences due to the kind of sample (solid or dissolved) are negligible. A “quasi-homogeneity” investigation of the local distributions of amount-of substance fractions x(ⁱSi) in the solid sample shows a uniform distribution within the limits of uncertainties. A measurement protocol of isotope ratios of natural silicon was developed using scanning LA-MC-ICP-MS, applying 10¹³ Ω resistors for Faraday detector readings of highest sensitivity, τ-correction, measurements of interference free (high resolution) Si signals, and strong depletion of the NO⁺ interference near the ³⁰Si⁺ signal.