Comparison and influence of 1011 and 1013 ohm resistors used for MC-ICP-MS determination of isotope ratios in highly enriched silicon†

The molar mass M and isotopic composition of a silicon material highly enriched in ²⁸Si were measured for the first time using high ohmic (10¹³ Ω) resistors in the feedback loop of amplifiers connected to the Faraday detectors of a multicollector-inductively coupled plasma mass spectrometer (MC-ICP-MS). In the context of the realization and dissemination of the SI units kilogram and mole via the X-ray crystal density (XRCD) method, it is of high importance to maintain and improve the state-of-the art technique to determine M with the lowest possible associated uncertainty. The applications and influences of 10¹¹ Ω and 10¹³ Ω resistors for ion detection were compared using the crystal Si28-33Pr11 exhibiting a large range (≈10²) of the ratio ³⁰Si/²⁹Si. The low abundance of ³⁰Si hampers the measurement and thus enlarges the uncertainty. The use of 10¹³ Ω resistors enables a fourfold dilution of the initial Si sample stock solutions from w_x(Si) = 4536 μg g⁻¹ down to 1134 μg g⁻¹ preserving the MS equipment (ion lenses, slits etc.). A lower limit of w_x(Si) ≈ 1134 μg g⁻¹ for ion currents I = 3.4 fA (corresponding to U = 0.34 mV: gain corrected for R = 10¹³ Ω) for ³⁰Si⁺ was determined still maintaining the ability to yield u_rel(M) = 4 × 10⁻⁹. This will also enable the use of smaller sample sizes, which will considerably reduce costs and time and thus improve this method strongly. Tau correction for 10¹³ Ω resistors was studied showing no significant influence in case of the continuous beam experiments.