Comparison and influence of 10 and 10 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 28 Si were measured for the first time using high ohmic (10 13 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 10 11 Ω and 10 13 Ω resistors for ion detection were compared using the crystal Si28-33Pr11 exhibiting a large range ( 10 2 ) of the ratio 30 Si/ 29 Si. The low abundance of 30 Si hampers the measurement and thus enlarges the uncertainty. The use of 10 13 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 = 10 13 Ω) for 30 Si + 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 10 13 Ω resistors was studied showing no significant influence in case of the continuous beam experiments. Use of high ohmic feedback resistors connected with Faraday cups enables strong reduction of sample size (here: Si crystal slices enriched in 28 Si) and proper signal quality while maintaining the measurement uncertainty.