High-precision, mass dependent Si isotope measurements via the critical mixture double-spiking technique.

We have developed a new method for measuring mass dependent Si isotope fractionation via critical mixture double-spiking. Samples need to be spiked before column chemistry to guarantee full equilibrium between the sample and double-spike ( 29 Si- 30 Si spike). An iterative addition of the double-spike to the sample, usually 2-4 times, is needed to generate a solution very close to the critically spiked mixture. We use a double-pass cyclonic quartz spray chamber, as it gives the highest signal-to-noise ratio. In conjunction with 6 μg ml -1 Si solution to yield intense Si isotope beams, this setup results in an ∼25 V (with 10 11 Ω resistor) signal on 28 Si + , while on-peak noise is less than 0.06 V. A typical sample analysis comprises 8 repeats ( n = 8) of an individual sample measurement (for each repeat n = 1, 168 second analysis time) normalised to bracketing measurements of critically double-spiked NIST SRM 8546 (commonly known as NBS28). Each of these n = 8 analyses consumes about 13 μg of sample Si and yields a mean δ 30/28 Si with a precision of approximately ±0.03‰ (2 s.e., 2 × standard error of the mean). Over a 16 month period, the reproducibility of the 11 mean δ 30/28 Si values of such n = 8 analyses of the silicate reference material BHVO-2 is ±0.03‰ (2 s.d., 2 × standard deviation), which is 2 to 8 times better than the long-term reproducibility of traditional Si isotope measurement methods (∼±0.1‰, 2 s.d., δ 30/28 Si). This agreement between the long-term and short-term variability illustrates that the data sample the same population over the long and short terms, i.e. , there is no scatter on the timescale of 16 months additional to what we observe over twenty hours (the typical timescale in one analytical session). Thus, for any set of n repeats, including n >8, their 2 s.e. should prove a useful metric of the reproducibility of their mean. Three international geological reference materials and a Si isotope reference material, diatomite, were characterised via the critical mixture double-spiking technique. Our results, expressed as δ 30/28 Si NBS28 , for BHVO-2 (-0.276 ± 0.011‰, 2 s.e., n = 94), BIR-1 (-0.321 ± 0.025‰, 2 s.e., n = 27), JP-1 (-0.273 ± 0.030‰, 2 s.e., n = 19) and diatomite (1.244 ± 0.025‰, 2 s.e., n = 20), are consistent with literature data, i.e. , within the error range, but much more precise.