In situ determination of precise stable Si isotope ratios by UV-femtosecond laser ablation high-resolution multi-collector ICP-MS

A new method for the in situ measurement of stable Si isotope ratio using UV-femtosecond laser ablation connected to a multiple-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) has been established. The use of medium mass resolution mode (with a resolving power m/Delta m = 8000) permits to resolve spectral interferences on Si-28, Si-29 and Si-30 allowing for determination of the Si-29/Si-28 and Si-30/Si-28 ratios with high accuracy and precision on wide, interference-free plateaus. A three-isotope plot demonstrated that interferences, if existing, are negligible for Si isotope ratios. The laser spot size is 35 mu m and measurements are made using square rasters of 150 mu m side length. Different types of elemental Si, Si-oxide and silicate matrices have been analysed by laser ablation using the international standard NBS28 as the bracketing standard. Thus 629 Si and 630 Si have been determined for the silicon isotope standard IRMM-017 (delta Si-30 +/- 2 S.D. = -1.26 +/- 0.24 parts per thousand; n = 89) and BigBatch (delta Si-30 = -10.55 +/- 0.42 parts per thousand; n = 15), as well as San Carlos Olivine (delta Si-30 = -0.8 +/- 0.1 parts per thousand; n = 14), Caltech Rose Quartz (delta Si-30 = 0.10 +/- 0.13 parts per thousand; n = 14) sponge needles (Stylocordyla borealis; delta Si-30 = -2.19 +/- 0.32 parts per thousand; n = 14) and JER-diopside glass (delta Si-30 = 0 +/- 0.09 parts per thousand; n = 14) samples. The overall repeatability achievable is 0.15 parts per thousand (2 S.D.) on delta Si-29 and 0.24 parts per thousand (2 S.D.) on delta Si-30. The silicon isotope standard IRMM-018 has also been measured and was confirmed to be heterogeneous. The two isotope ratios follow an equilibrium mass-dependent fractionation law which can be represented as delta Si-30 = 1.93 x delta Si-29. Published extractions methods have been used to wet-chemically purify Si from the JER-diopside glass. The Si extracted was presented as Si-gel and ablated like a solid. The results show excellent agreement with in situ measurements of this glass that confirms that this technique can be employed to a wide variety of matrices, including Si purified from solution. The new technique presents a viable alternative to solution MC-ICP-MS for bulk measurements and the most precise technique so far for in situ measurement of Si isotope ratios. (C) 2008 Elsevier B.V. All rights reserved.