Analysis
Sr Isotope Analytical Procedure
Sr isotopes are measured on a multi-collector thermal ionization mass spectrometer, which allows the simultaneous measurement of 84Sr, 85Rb, 86Sr, 87Sr, and 88Sr. Sr isolated from a sample is loaded in a nitric acid solution onto a tantalum filament along with a TaCl loading matrix with helps control the ionization of the Sr from the filament. This is dried by heating the filament by passing a current through the filament. After loading the filament into the mass spectrometer, Sr is ionized by heating the filament to approximately 1500C.
As 86Sr and 88Sr are not radioactive nor radiogenic, the 86Sr/88Sr ratio is constant in all materials and throughout geologic time. Therefore it can be used as an internal monitoring ratio during mass spectrometer analysis. This is essential in order to obtain high precision data, as there is a mass bias induced by the ionization of Sr in the mass spectrometer. An in-run correction for this mass bias is determined by measuring the offset of the known natural 86Sr/88Sr from the measured 86Sr/88Sr, and this mass bias correction is applied to the 87Sr/86Sr ratio. The 85Rb is monitored as there is an isobaric interference between 87Rb and 87Sr. The signal contributed from 87Rb on mass 87 can be calculated from estimated 85Rb/87Rb ratio of the material being analyzed, and that 87Rb contribution can be subtracted from the total mass 87 signal, leaving just the 87Sr intensity.
This correction is typically small, as Rb (and all other elements) are separated from Sr by ion-exchange isolation of the sample Sr prior to analysis. The separation procedure involves a sample digestion process (typically using nitric acid), and passing the dissolved sample over a Sr-specific ion exchange material (Eichrom Industries) in an ion-exchange column. Sr is quantitatively isolated on the ion exchange media in acid normalities of cca 3N, while most other ions pass through. Sr can then be eluted from the column with pure water. The eluant is evaporated to dryness, then taken up in a small amount of nitric acid and loaded directly onto a Tantalum ribbon for mass spectrometry.