Complete separation of Sr for the isotopic analysis of Mg-rich samples, such as ultramafic rocks and their constituent minerals, was established by adopting a combination of cationexchange chromatography in H⁺ form and pyridinium form with a DCTA complex using extremely small volumes of cation-exchange resin (1ml and 0.5ml respectively), This method made it possible to efficiently separate nanogram sizes of Sr from coexisting large amounts of Mg and Ca with a reduced total elution volume of only 24ml. The method also resulted in the reduction of procedural blanks for Sr and Rb to 32 and 25 pg, respectively.
Applying static multicollection mass spectrometry with Ta-oxide as an ionization activator on a V-shaped W single filament, the isotopic composition of 10 to 20 ng Sr samples separated from 40 to 55mg of ultramafic rock standard (PCC-1) was determined with analytical precision of<0.005% and reproducibility of<0.006%. These precise analyses were performed with a high ⁸⁸Sr⁺ ion beam intensity (>1.5×10^-11 A). Our efficient chemical separation procedure also ensured the absence of Ca and Mg interference to Sr ionization. The Ta-oxide readily eliminated Rb prior to the Sr isotopic analysis in the mass spectrometer. This improved the analytical reliability of isotope dilution mass spectrometry (IDMS) for the simultaneous determination of Sr isotopic composition and concentration. The measured isotopic compositions of spiked PCC-1 agreed within error with those obtained from unspiked measurements, Rb/Sr ratio analyses for PCC-1 using IDMS provided analytical reproducibilities of better than 2%. These achievements indicate that our IDMS technique is capable of yielding trace Rb and Sr concentrations simultaneously with Sr isotopic composition in Mg-rich samples with an analytical reliability similar to that obtained from larger samples ( ?? 1 μg) of common silicate rock samples.