High-precision measurement of magnesium isotopes by multiple-collector inductively coupled plasma mass spectrometry

Multiple-collector inductively coupled plasma mass spectrometry has been us ed for the precise measurement of variations in the isotopic composition of Mg in a range of materials. The contributions of C-C, C-N, and Mg-H molecu lar species to the mass spectrum in the Mg mass region are minimised. Varia tions in sample Mg-26/Mg-24 and Mg-25/Mg-24 ratios are expressed as delta M g-26 and delta Mg-25 units, which are deviations in parts per 10(3) from th e same ratio in the SRM 980 Mg standard. The long-term repeatability of the Mg-26/Mg-24 and Mg-25/Mg-24 ratios of a sample Mg solution relative to the SRM 980 Mg isotope standard are 0.12 parts per thousand, and 0.06 parts pe r thousand, respectively, at 95% confidence. The addition of Na, Al, and Ca in a solution of Mg having a known isotopic composition induces 0.2 parts per thousand -1 parts per thousand increase of delta Mg-26. This chemical b ias is a result of a mass-dependent process and is observed to be greater w ith Ca than Na. Isobaric interference from doubly charged Ca-48 ions on mas s 24 is observed to be significant when [Ca]/[Mg] greater than or equal to 0.5. The results obtained on nine terrestrial material show a variation of Mg-isotopes of 4 parts per thousand in delta Mg-26. When plotted in a three -isotope diagram, all the data fall on a single mass fractionation line. Th e excess of Mg-26 has been determined by the deviation from that mass-depen dent relationship, and its long-term repeatability is 0.06 parts per thousa nd at 95% confidence. (C) 2001 Elsevier Science B.V.