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

Multiple-collector inductively coupled plasma mass spectrometry has been used 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 molecular species to the mass spectrum in the Mg mass region are minimised. Variations in sample ²⁶Mg/²⁴Mg and ²⁵Mg/²⁴Mg ratios are expressed as δ²⁶Mg and δ²⁵Mg units, which are deviations in parts per 10³ from the same ratio in the SRM 980 Mg standard. The long-term repeatability of the ²⁶Mg/²⁴Mg and ²⁵Mg/²⁴Mg ratios of a sample Mg solution relative to the SRM 980 Mg isotope standard are 0.12‰ and 0.06‰, respectively, at 95% confidence. The addition of Na, Al, and Ca in a solution of Mg having a known isotopic composition induces 0.2‰-1‰ increase of δ²⁶Mg. This chemical bias is a result of a mass-dependent process and is observed to be greater with Ca than Na. Isobaric interference from doubly charged ⁴⁸Ca ions on mass 24 is observed to be significant when [Ca]/[Mg] ≥ 0.5. The results obtained on nine terrestrial material show a variation of Mg-isotopes of 4‰ in δ²⁶Mg. When plotted in a three-isotope diagram, all the data fall on a single mass fractionation line. The excess of ²⁶Mg has been determined by the deviation from that mass-dependent relationship, and its long-term repeatability is 0.06‰ at 95% confidence. © 2001 Elsevier Science B.V.