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

Abstract

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 CC, CN, and MgH 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.