Measurement of lithium isotope ratios by quadrupole-ICP-MS: application to seawater and natural carbonates

Abstract

We present an improved method for lithium isotope ratio (⁷Li/⁶Li) determinations with low total lithium consumption (<0.2 ng/quintuplicate analyses), high column yields (>99.98%), high isotope ratio precision (<±0.8‰, 2σ), and low blanks (1.0 ± 0.5 pg). We refine a single step ion chromatographic method to quantitatively recover and separate lithium from all matrix elements using small volume resin (2 ml/3.4 meq AG 50W-X8) and low volume elution (6 ml, 0.5 N HCl) with low procedural blanks (<500 fg/ml). We optimize the procedure for analyses of natural carbonates (foraminifera) containing 1 to 2 ppm lithium. This lithium separation method is applicable to other natural samples (e.g. seawater, pore-waters, mineral grains) by appropriate scaling. Isotope ratio measurements are made by a single collector Quadrupole ICP-MS (Agilent 7500cs) using cool plasma (600 W), soft extraction, peak jumping, and pulse detection mode with sample-standard bracketing. The precision is better than ±0.8‰ (2σ) for L-SVEC lithium standards and better than ±1.5‰ (2σ) for natural samples. We report a high matrix tolerance limit for sodium (∼0.6 mol/mol, Li/Na) and calcium (<20 µmol/mol, Li/Ca) for our Quadrupole ICP-MS method. Our seawater δ⁷Li value (30.75 ± 0.41‰, 2σ, n = 10) is the same as that reported by other workers (∼31.0 ± 0.5‰). Species-specific and bulk sample δ⁷Li analyses of two size fractions of core-top foraminifera yield values similar to modern seawater.