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

We present an improved method for lithium isotope ratio (7Li/(6)Li) determinations with low total lithium consumption(<0.2 ng/quintuplicate analyses), high column yields (>99.98%), high isotope ratio precision (<+/- 0.8 parts per thousand, 2 sigma), 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 parts per thousand (2 sigma) for L-SVEC lithium standards and better than +/- 1.5 parts per thousand (2 sigma) for natural samples. We report a high matrix tolerance limit for sodium (similar to 0.6 mol/mol, Li/Na) and calcium (< 20 mu mol/mol, Li/Ca) for our Quadrupole ICP-MS method. Our seawater delta(7)Li value (30.75 +/- 0.41 parts per thousand, 2 sigma, n = 10) is the same as that reported by other workers (similar to 31.0 +/- 0.5 parts per thousand). Species-specific and bulk sample delta(7)Li analyses of two size fractions of core-top foraminifera yield values similar to modern seawater.