Precise determination of lithium isotope ratios at the sub-nanogram level by QQQ-ICP-MS: application to natural waters and carbonates

Abstract

We present an improved method for the high precision (≤0.6‰, 2σ) determination of lithium isotope ratios (⁷Li/⁶Li) in sub-nanogram samples by single collector triple quadrupole ICP-MS (Agilent® 8900 QQQ-ICP-MS). We utilized cool plasma (600 W), soft ion extraction, and rapid sample standard bracketing with exclusive pulse mode of detection for both isotopes for precise δ⁷Li determination at ≤0.15 ng-Li per analysis. Our long-term averages of NIST-L-SVEC-8545 and 6Li–N-SRM (Li6–N) are 0.04 ± 0.41‰ (2σ, n = 96) and −8.27 ± 0.29‰ (2σ, n = 15) respectively. The average δ⁷Li value of seawater samples, column processed at 1.75 to 3.0 ng-Li, is 31.34 ± 0.56‰ (2σ, n = 49), which is identical to published values. Our intermediate precision of δ⁷Li determination of mass limited carbonate samples, determined on a homogenized coral specimen, is ±0.62‰ (2σ, n = 12). Additionally, we report a high tolerance for sodium, up to 100 ng mL⁻¹ per ng mL⁻¹ of Li, during mass spectrometric δ⁷Li determination. A new single-step ion chromatographic method has been developed for quantitative separation of Li from matrix elements at low blanks and high yield. We utilized high aspect ratio Savillex® PFA columns (1.5 mL resin volume and 20 cm resin height) and macro-porous cation exchange resin (Bio-Rad® AG MP-50) for chromatography. Quantitative elution of Li was achieved in 6 mL of 0.5 mol L⁻¹ HCl at an average recovery of 101.0 ± 1.2% (n = 20), low cumulative Li (<0.6 pg) and crustal contaminant blanks (<1.5 ng). A 4 mL separation between the tailing-edge of Li and the leading-edge Na peak allows for quantitative separation of Li from Na from multiple types of natural samples.