Precise measurement of stable potassium isotope ratios using a single focusing collision cell multi-collector ICP-MS

Abstract

High precision potassium isotope ratio measurements were made using a collision-cell equipped single focusing Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS). Interferences on ⁴¹K from ⁴⁰ArH⁺ were largely suppressed through collision with He gas atoms, and reaction with H₂ or D₂ gas molecules in the collision cell under optimum collision gas flow conditions. Using H₂ or D₂ as the collision gas, we distinguish charged argon–deuterium molecules (ArD⁺) generated in the collision cell from argon hydride (ArH⁺) generated in the plasma or in the interface region (referred to as “plasma-related AH⁺” hereafter), and demonstrate, for the first time, that both plasma-related and collision cell-generated ArH⁺ are important sources of ArH⁺ that interfere with ⁴¹K⁺ in collision-cell ICP-MS instruments that use H₂ as a collision gas. The use of D₂ instead of H₂ as a reactive gas in the collision cell resulted in better overall performance in K isotope ratio measurements. By combining these mass spectrometry methods with chemical purification of K by ion exchange chromatography, we achieved an internal precision of <±0.07‰ (2 standard error) and an external reproducibility of <±0.21‰ (2 standard deviation, or 95% confidence) in the ⁴¹K/³⁹K ratio measurement for geological and biological samples. With the improved precision, it is possible to distinguish a ∼1.3‰ variation in K isotope compositions (⁴¹K/³⁹K ratios) among seawater, igneous rocks, and biological samples. The K isotope system is likely to be beneficial in providing a better understanding of potassium cycling during continental weathering and the uptake of nutrients by plants.