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

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 K-41 from (ArH+)-Ar-40 were largely suppressed through collision with He gas atoms, and reaction with H-2 or D-2 gas molecules in the collision cell under optimum collision gas flow conditions. Using H-2 or D-2 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 K-41(+) in collision-cell ICP-MS instruments that use H-2 as a collision gas. The use of D-2 instead of H-2 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 parts per thousand (2 standard error) and an external reproducibility of <+/- 0.21 parts per thousand (2 standard deviation, or 95% confidence) in the K-41/K-39 ratio measurement for geological and biological samples. With the improved precision, it is possible to distinguish a similar to 1.3 parts per thousand variation in K isotope compositions (K-41/K-39 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.