High-precision potassium isotopic measurements by collision cell equipped MC-ICPMS

Abstract

We report techniques for measuring 41K/39K ratios using a Nu Instruments Sapphire double-focusing multi-collector ICPMS, equipped with a hexapole collision/reaction gas cell. Large Ar-hydride spectral interferences, as well as the 40Ar+ ion beam from the Ar-plasma, are effectively eliminated through a series of reactions with H2 gas in the collision/reaction cell. This allows the 39K+ and 41K+ ion beams to be measured directly and without offsetting peak centers. With optimized operating conditions, 41K/39K can be routinely obtained with an external reproducibility of 0.07 per mil (2 standard deviations), facilitating resolution of small K isotopic variations among terrestrial samples. The signal intensity can be further increased (1200 V/ppm for 39K) by utilizing a desolvating nebulizer (Apex Omega) for introducing samples, making measurements of low-K samples possible. We evaluate the effects of matrix elements, mismatch of K concentration, and mismatch of acid molarity on the accuracy of sample-standard bracketing measurements. A relatively straightforward method for accurate determination of K isotopic ratios is likely to accelerate development and utilization of a wide range of potassium studies, such as into its global geochemical cycle.