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

We report techniques for measuring 41K/39K ratios using a Nu Instruments Sapphire double-focusing multicollector 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.

Automated summary

The techniques for measuring 41K/39K ratios were reported using a Nu Instruments Sapphire double-focusing multicollector ICPMS, equipped with a hexapole collision/reaction gas cell. The method effectively eliminates spectral interferences and ion beams, allowing for direct and accurate measurement of potassium isotopic ratios. Optimized conditions enable high reproducibility and facilitate the study of small potassium isotopic variations in terrestrial samples.