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.

我们报告了使用配备六极碰撞/反应气室的Nu Instruments蓝宝石双聚焦多收集器ICPMS测量⁴¹ K / ³⁹ K比率的技术。通过在碰撞/反应池中与H ₂气体进行一系列反应，有效消除了较大的氢化物光谱干扰以及来自Ar等离子体的⁴⁰ Ar ⁺离子束。这样就可以直接测量³⁹ K ⁺和⁴¹ K ⁺离子束，而不会偏移峰中心。在优化的工作条件下，⁴¹ K / ³⁹可以常规获得K，外部重现性为每密耳0.07（2个标准差），这有助于解决陆地样品之间较小的K同位素变化。通过使用去溶剂化雾化器（Apex Omega）引入样品，可以进一步提高信号强度（对于³⁹ K为1200 V / ppm ），从而可以进行低K样品的测量。我们评估了基质元素，钾浓度的不匹配以及酸摩尔浓度的不匹配对样品标准方括号测量的准确性的影响。准确测定K同位素比的相对简单的方法可能会加快钾的广泛研究的开发和利用，例如进入其全球地球化学循环。