Application of a Continuous-Acquisition-Method to potassium isotope measurements by multi-collector inductively coupled plasma mass spectrometry

Rationale The very small mass difference between K-41 and (ArH+)-Ar-40 makes the flat, hydride interference-free peak shoulders very narrow (0.002-0.003 m/z unit), bringing a number of analytical challenges when measuring K isotopic compositions by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). In traditional Sequence Run mode, the parameters are loaded every line of the sequence which can introduce tiny drifts of tune parameters and mass peaks. This may occasionally lead to the failure of K isotope measurements when mass drifts exceed 0.002 m/z unit. It is thus essential to keep the tune parameters, especially the magnet current, very stable to achieve high-precision K isotopic compositions. Method We developed a Continuous-Acquisition-Method (CAM) MC-ICP-MS Run mode to improve the stability when determining K isotopes. Two sets of experiments were designed: (a) Stability test: measuring a single pure K solution (viz. NIST-999c) for similar to 3 h and comparing the stability of the two run modes; and (b) GSB-K test: measuring our inhouse pure K standard solution (GSB-K) in both run modes and comparing the accuracy and precision. Results The traditional Sequence Run mode only kept the MC-ICP-MS system stable for the first similar to 1.5 h during the similar to 3-h test, with an offset of the mass peaks of similar to 0.003 m/z unit. The CAM Run mode yielded higher stability during the whole test (similar to 3 h), with a peak shift m/z unit. Measurement of the GSB-K standard solution in Sequence Run and CAM Run modes gives identical delta K-41 values when the magnet was kept stable, with the CAM Run mode offering a better precision and keeping the instrument stable for longer time. Conclusions The MC-ICP-MS CAM Run mode shows higher stability and better precision. It is, therefore, good for high-precision K isotope measurements.

Automated summary

The study developed a Continuous-Acquisition-Method (CAM) MC-ICP-MS Run mode to improve stability when determining K isotopes. Experiments showed that the CAM Run mode provided higher stability and better precision compared to the traditional Sequence Run mode, making it suitable for high-precision K isotope measurements.