High-precision potassium isotope analysis using the Nu Sapphire collision cell (CC)-MC-ICP-MS

Significantly Enhanced Robustness of K Isotope Analysis by Collision Cell MC-ICP-MS and Its Application to the Returned Lunar Samples by China’s Chang’e-5 Project

“…One is to run MC-ICP-MS under high mass resolution mode, with cold plasma ,,,,,, or without cold plasma, ,, to physically resolve 40 Ar 1 H + from 41 K + in the mass spectrometer. The other approach is to remove 40 Ar 1 H + chemically by collision and reaction with gas molecules in the collision cell of MC-ICP-MS (e.g., refs , , , , , , and ).…”

“…Sensitivity of 1000–2000 V per μg/g for K can be achieved using a new generation collision cell MC-ICP-MS (CC-MC-ICP-MS), ,,, compared to the typically sensitivity of <30 V per μg/g for K on HR-MC-ICP-MS without a collision cell. ,,, The >1000 V per μg/g sensitivity means that, from a counting statistics point of view, precise K isotope ratio measurements can be achieved for a diluted K solution with a concentration as low as 10 ng/g (i.e., 10–20 V total K signal intensity) on CC-MC-ICP-MS. However, the accuracy of K isotope analysis by CC-MC-ICP-MS can be compromised by Ca in the sample solutions, where a [Ca]/[K] of 0.01 can cause a 0.03–0.10‰ offset in measured 41 K/ 39 K ratios. ,,, Because Ca is a major contaminant that ubiquitously exists in reagents, acids, water, containers, reaction vessels, pipet tips, and air dusts, ng/g to sub-ng/g level background of Ca is common in K analyte solutions, even after careful chemical procedures of purification. Therefore, although sufficient precision of the K isotope ratio measurement is achievable on a 10 ng/g K solution using CC-MC-ICP-MS, the Ca concentration in that 10 ng/g K solution needs to be below 0.1 ng/g (i.e., [Ca]/[K]<0.01) to guarantee the accuracy of K isotope analysis.…”

“…High sensitivity is the main advantage of the collision cell approach over the high mass resolution approach in K isotope analysis by MC-ICP-MS. Sensitivity of 1000−2000 V per μg/g for K can be achieved using a new generation collision cell MC-ICP-MS (CC-MC-ICP-MS), 8,10,13,18 compared to the typically sensitivity of <30 V per μg/g for K on HR-MC-ICP-MS without a collision cell. 11,17,19,27 The >1000 V per μg/g sensitivity means that, from a counting statistics point of view, precise K isotope ratio measurements can be achieved for a diluted K solution with a concentration as low as 10 ng/g (i.e., 10−20 V total K signal intensity) on CC-MC-ICP-MS.…”

“…However, the accuracy of K isotope analysis by CC-MC-ICP-MS can be compromised by Ca in the sample solutions, where a [Ca]/[K] of 0.01 can cause a 0.03−0.10‰ offset in measured 41 K/ 39 K ratios. 8,10,13,18 Because Ca is a major contaminant that ubiquitously exists in reagents, acids, water, containers, reaction vessels, pipet tips, and air dusts, ng/g to sub-ng/g level background of Ca is common in K analyte solutions, even after careful chemical procedures of purification. Therefore, although sufficient precision of the K isotope ratio measurement is achievable on a 10 ng/g K solution using CC-MC-ICP-MS, the Ca concentration in that 10 ng/g K solution needs to be below 0.1 ng/g (i.e., [Ca]/[K]<0.01) to guarantee the accuracy of K isotope analysis.…”

“…It is extremely challenging, if not impossible, to keep the Ca concentration in solutions at a sub-ng/g to pg/g level routinely for a regular isotope laboratory. To date, most CC-MC-ICP-MS laboratories run K isotope analysis on solutions that contain 150−250 ng/g K. 8,10,13,18 In order to take full advantage of the high sensitivity of CC-MC-ICP-MS and increase the robustness of CC-MC-ICP-MS for K isotope analysis on precious and small samples (e.g., extraterrestrial specimens), it is important to address the inaccuracy induced by Ca during K isotope analysis. In this report, we show that when D 2 rather than H 2 is used as the collision gas, the susceptibility of measured 41 K/ 39 K ratios to Ca is reduced to a negligible level; thus, the robustness of K isotope analysis by CC-MC-ICP-MS is greatly enhanced.…”