Measurement of 87Sr/86Sr Isotope Ratios in Rocks by ICP-QQQ in Mass-Shift Mode

Triple quadrupole ICP-MS (ICP-QQQ) instrumentation is increasingly used for isotope ratio (IR) analysis studies. Geologists use radioactive isotopes that have long half-lives to date rocks and sediments, often with the choice of dating method dependent on the type of samples under investigation. For example, rubidium (Rb) is abundant in many potassium-containing rocks, so the Rb-strontium (Sr) β-decay scheme is used in geochronology studies of igneous, and sometimes metamorphic rocks and minerals.

Strontium (Sr) has four naturally occurring isotopes: 84Sr (0.56% abundance), 86Sr (9.86% abundance), 87Sr (7.00% abundance), and 88Sr (82.58% abundance). While these are the four stable isotopes of Sr, the abundance of 87Sr varies over long periods of time due to the formation of radiogenic 87Sr following β-decay of 87Rb. 

The level of 87Sr compared to 86Sr is dependent on the original concentration of Rb in the environmental or geological system. Therefore, 87Sr/86Sr IR measurements can also serve as a tracer of ecosystem processes. Traditional IR measurement techniques such as thermal ionization mass spectrometry (TIMS) or multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) offer excellent accuracy and high precision for most IR studies. However, they are less efficient for the measurement of β decay product isotopes, due to isobaric interferences of the daughter isotope by their respective parent isotope. For example, 87Sr must be separated from 87Rb before analysis by TIMS or MC-ICP-MS to avoid the isobaric spectral overlap at mass 87. This process is labor-intensive and time consuming, especially for large sample numbers that are typical of geological and environmental survey studies. By contrast, no Rb/Sr separation is needed before analysis using ICP-MS/MS methodology, simplifying the analysis and improving sample throughput. ICP-MS/MS methods use “chemical resolution” to resolve isobaric interferences, as demonstrated in previous studies using Agilent ICP-QQQ instrumentation.

The aim of this study was to develop a fast, high throughput, routine method suitable for the accurate determination of 87Sr/86Sr IR with precision below 0.1% RSD. To achieve the objective, an Agilent 8900 ICP-QQQ was operated in MS/MS mass-shift mode using O2 as the cell gas.

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