In-situ Rb-Sr dating using LA-MC-ICP-MS/MS: the effect of YO+ and ZrO+ interferences and their correction.

The ⁸⁷Rb-⁸⁷Sr system is a versatile chronological tool and geochemical tracer. Unlike Rb-Sr measurements by thermal ionization mass spectrometry, in-situ laser ablation (LA) mass spectrometry offers high sample throughput and minimizes sample destruction. This method, however, is compromised by the presence of multiple isobaric interferences on Sr isotopes. This problem can be overcome using multi-collector inductively-coupled-plasma mass spectrometers (MC-ICP-MS) equipped with the recently developed pre-cell mass filter and collision/reaction cell (CRC), such as the Thermo Scientific^TM Neoma^TM MS/MS-MC-ICP-MS. This instrument allows the simultaneous measurements of on-mass and mass-shifted Rb and Sr isotopes using SF₆ reaction-gas [1-4]. Here we show, however, that for Y- and Zr-bearing phases, ⁸⁹Y¹⁶O⁺, ⁹⁰Zr¹⁶O⁺, and ⁹¹Zr¹⁶O⁺ can form in the CRC, when O₂/H₂O enters the CRC either from impurities in the reaction gas or molecules that formed in the plasma. These oxides interfere with ⁸⁶SrF⁺, ⁸⁷SrF⁺, and ⁸⁸SrF⁺, respectively, and cause systematic errors on measured ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratios. To correct for these effects, we synthesized glasses, doped with Rb, Sr, Y and Zr. The glasses can be used to correct for YO⁺- and ZrO⁺-induced systematic errors on measured ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratios. The effects of these interferences and their correction, using our synthesized glasses, on in-situ Rb-Sr dating using LA-MC-ICP-MS/MS will be discussed at the meeting.

[1] Craig et al. (2021), Anal. Chem. 93, 30, 10519-10527;

[2] Dauphas et al. (2022), JAAS 37, 2420-2441;

[3] Telouk et al. (2024), JAAS 39, 879-887;

[4] Huang et al. (2025), Spectrochim. Acta B 224, 107117