High-precision simultaneous measurement of ¹⁸⁷Os, ¹⁸⁶Os, and ¹⁸⁴Os using 10¹³ Ω amplifiers and CDDs on NTIMS

High-precision measurements of ¹⁸⁴Os/¹⁸⁸Os, ¹⁸⁶Os/¹⁸⁸Os, and ¹⁸⁷Os/¹⁸⁸Os ratios are significant in the fields of geochemistry and cosmochemistry. However, no high-precision measurement technique exists for simultaneously obtaining all three ratios using a static method for samples with an Os content of <1 ng or ¹⁸⁶OsO₃^‾ and ¹⁸⁷OsO₃^‾ ion-beam intensities of <150 mV. This greatly limits research on rare samples with small sample sizes or low Os contents, such as Lunar, Martian, or old Earth samples. This paper reports a static method, which could achieve the simultaneous measurement of 9 Faraday cups (FCs) with high-signal/noise ratio 10¹² Ω amplifiers and 10¹³ Ω amplifiers and two compact discrete dynodes (CDDs). By analyzing two calibration solutions, a precision value of less than 3‰ (2RSD) could be achieved for ¹⁸⁴Os/¹⁸⁸Os ratios, even if the ¹⁸⁴OsO₃^‾ intensity was as low as 1000 cps. The precision values for the ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios were similar and could be better than 0.066‰ (2RSD) when the intensities of ¹⁸⁶OsO₃^‾ and ¹⁸⁷OsO₃^‾ were greater than 30 mV, which can be obtained with conventional 10¹¹ Ω amplifiers only at signals larger than 150 mV. Three geological reference materials were used in this study. The precision values of ¹⁸⁴Os/¹⁸⁸Os, ¹⁸⁶Os/¹⁸⁸Os, and ¹⁸⁷Os/¹⁸⁸Os ratios reached 2, 0.061, and 0.050‰ (2RSD), respectively, when the maximum Os amount was approximately 12 ng, and 87, 15, and 10‰ (2RSD), respectively, when the maximum Os amount was as low as approximately 66 pg. Additionally, our results show that changes in the volume of oxygen added during measurement can cause significant variations in the oxygen isotope composition. This static measurement method not only avoids the nonlinear signal change of the instrument during the analysis process but also realizes the accurate removal of the oxygen isobaric interference in the run, which could improve the precision of ¹⁸⁴Os/¹⁸⁸Os, ¹⁸⁶Os/¹⁸⁸Os, and ¹⁸⁷Os/¹⁸⁸Os ratios for small-size/low-signal samples.