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.