Meteorite studies suggest that each solar system object has a unique oxygen isotopic composition. Chondrites, the most primitive of meteorites, have been believed to be derived from asteroids, but oxygen isotopic compositions of asteroids themselves have not been established. We measured, using secondary ion mass spectrometry, oxygen isotopic compositions of rock particles from asteroid 25143 Itokawa returned by the Hayabusa spacecraft. Compositions of the particles are depleted in (16)O relative to terrestrial materials and indicate that Itokawa, an S-type asteroid, is one of the sources of the LL or L group of equilibrated ordinary chondrites. This is a direct oxygen-isotope link between chondrites and their parent asteroid.
Thermal ionization mass spectrometry (TIMS) was used to directly quantify an ultratrace of radioactive 90 Sr in microliter droplet samples. No chemical separation was required in removing isobaric interferences on M = 90 such as 90 Zr and organic molecules in the mass spectrum because the difference in evaporation and ionization (emission) temperature among organic molecules, Zr and Sr, allows us to control the emission manner and significantly suppress the isobaric interferences. Direct quantification was achieved by improving the intercalibration of Faraday cups and ion counting in an isotope dilution (ID) method. Furthermore, the use of a total evaporation method (TE) enhanced the detection efficiency by the complete collection of the 90 Sr ion beam from the samples and minimized the complexity of the fractionation effect in the isotope ratio calculation. In this study, 1 fg of 90 Sr (equal to activity of 5 millibecquerel (mBq)) in a very low-volume sample with 10 8 times greater isobaric interference from 90 Zr was successfully measured using the proposed ID-TE-TIMS method. The limit of detection was 0.029 fg (equal to 0.15 mBq) without any preconcentration. To demonstrate the wide usability of this method, low-volume samples of tears, eyelashes, saliva, environmental standards, and water samples (i.e., seawater and ground water) were analyzed within 1 h. The relationship of the measured values between this ID-TE-TIMS method and a radiometric analysis was shown to have good linearity.
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