Isotopic Analysis of Ca from Extraterrestrial Micrometer-Sized SiC by Laser Desorption and Resonant Ionization Mass Spectroscopy

The isotopic composition of calcium in a 2.8 μm SiC grain, isolated from the Murchison CM2 meteorite, has been measured by resonant ionization mass spectroscopy with a two-color (2ω + ω) ionization scheme. The sample was atomized by laser-induced thermal desorption by a pulsed N₂ gas laser and mass-analyzed in a time-of-flight mass spectrometer (TOFMS). The Ca ionization scheme results in excellent discrimination against isobaric interferences from SiC, SiO, C₄, and Ti. To investigate possible mass-dependent variations in the postionization efficiency or TOFMS transmission, we have also measured the isotopic abundance of Ca from a terrestrial SiC sample (∼130 ppm Ca concentration) using the same experimental parameters. Furthermore, we have measured the isotopic composition of Ca ion-sputtered with Ar⁺ from a pure terrestrial Ca target to determine isotopic variations of the ionization efficiency. Within statistical uncertainty, the isotopic abundance of Ca from the meteoritic SiC grain does not reveal any deviation from the values obtained from Ca in terrestrial SiC. However, the isotopic pattern of Si, measured with nonresonant ArF laser postionization, shows a significant enhancement of ²⁹Si and ³⁰Si. This clearly indicates the extraterrestrial origin of the analyzed SiC grain. The results of this work are encouraging; the combination of pulsed laser desorption, resonant ionization, and TOFMS is useful for isotopic analysis of elements at trace concentrations in micrometer-sized particles. This is particularly interesting for the study of presolar interstellar grains, where isotopic anomalies relative to terrestrial composition are large and conventional methods, such as SIMS, can suffer from unresolvable isobaric interferences.