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

Abstract

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 an...