ISOTOPIC ANALYSIS OF CA FROM EXTRATERRESTRIAL MICROMETER-SIZED SIC BYLASER-DESORPTION AND RESONANT IONIZATION MASS-SPECTROSCOPY

The isotopic composition of calcium in a 2.8 mu m SiC grain, isolated from the Murchison CM2 meteorite, has been measured by resonant ioniza tion mass spectroscopy with a two-color (2 omega + omega) ionization s cheme, The sample was atomized by laser-induced thermal desorption by a pulsed N-2 gas laser and mass-analyzed in a time-of-flight mass spec trometer (TOFMS). The Ca ionization scheme results in excellent discri mination against isobaric interferences from SiC, SiO, C-4, and Ti, To investigate possible mass-dependent variations in the postionization efficiency or TOFMS transmission, we have also measured the isotopic a bundance of Ca from a terrestrial SiC sample (similar to 130 ppm Ca co ncentration) using the same experimental parameters, Furthermore, we h ave measured the isotopic composition of Ca ion-sputtered with Ar+ fro m a pure terrestrial Ca target to determine isotopic variations of the ionization efficiency. Within statistical uncertainty, the isotopic a bundance of Ca from the meteoritic SIC grain does not reveal any devia tion from the values obtained from Ca in terrestrial SiC. However, the isotopic pattern of Si, measured with nonresonant ArF laser postioniz ation, shows a significant enhancement of Si-29 and Si-30. This clearl y 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 ana lysis of elements at trace concentrations in micrometer-sized particle s, This is particularly interesting for the study of presolar interste llar grains, where isotopic anomalies relative to terrestrial composit ion are large and conventional methods, such as SIMS, can suffer from unresolved isobaric interferences.