MICROSCOPIC ORGANIC-ANALYSIS USING 2-STEP LASER MASS-SPECTROMETRY - APPLICATION TO METEORITIC ACID RESIDUES

We have developed a microanalytical instrument to analyze the organic constituents of particulates and inhomogeneous samples with a spatial resolution of approximately 40-mu-m. Our method, two-step laser desorption/laser ionization mass spectrometry (L2MS), uses an infrared laser to volatilize constituent molecules intact and an ultraviolet laser to ionize desorbed molecules in a selective manner with little or no fragmentation. In particular, our instrument is currently tuned to detect polycyclic aromatic hydrocarbons (PAHs), which have a strong absorption cross section at the ionization laser wavelength of 266 nm. The detection limit of our instrument is approximately 8 fmol for the compound coronene (C24H12). We demonstrate the use of this instrument to analyze acid residues from six meteorites; the samples consist of particles that are about 200-mu-m in diameter. Quantitative and qualitative trends in PAH composition are identified for different classes of meteorites. Significant differences are seen between the carbonaceous and ordinary chondrites. Moreover, within a single class of ordinary chondrites we see an increase in alkylation with increasing petrographic type (thermal metamorphism).