The laser microprobe: A technique for extracting carbon, nitrogen, and oxygen from solid samples for isotopic measurements

Abstract

The laser microprobe extraction technique has been adapted for the determination of concentrations and stable isotopic compositions of carbon, nitrogen, and oxygen. The power from a focused laser beam is used to selectively pyrolyse, or combust, specific areas of a solid sample of interest, which is located within a vacuum extraction line. Gases released from the sample are collected, purified, and then admitted to an appropriate mass spectrometer as either molecular nitrogen (for δ15N measurements) or carbon dioxide (for δ13C and δ18O measurements). The minimum amounts of gas that can be measured Isotopically are less than a nanomole. A number of samples have been analysed in order to evaluate the efficacy of the laser microprobe including two carbonate minerals (calcite and siderite), graphite, and titanium nitride. Initial studies on the distribution of nitrogen in an iron meteorite (Uwet) and a carbonaceous chondrite (Murchison) have been undertaken. Nitrogen in Uwet was found to be concentrated in the phosphide mineral schreibersite rather than in the kamacite. Laser extractions of the dark matrix of Murchison reveal 620 to 790 pprn of nitrogen with a δ15N = +38.5 to +44.7‰ (in agreement with stepped heating extractions of bulk Murchison) while much lower concentrations, down to 80 ppm (and δ15N = +12 to +60‰), were found in the light‐coloured high‐temperature inclusions. For carbon‐containing phases, there appears to be an isotope fractionation associated with the formation of CO2 and CO, However, preliminary results suggest that the extent of isotope fractionation may be reproducible, allowing suitable corrections to be applied to the raw data.