Developing Micro-Raman Mass Spectrometry for Measuring Carbon Isotopic Composition of Carbon Dioxide

Abstract

We investigated the applicability of micro-Raman spectroscopy for determining carbon isotopic compositions (13C/12C) of minute CO2 fluid inclusions in minerals. This method is nondestructive and has sufficiently high spatial resolution (1 μm) to measure each fluid inclusion independently. Raman spectra of CO2 fluid have 12CO2-origin peaks at about 1285 cm−1 and 1389 cm−1 (v12− and v12+) and a 13CO2-origin peak at about 1370 cm−1 (v13+). The relationship between carbon isotopic compositions and peak intensity ratios of v12+ and v13+ was calibrated. Considering several factors affecting the peak intensity ratio, the error in obtained carbon isotopic composition was 2% (20‰). The reproducibility of the intensity ratio under the same experimental environment was 0.5% (5‰). Within these error values, we can distinguish biogenic CO2 from abiogenic CO2.