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

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.