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 (v_-¹² and v₊^[12]) and a ¹³CO₂-origin peak at about 1370 cm^-1 (v₊^[13]). The relationship between carbon isotopic compositions and peak intensity ratios of v₊^[12] and v₊^[13] 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 CO ₂.