Raman spectroscopic evaluation of precision of oxygen isotope ratio of carbon dioxide

抄録

We measured the Raman spectra of CO₂ fluid at 30 MPa and room temperature (19.1–21.3°C). The spectra showed peaks attributed to the CO₂ combining various isotopes such as ¹²C¹⁶O₂, ¹³C¹⁶O₂, and ¹²C¹⁶O¹⁸O. The relative ratio of ¹²C¹⁶O₂ to ¹²C¹⁶O¹⁸O peaks represents the oxygen isotope ratio of CO₂. To evaluate the precision of the peak ratios, we measured the CO₂ Raman spectra at different exposure times. We examined the standard deviation (1 σ) of the peak intensity ratios and area ones for 20 measurements at each exposure time. The standard deviations of the intensity ratios and area ones were 2.7 and 3.1%, respectively, at a maximum exposure time of 494 s, conversion to peak intensity of ¹²C¹⁶O₂ yields about 2,000,000 counts. The uncertainties are 2.5–3 times greater than expected from the noise of a CCD camera and photon statistics. The oxygen isotope ratios (δ¹⁸O) of natural samples have a range of variation of about 16.6%. Compared to that value, the precision we obtained from this study is very small. Raman spectroscopy can be combined with microscopy to analyze areas as small as approx. 1 μm in diameter. Therefore, oxygen isotope measurement using Raman spectroscopy has potential for application to natural samples as a new method for small CO₂ fluids such as fluid inclusions.