Raman Spectral Characteristics of 12co2/13co2 and Quantitative Measurements of Carbon Isotopic Compositions from 50 to 450°C and 50 to 400 Bar

Abstract

The carbon isotopic composition of CO2 is traced to its different origins and widely used in the fields of geology, biology, and chemistry. Raman spectroscopy can be performed in situ, is nondestructive, and requires no sample preparation; these characteristics enable Raman spectroscopy to be considered as a new alternative method to measure the carbon isotopic composition of CO2. In this work, Raman spectra of high-purity 13CO2, 12CO2, and six 12CO2-13CO2 binary mixtures with known mixing ratios were collected using a High Pressure Optical Cell (HPOC) at 50-450°C and 50-400 bar. The results showed that the characteristic peak positions of both 13CO2 and 12CO2 shift to blue with increasing temperature and decreasing pressure, but the peak positions of 13CO2 showed a larger shift. The peak position difference of the corresponding characteristic peaks between 13CO2 and 12CO2 is more than 15 wavenumbers under the above temperatures and pressures and can be distinguished. However, ν-13 overlays νH.B.12 near 1265 cm-1, ν+12 overlaps νH.B.13 near 1288 cm-1. The existence of 13CO2 causes a change in the splitting of the Fermi diad of 12CO2 and affects the establishment of CO2 Raman densitometers. The positive correlation obtained between the peak intensity ratio and the content ratio is affected by temperature, pressure, and 13CO2 content. I+13/I+12 and I-13/I-12 were selected as the quantitative indexes to establish Raman quantitative analysis models for the determination of the carbon isotope composition of CO2, which can be applied to various carbon isotope labeling experiments and field measurements.

Keywords: Raman spectroscopy, In situ measurement, Carbon dioxide, Carbon isotopic, quantitative analysis