Correction of drift in Raman ¹³CO₂/¹²CO₂ by bandwidth fluctuation: implications for practical application of Raman...

Mass spectrometry using the micro-Raman spectroscopic technique has been developed to measure ¹³CO₂/¹²CO₂. A necessary step required to improve the uncertainty of this method is to mitigate the influence of external fluctuations on the device: so-called drift. To offset the drift, we examined factors interacting with drift by measuring 10 MPa CO₂ in a high-pressure optical cell. The Raman spectra of CO₂ obtained while changing the width of the entrance slit to a spectrometer showed that ¹³CO₂/¹²CO₂ is correlated linearly with the ¹³CO₂ bandwidth, suggesting that the ¹³CO₂/¹²CO₂ drift can be corrected using the bandwidth. We performed 60 consecutive measurements of Raman spectra of 10 MPa CO₂ in triplicate, with exposure times of 3 min. A linear correlation was found between the ¹³CO₂/¹²CO₂ and ¹³CO₂ bandwidths in all three measurements. Then the slopes of the calibration curves of those correlations were used to correct for the ¹³CO₂/¹²CO₂ drift. The correction was effective for both the precision of the ¹³CO₂/¹²CO₂ intensity ratio and that of the area ratio, especially improving the precision of the area ratio by a factor of 2-3. The precision of the corrected ¹³CO₂/¹²CO₂ area ratio was better than ±3‰ as the standard deviation.