Precision evaluation of nitrogen isotope ratios by Raman spectrometry

We measured Raman spectra of N₂ fluids obtained at 0.1-25 MPa at room temperature. The ¹⁴N¹⁵N peak in the Raman spectrum of a low-pressure N₂ fluid is difficult to detect because of the prevalence of a group of peaks attributed to rotational vibration of ¹⁴N₂. The Raman peaks of ¹⁴N¹⁵N and ¹⁴N₂ of N₂ fluid at 25 MPa were acquired at various exposure times. The mean values and standard deviations of the peak height ratios and peak area ones of ¹⁴N¹⁵N and ¹⁴N₂ were examined for each time. The standard deviations of the peak height ratios and peak area ones were 2.2% and 1.9%, respectively, for 20 spectra acquired with peak height of 1 million counts of ¹⁴N₂. The uncertainties are about two times higher than those predicted from the noise of a CCD. Improvement of the pixel resolution can enhance the precision of the nitrogen isotope ratios by Raman spectroscopy.