Precision evaluation of nitrogen isotope ratios by Raman spectrometry.

We measured Raman spectra of N fluids obtained at 0.1-25 MPa at room temperature. The NN peak in the Raman spectrum of a low-pressure N fluid is difficult to detect because of the prevalence of a group of peaks attributed to rotational vibration of N. The Raman peaks of NN and N of N 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 NN and N 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 N. 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.