ABSTRACT

We measured Raman spectra of calcite at various exposure times using a spectrometer with two gratings (1800 and 1200 grooves/mm) to evaluate the precision of the relative ratio of Raman peaks (¹²C¹⁶O¹⁶O¹⁸O/¹²C¹⁶O₃), which is attributed to ¹⁸O/¹⁶O. Preliminary calculations showed that the uncertainty in the Raman peak ratio is a function of the Raman peak intensity and the pixel resolution. The relation between the Raman peak ratio and the exposure time obtained with two gratings showed that the precisions of both the intensity and area ratio of the Raman peaks improved with increasing exposure time for both gratings, which is consistent with the calculations. However, for exposure time greater than approximately 90 s, the rate of improvement in precision with increasing exposure time decreased for both gratings. Continuous monitoring of the Raman peak ratio for more than 60 hr revealed that the decrease in precision improvement can be attributed to external factors such as room temperature fluctuation. Nevertheless, we obtained the best precision values of 2.69 ± 0.84‰ and 3.26 ± 0.74‰, respectively, for the ¹²C¹⁶O¹⁶O¹⁸O/¹²C¹⁶O₃ intensity and area ratio when measuring using a 1200 grooves/mm grating and 900 s exposure time. Those findings are 70 times better than the best precision achieved to date. They might be applicable to studies using natural calcites with large oxygen isotopic variation.