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[SGC33-01] Challenges in measuring oxygen isotope ratios using Raman mass spectrometry
Keywords:Raman mass spectrometry, calcite, error, oxygen isotope
Carbonate minerals are a group of minerals widely distributed on Earth and in space. Oxygen isotope ratios of carbonates have been widely used as tracers of present and past geological and environmental variations (e.g., Wang et al., 2008) Indeed, changes in oxygen isotope ratios in response to changes in the surrounding environment are recorded in stripes as the minerals grow. Carbonate minerals sometimes have growth zonings of about 1 mm width. Therefore, to accurately study changes in formation conditions, it is necessary to measure oxygen isotope ratios with a spatial resolution of about 1 mm. Previously, oxygen isotope ratios of carbonates have been analysed by stable isotope ratio mass spectrometry (IR-MS) and secondary ion mass spectrometry (SIMS). However, these methods are not suitable because they have a spatial resolution of more than a few mm. This study focused on Raman spectroscopy to resolve the spatial resolution. It has been reported that the precision of the oxygen isotope ratio of calcite is 210‰, making its application in nature difficult (McKay et al., 2013). Recently, however, isotope ratio analyses of carbon dioxide and nitrogen have been reported (e.g. Yamamoto and Hagiwara, 2022; Hagiwara et al., 2023). Some of these have reached natural applications. We therefore re-examined the oxygen isotope ratios of calcite again.
We measured the Raman spectra of calcite after various exposure times at two pixel resolutions in order to assess the precision of the Raman spectra attributed to CO3 (12C16O16O18O and 12C16O3). Calculations performed to assess the influence of the measurement conditions show that the precision of the oxygen isotope ratio is a function of pixel resolution, Raman peak intensity and peak width (Hagiwara et al., 2023). obtained at two gratings (1200 and 1800 lines/mm). The relationship between the precision of the Raman spectra and exposure time showed that the precision of both the Raman spectra intensity and area ratio improved with increasing exposure time, in agreement with the calculated results. However, the rate of increase in precision with increasing exposure time decreased for both gratings when the exposure time exceeded about 90 s. When measured with 1200 lines/mm grating and an exposure time of 900 s, the accuracy of the 12C16O16O18O/12C16O3 spectra was 2.69 ± 0.84‰ for the intensity and area ratio, respectively, 2.69 ± 0.84 ‰ and 3.26 ± 0.74 ‰ precision was obtained.
However, the overall precision of the 12C16O16O18O/12C16O3 intensity and area ratios monitored over 60 hours with an exposure time of 900 s was 19.1‰ and 20.9‰ for the intensity and area ratios. The highest precision of 1.75‰ was observed for each of the 20 results. This is consistent with the 12C16O16O18O/12C16O3 intensity and area ratio are affected by changes in room temperature and voltage during long-term measurements. Therefore, room temperature and voltage should be kept constant..
These results are 70 times better than the best precision obtained so far and could be applied to studies with natural calcite, which has a large oxygen isotope distribution.
We measured the Raman spectra of calcite after various exposure times at two pixel resolutions in order to assess the precision of the Raman spectra attributed to CO3 (12C16O16O18O and 12C16O3). Calculations performed to assess the influence of the measurement conditions show that the precision of the oxygen isotope ratio is a function of pixel resolution, Raman peak intensity and peak width (Hagiwara et al., 2023). obtained at two gratings (1200 and 1800 lines/mm). The relationship between the precision of the Raman spectra and exposure time showed that the precision of both the Raman spectra intensity and area ratio improved with increasing exposure time, in agreement with the calculated results. However, the rate of increase in precision with increasing exposure time decreased for both gratings when the exposure time exceeded about 90 s. When measured with 1200 lines/mm grating and an exposure time of 900 s, the accuracy of the 12C16O16O18O/12C16O3 spectra was 2.69 ± 0.84‰ for the intensity and area ratio, respectively, 2.69 ± 0.84 ‰ and 3.26 ± 0.74 ‰ precision was obtained.
However, the overall precision of the 12C16O16O18O/12C16O3 intensity and area ratios monitored over 60 hours with an exposure time of 900 s was 19.1‰ and 20.9‰ for the intensity and area ratios. The highest precision of 1.75‰ was observed for each of the 20 results. This is consistent with the 12C16O16O18O/12C16O3 intensity and area ratio are affected by changes in room temperature and voltage during long-term measurements. Therefore, room temperature and voltage should be kept constant..
These results are 70 times better than the best precision obtained so far and could be applied to studies with natural calcite, which has a large oxygen isotope distribution.