9:45 AM - 10:00 AM
[PCG20-04] Isotopic Measurement of Ammonia Using Cavity Ring-Down Spectroscopy (CRDS) with a 1.49 µm Laser
Keywords:CRDS, NH3, Isotopic Ratio
1. Introduction
In planetary exploration, understanding the distribution and origin of volatile substances such as water and ammonia (NH3) is one of the key challenges. Recent remote sensing data suggest that such substances may have been preserved for long periods within permanently shadowed regions of lunar south pole craters. These materials are important both as resources for future lunar exploration and for scientific purposes. Particularly, measurements of the isotopic ratios of water and ammonia are expected to provide constraints on the origin of volatile substances in the Moon-Earth system. The CRDS (Cavity Ring-Down Spectroscopy) system being developed for planetary exploration, by combining high-reflectivity mirrors (R = 99.995% or higher), has achieved an effective optical path length of over 1 km with a compact 5 cm cavity. This high-sensitivity measurement allows the detection of water molecules at the 1 ng level in a 15 cm3 cell. CRDS is expected to be applied as a compact and highly sensitive spectroscopic instrument for planetary exploration in the future.
This study focuses on ammonia, a crucial substance from the perspective of the origin of life, and aims to evaluate the isotopic measurement performance of NH3 using CRDS with a 1.49 µm band laser. Specifically, the study investigates the fractionation of ammonia's stable isotopes (15NH3 , 14NH3 , NDH2, NH2D, ND2) and the precision of isotopic ratio measurements, evaluating the potential application of CRDS in planetary exploration.
2. Experimental Methods
In this study, a CRDS system with a 1.49 µm band laser as the light source was used to measure pure 14NH3 gas, 15NH3 gas, ND3 gas, and a mixture of heavy water and ammonia water. The CRDS system employed a cavity length of 50 cm, and with high-reflectivity mirrors (R = 99.995% or higher), the system achieved an effective optical path length of over 10 km. There is limited database information available on the spectroscopic spectra of ammonia isotopes in the near-infrared range, and almost no existing data for the wavelength range used in this study. Therefore, this study first identified the spectral peaks of each isotope and then used samples with known isotopic ratios to evaluate the precision of the isotopic ratio measurements by the CRDS system.
3. Results and Discussion
For the nitrogen isotopic ratio (15N/14N), the peaks from 15NH3 and 14NH3were successfully resolved, and based on this, isotopic ratios were measured, and a calibration line for isotopic ratio measurement precision was established. This demonstrated that the CRDS system could easily distinguish between the nitrogen isotopic ratio of Earth (δ15N = 0‰) and that of the outer solar system (e.g., comets, Kuiper Belt objects, δ15N≈ 800‰). For the deuterium isotopes, the study measured a mixture of heavy water and ammonia water (ammonia water: heavy water = 1:0.2 ~ 1:3), discovering an increasing peak corresponding to the concentration of heavy water, which was successfully identified as the deuterium isotopic peak of ammonia. However, the assignment of absorption lines to specific deuterium isotopes (NDH3, NH3D, ND3) and the establishment of a calibration line for these peaks are still ongoing. Comparative studies with FTIR and other instruments with wider spectral measurement ranges are being conducted. Further experiments are planned to improve measurement precision and establish calibration lines.
4. Conclusion and Future Outlook
This study evaluated the performance of ammonia isotopic measurements using CRDS with a 1.49 µm band laser, showing that nitrogen isotopic fractionation is achievable with high precision. However, the assignment of absorption lines and establishment of calibration lines for deuterium isotopes remain open issues. Future work will focus on advancing the measurement of deuterium isotopic ratios using CRDS.
In planetary exploration, understanding the distribution and origin of volatile substances such as water and ammonia (NH3) is one of the key challenges. Recent remote sensing data suggest that such substances may have been preserved for long periods within permanently shadowed regions of lunar south pole craters. These materials are important both as resources for future lunar exploration and for scientific purposes. Particularly, measurements of the isotopic ratios of water and ammonia are expected to provide constraints on the origin of volatile substances in the Moon-Earth system. The CRDS (Cavity Ring-Down Spectroscopy) system being developed for planetary exploration, by combining high-reflectivity mirrors (R = 99.995% or higher), has achieved an effective optical path length of over 1 km with a compact 5 cm cavity. This high-sensitivity measurement allows the detection of water molecules at the 1 ng level in a 15 cm3 cell. CRDS is expected to be applied as a compact and highly sensitive spectroscopic instrument for planetary exploration in the future.
This study focuses on ammonia, a crucial substance from the perspective of the origin of life, and aims to evaluate the isotopic measurement performance of NH3 using CRDS with a 1.49 µm band laser. Specifically, the study investigates the fractionation of ammonia's stable isotopes (15NH3 , 14NH3 , NDH2, NH2D, ND2) and the precision of isotopic ratio measurements, evaluating the potential application of CRDS in planetary exploration.
2. Experimental Methods
In this study, a CRDS system with a 1.49 µm band laser as the light source was used to measure pure 14NH3 gas, 15NH3 gas, ND3 gas, and a mixture of heavy water and ammonia water. The CRDS system employed a cavity length of 50 cm, and with high-reflectivity mirrors (R = 99.995% or higher), the system achieved an effective optical path length of over 10 km. There is limited database information available on the spectroscopic spectra of ammonia isotopes in the near-infrared range, and almost no existing data for the wavelength range used in this study. Therefore, this study first identified the spectral peaks of each isotope and then used samples with known isotopic ratios to evaluate the precision of the isotopic ratio measurements by the CRDS system.
3. Results and Discussion
For the nitrogen isotopic ratio (15N/14N), the peaks from 15NH3 and 14NH3were successfully resolved, and based on this, isotopic ratios were measured, and a calibration line for isotopic ratio measurement precision was established. This demonstrated that the CRDS system could easily distinguish between the nitrogen isotopic ratio of Earth (δ15N = 0‰) and that of the outer solar system (e.g., comets, Kuiper Belt objects, δ15N≈ 800‰). For the deuterium isotopes, the study measured a mixture of heavy water and ammonia water (ammonia water: heavy water = 1:0.2 ~ 1:3), discovering an increasing peak corresponding to the concentration of heavy water, which was successfully identified as the deuterium isotopic peak of ammonia. However, the assignment of absorption lines to specific deuterium isotopes (NDH3, NH3D, ND3) and the establishment of a calibration line for these peaks are still ongoing. Comparative studies with FTIR and other instruments with wider spectral measurement ranges are being conducted. Further experiments are planned to improve measurement precision and establish calibration lines.
4. Conclusion and Future Outlook
This study evaluated the performance of ammonia isotopic measurements using CRDS with a 1.49 µm band laser, showing that nitrogen isotopic fractionation is achievable with high precision. However, the assignment of absorption lines and establishment of calibration lines for deuterium isotopes remain open issues. Future work will focus on advancing the measurement of deuterium isotopic ratios using CRDS.