Analysis of the Stable Isotope Ratios (18O/16O, 17O/16O, and D/H) in Glacier Water by Laser Spectrometry

A Highly Sensitive TDLAS-Based Water Vapor Isotopes Sensor Using a Quantum Cascade Laser

Based on tunable diode laser absorption spectroscopy (TDLAS), a water isotopes detection system was developed to detect the isotopic abundance of water vapor in the atmosphere. A single 1483.79 cm-1 quantum cascade laser (QCL) and a 3120 cm optical path multi-pass cell (MPC) were adopted in the detection system. The selected spectral range, as well as the laser technology used, is particularly interesting for the real-time monitoring of water vapor isotopes in the atmosphere. In this study, a single laser can be used to perform high-sensitivity, rapid investigations of H2O, H218O, H217O, and HDO absorption lines. Finally, we measured the abundance values of three isotopes of water vapor in the atmosphere and compared them with data from the Global Network of Isotopes in Precipitation (GNIP) website, dedicated to exploring the possibility of in situ monitoring of H₂O isotopes in the atmosphere.

High-Precision and Real-Time Measurement of Water Isotope Ratios Based on a Mid-Infrared Optical Sensor

A compact spectrometer based on a mid-infrared optical sensor has been developed for high-precision and real-time measurement of water isotope ratios. The instrument uses laser absorption spectroscopy and applies the weighted Kalman filtering method to determine water isotope ratios with high precision and fast time response. The precision of the measurements is 0.41‰ for δ18O and 0.29‰ for δ17O with a 1 s time. This is much faster than the standard running average technique, which takes over 90 s to achieve the same level of precision. The successful development of this compact mid-infrared optical sensor opens up new possibilities for its future applications in atmospheric and breath gas research.

In situ measurement of water vapor isotope ratios in air with a laser-based spectrometer

Simultaneous measurement of H₂¹⁷O/H₂¹⁶O, H₂¹⁸O/H₂¹⁶O, and HDO/H₂¹⁶O in air with a compact spectrometer based on a mid-infrared distributed feedback (DFB) laser was described. The obtained mixing ratios of H₂¹⁶O, H₂¹⁷O, and H₂¹⁸O agreed reasonably well with those measured by a hygrometer. The precision and repeatability of the spectrometer were analyzed. Indoor air tests demonstrated that its 220-s precision was 0.08 ‰, 0.06 ‰, and 0.14 ‰ for δ¹⁸O, δ¹⁷O, and δ²H respectively. The measured values of δ¹⁸O, δ¹⁷O, and δ²H in indoor air were highly correlated with the water vapor mixing ratios. The compact spectrometer provides in situ measurements of water vapor isotopes with high precision and fast time response, which opens new possibilities for its application in atmospheric and hydrological research in the future.

Optically Switched Dual-Wavelength Cavity Ring-Down Spectrometer for High-Precision Isotope Ratio Measurements of Methane δD in the Near Infrared

We report a spectrometer employing optically switched dual-wavelength cavity ring-down spectroscopy (OSDW-CRDS) for high-precision measurements of methane isotope ratios. A waveguide optical switch rapidly alternated between two wavelengths to detect absorption by two isotopologues using near-infrared CRDS. This approach alleviated common-mode noise that originated primarily from temperature and frequency fluctuations. We demonstrated the measurement of δD in natural abundance methane to a precision of 2.3 ‰, despite the lack of active temperature or frequency stabilization of the cavity. The ability of alternating OSDW-CRDS to improve the isotope precision in the absence of cavity stabilization were measured by comparing the Allan deviation with that obtained when frequency-stabilizing the cavity length. The system can be extended to a wide variety of applications such as isotope analysis of other species, kinetic isotope effects, ortho-para ratio measurements, and isomer abundance measurements. Furthermore, our technique can be extended to multiple isotope analysis or two species involved in kinetics studies through the use of multiport or high-speed optical switches, respectively.