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Home > Teams > LAsers, Molecules and Environment > Themes > Measurement of the isotopic ratios

Measurement of the isotopic ratios

Roberto GRILLI

The quantification of isotopic species of small molecules presents an important analytical tool in a wide variety of research fields, including, e.g., atmospheric chemistry, biomedicine, and hydrology. The conventional method to measure the so-called isotope ratios (e.g., [13C]/[12C] in CO2, or [D]/[H] in H2O) is isotope ratio mass spectrometry (IRMS). These instruments have benefitted from over 50 years of commercial development and can reach extreme levels of precision and sensitivity, at the cost of complex and expensive instrumentation and infrastructure. We have been at the forefront of modern developments using laser techniques [Kerstel et al. 1999, Kerstel 2004] to provide an alternative that addresses some of the most important drawbacks of IRMS: cost, size, restriction to a dedicated laboratory, necessity of a skilled operator, and batch mode operation. We have successfully applied our earlier prototype instruments to, among others, biomedicine and climate research. Here we present current research projects of our team.
For background information on isotope research with lasers (stable isotope ratio infrared spectroscopy, or SIRIS), or information on older research projects (notably those carried out at the Groningen Center for Isotope Research), please look here.

Based on significant technological advances and unconventional approaches, the SUBGLACIOR project aims at revolutionizing ice core research by inventing, constructing and testing an in-situ probe to evaluate if a target site is suitable to recover ice as old as 1.5 million years, all within a single season in Antarctica. The SUBGLACIOR probe will make its own way into the ice and, relying on innovative laser technology, patented by the UJF, will measure in real time and down to bedrock the depth profiles of the ice deuterium isotopic signal (\deltaD), as well as the trapped CH4 gas concentration. The deuterium isotope record of the melted ice will deliver the baseline climatic signal in the deep ice. Its evolution with depth will differentiate between ice of interglacial and glacial conditions, and enable its comparison it to marine reference records. Atmospheric CH4 shows large changes between glacial and interglacial states. It is an indirect tracer of northern hemisphere climate. Being recorded in trapped bubbles and clathrates, its changes are shifted with depth compared with concomitant climatic changes recorded in \deltaD of H2O because of firnification processes. The observation of such a depth shift is a primary indicator that the ice layers are still in good stratigraphic order. The probe would become a central tool to investigate the best possible sites for the "oldest ice" challenge of the IPICS international project, which involves 25 nations.
SUBGLACIOR builds on the substantial technological progress in laser physics provided by the development of an ultra-sensitive trace gas detection technique invented by the LIPhy and its applications in the atmospheric monitoring domain. Using a near-to-mid infrared implementation of OFCEAS, it is now possible to accurately and precisely measure trace gas concentrations as well as the water isotopic composition on very small gas flows and with a very compact instrument. In fact, a major challenge for the LAME team involved in this project has been to minimize the OFCEAS spectrometer to make it fit in a tube of only 50 mm internal diameter. The miniaturized OFCEAS spectrometer successfully measured methane profiles down to a depth of 600 m during an oceanic campaign in the Mediterranean off the coast of Toulon form 10 to 13 July 2014.

Carbon Isotopes in Ice Cores
We are developing an ultra-sensitive, laser-based spectrometer for the 13C analysis of atmospheric carbon dioxide in gas trapped in bubbles in ice-cores, and to couple this device to the dry gas extraction instruments of the Grenoble glaciology laboratory (LGGE). This requires the analysis of exceedingly small samples of CO2 (nano-mols) while maintaining an extremely high level of precision ( 0.05 per mil).
Compared to the current 13C measurement technique of isotope ratio mass spectrometry, the quantum cascade laser system under development will provide both a higher measurement precision (necessary to resolve the small natural 13C variations) and a more than two orders of magnitude higher sample throughput, enabling us to improve the temporal resolution of the ice core signal, while at the same time running more samples for improved statistics. This is expected to lead to a true revolution in the field of ice-core paleo-climatology, which has been engaged for over one decade in a struggle to understand the mechanisms that control the natural variability of the carbon dioxide greenhouse gas between glacial and interglacial cycles. Considering the strong correlation between the carbon cycle and climate, and in light of the post-industrial revolution, anthropogenic increase of the CO2 concentration, this is becoming an ever more urgent issue.

Atmospheric Water Vapor
In the context of a globally warming climate it is crucial to study the climate variability in the past and to understand the underlying mechanisms (IPCC 2007). Precipitation deposited on the polar ice caps provides a means to retrieve information on temperature changes and atmospheric composition on time scales from one to almost one million years, with sub-annual resolution in the most recent centuries.
It is now generally accepted that the water oxygen and hydrogen isotope signals (\delta18O and \delta2H) in ice cores act as proxies of paleo-temperatures, and numerous ice-cores have been drilled and analyzed in the last decades. For a correct interpretation of the resulting datasets, the temporal relationship between the isotope signal and the (local, paleo) temperature needs to be established over the entire time scale spanned by the age of the ice. However, this calibration of the paleo-thermometer remains problematic and attempts are ongoing to provide a more physical basis of the isotope – temperature relation.
To address this principal problem with the interpretation of ice core isotope signals in terms of paleoclimate, one can try to model the global water cycle to the extent that the isotope signals in precipitation can be adequately reconstructed. Our work (the PhD projects of Janek Landsberg and Mathieu Casado) will contribute to this approach by measuring the isotopic composition of moisture carried towards and deposited on Antarctica, thus constraining the numerical models. For this, a spectrometer has been developed specifically for the measurement of water vapor isotope ratios at the very low humidity levels encountered in Antarctica.

Cloud Microphysics
Ice cloud formation and dynamics play a significant role in Greenhouse warming in the Arctic. Interaction with aerosols affecting the ozone balance and a strong radiative impact due to absorption of upwelling infrared radiation and reflection of sunlight back to space are only two examples that emphasize the importance of understanding the underlying physical mechanisms.
The ISOCLOUD project was initiated by researchers from Chicago (PI: E. Moyer), Karlsruhe (O. Mueller and H. Saathoff), and PTB/Darmstadt (V. Ebert), in order to study the underlying mechanisms of cloud formation in the controlled environment of the AIDA cloud simulation chamber, employing ultra-high sensitivity optical instruments for in-situ measurements of water vapor isotopologues. LAME joined the project with a unique, ultra-sensitive extractive water isotope ratio laser spectrometer based on the OFCEAS detection technique (thesis of Janek Landsberg, 2014).

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