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SARA: a analyzer of gas traces

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Environmental monitoring requires the analysis of chemical species present in small concentration. This same need exists in the industrial, medical, agro-alimentary domains and more. Our method based on a diode laser and a light trap (an optical resonator) allows to quantify in real time (1s) the traces of several molecules: e.g. those responsible for the greenhouse effect, some atmospheric pollutants, and also some products of human metabolism which can be found in the exhaled air.

Experimental technique

The method of Spectroscopy by Amplified Resonant Absorption (*) - SARA - attains absorption path lengths > 10 km with a simple and compact optical setup. Our implementation of this method, which exploits optical feed-back (OF-CEAS), is covered by two patents. Compared to concurrent methods, notably those based on laser absorption in a multipass cell, a 100 times longer path is obtained with a sample volume 100 times smaller.
(*)also : « Cavity-Enhanced Absorption Spectroscopy »

Two ultrasensitive measurement schemes are simultaneously exploited in our instruments...

Experimental scheme for OF-CEAS (complex physics underlying a very simple layout)


With LSCE (Laboratoire des Sciences du Climat et de l’Environnement) we compared an OFCEAS instrument measuring methane to a chromatographer (calibrated every 30 minutes). Ambient air was sampled on the building roof. Agreement has remained within 1% during 11 days with a single initial calibration of our device.
By the side, a zoom over a 1 day window for a series of tests of the same kind but concerning carbon monoxide, CO. Agreement remained within 10 ppb over almost one month of continuous measurements.
The linear response for the measurement of concentrations by OF-CEAS was demonstrated over a dynamic range of 2 decades. The available dynamic range is actually in excess of 4 decades.


Measurements aboard an airplane

Understanding the origin of the (very low) water vapor level in the stratosphere constitutes today a major challenge. Isotopic ratios of one species depend on its formation mechanism. Their measurement permits then to determine the origins of a species. A transfert of the OF-CEAS technique towards the CIO laboratory in Groningen University (Netherlands) took place and allowed to start testing the measurement of water isotopic ratios directly in the stratosphere. After test flights during 2004-2005 with NASA, a device was finally installed and worked on the European stratospheric airplane Geophysica at Burkina Faso (campaign AMMA 2006). Photos and results below.

Another hot subject is the monitoring of greenhouse gases like méthane, CO2 and CO: During 2004, aboard a NASA DC8 aircraft, an OF-CEAS instrument recorded external methane concentration during flight. The concentration peak visible on the figure to the right is associated to a passage (at low altitude) close to a cow farming site...

With LSCE, airborne measurements of CO above Orléans forest obtained in 2006. Here we compare OF-CEAS real-time results (yellow trace) with results from a chromatographer (circles) obtained over 11 air samples bottled during the flight.

Detection limits for different species

Hydro-Fluoric acid

Inspectors of IAEA need new sensitive tools for the surveillance of sensitive sites in those countries who signed the Nuclear Non-Proliferation Treaty (NPT).
Leaks of UF6, used in isotopic separation plants, would rapidly produce HF in air. With French CEA (Comité Energie Atomique), a portable system for detecting this specie is under investigation.

Volcanic emissions

Collaboration with University Naples 2 : Measurement tests of traces in the emitted gas from a fumerole in Solfatara volcan.
Perspective: detect changes in magmatic chamber of a volcan in order to foresee telluric and eruptive activities. A laser at 2.3 µm permitted to measure in parallel CH4, CO et NH3.


- CEA : DEN/DPC Service de Chimie Physique (Saclay, FR);
- CIO : Centre de Recherche des Isotopes, Univ. Groningen (NL);
- LSCE : Lab. Sciences du Climat et de l’Environnement - CEA/CNRS (Saclay, FR);
- NASA : Bay Area Environmental Research Institute, Californie (USA);
- Univ. Naples 2 : Department of Environmental Science (IT).

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