Cavity-Enhanced Absorption Spectroscopy
Detects Geothermal Gases

Lauren Rugani

Gas phase chromatography and mass spectrometry often are used to analyze volcanic emissions because of their ability to detect low levels of trace gases. However, these techniques are limited by their instrumental complexity and inability to withstand the conditions at an active volcanic site.

A minivan that carried instruments for the volcanic gas analysis is shown parked close to the main fumarole inside the crater of the inactive Solfatara volcano near Naples, Italy.

Researchers at Université J. Fourier de Grenoble in St. Martin d’Hères and Université Montpellier, both in France, and Seconda Università di Napoli in Caserta, Italy, have proposed a method for detecting volcanic emissions in situ based on distributed feedback diode lasers that are efficiently coupled with cavity-enhanced absorption spectroscopy by exploiting optical feedback. The team applied this method to probe for the presence of CO, NH₃ and CH₄ 20 m away from an active fumarole of the Solfatara volcano near Naples, Italy, where the gaseous emissions are dominated by H₂O and CO₂.

“The real-time continuous monitoring of volcanic emissions could permit the early recognition of changes in the activity status that can be a precursor to eruptive or seismic activity,” explained team member Daniele Romanini. Unlike gas phase chromatography or mass spectrometry, optical-feedback cavity-enhanced absorption spectroscopy does not require calibration standards and can operate unattended for several days in a nonlaboratory setting.

For the technique, they used an extended wavelength GaSb-based diode laser operated at 2.33 μm. This wavelength is in an atmospheric transmission window that does not allow the absorption of carbon dioxide or water but does permit simultaneous detection of the less prominent gases of interest.

Light from the diode laser was injected into a V-shaped high-finesse resonator cavity that contained three highly reflective dielectric mirrors. When the laser frequency matches one of the cavity resonances, optical field build-up occurs and may feed back to the laser, causing the laser emission to become narrower than the resonances. An absorption spectrum is obtained by detecting the maximum light transmitted when the cavity is filled at each mode.

The volcanic gas was passed through a gas dryer and cooler, condensing and collecting water vapor at the bottom of a cold trap and allowing the dry gas to be carried to the spectrometer.

Concentration measurements as a function of time showed that there were no variations in the levels of CO and CH₄ during the analysis, while observed variations in ammonia concentration were attributed to its solubility in water and retention in the trap. Changes in the concentration of one species did not affect the ability to detect absorption lines of other species in the same region. Measured concentrations for CO and methane were found to be 3 and 76 ppm, respectively.

Detection limits of 16 ppb, 14 ppb and 0.4 ppm (at 8 Hz) were obtained for simultaneous measurement of CO, NH₃ and CH₄, respectively. Altering the laser temperature to address nearby absorption lines can lower the detection limit for CH₄ or NH₃ by a factor of 10.

The small sample volume requires little gas flow and a light membrane pump for low-pressure operation, which narrows the absorption lines of different species. “This allows for selective measurements, with low sensitivity to the presence of other species than those being measured,” Romanini said.

The team is working to improve the detection limit and measurement stability of the device and has employed the technique in monitoring greenhouse gases. Applications also might include monitoring industrial toxic emissions and noninvasive medical diagnostics through breath analysis.

Optics Express, Nov. 13, 2006, pp. 11442-11452.

Cavity-Enhanced Absorption Spectroscopy Detects Geothermal Gases

Cavity-Enhanced Absorption Spectroscopy
Detects Geothermal Gases