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12 Aug 2008
A new design of ring-down spectrometer allows in situ measurements of methane, carbon dioxide, and a carbon dioxide isotope in the same instrument.
Cavity ring-down spectroscopy (CRDS) is one of several analytical techniques that can allow measurements of atmospheric species in a variety of ecological studies. Now a US team has developed a CRDS instrumentation platform that utilizes one ring-down cavity to measure three species of interest in environmental and industrial applications: CH4, CO2 and the 13CO2 isotope (Appl. Phys. B 92 2 259).
"By carefully selecting the spectral fingerprints and using wavelength multiplexing, we use one set of ring-down optics to measure three species," Chuji Wang of Mississippi State University told optics.org. "This means that the instrument is low in both cost and power consumption, and compact in size."
The team’s device is based on near-IR continuous-wave CRDS. It takes its measurements at different wavelengths through multiplexing two distributed feedback laser diodes with central wavelengths at 1597 and 1650 nm, both having a maximum output power of 20 mW. The spectrometer weighs 16 kg and measures 50 x 40 x 15 cm, allowing it to be transported by a single person and used for in situ analyses at different locations.
Gas concentrations are measured in less than one second, so results are obtained in almost real-time, and switching from measuring one species to another takes less than one minute. "Changing from measuring CH4 to CO2, for example, can be achieved using just the spectrometer’s touch-screen controls," said Wang. "If the spectrometer is connected to the internet, it can be changed remotely from a computer."
Other instruments based on CRDS or similar techniques that measure the concentrations of these three particular gases are already available, but Wang’s device has the advantage of measuring all three in a single instrument. "This spectrometer has been developed for the US DOE National Energy Technology Laboratory for its carbon sequestration applications," commented Wang. "It requires a portable, battery-powered, sensitive instrument for in situ simultaneous quantification of these three species in CO2 injection sites. Currently, no other instrument is available that can achieve this."
At present, the Mississippi team’s design does have an inbuilt application limitation, in that its detection sensitivity is relatively low and its measurement uncertainty higher than other conventional spectroscopic methods. Detection limits for CH4 and CO2 are 0.2 and 120 ppmv respectively, while the measurement uncertainty is better than ±4% of full-scale reading for both species.
"Our spectrometer uses relatively cheap mirrors and a short cavity length that compromise the detection sensitivity," noted Wang. "The current measurement uncertainty is actually reasonable, considering the absence of temperature control of the gas cell. Both parameters can certainly be improved in the future. What we have demonstrated here is a technology platform that could now be adapted to develop spectrometers for a variety of species in a range of applications."
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