A cavity ringdown spectrometer for methane isotope analysis using a 1.65 µm distributed feedback diode laser with...

Area of Science:

• Atmospheric Science
• Analytical Chemistry
• Spectroscopy

Background:

• Methane (CH4) is a potent greenhouse gas, and isotopic analysis (12CH4 and 13CH4) is crucial for source apportionment.
• High-precision methane isotope analysis requires sensitive and stable instrumentation.

Purpose of the Study:

• To develop a 1.65 µm cavity ringdown methane spectrometer for precise methane isotope analysis.
• To improve instrument sensitivity and detection limits for ambient air measurements.

Main Methods:

• Implemented optical feedback with an external fiber cavity and retroreflector to reduce laser linewidth and enhance sensitivity.
• Utilized cavity ring-down spectroscopy (CRDS) for high-resolution absorption measurements.
• Evaluated detection limits using Allan-Werle plots and baseline noise analysis at 100 Torr gas pressure.

Main Results:

• Achieved a detection limit at the parts-per-trillion (ppt) level for both 12CH4 and 13CH4 concentrations.
• Observed residual periodic background oscillations limited detection to parts-per-billion (ppb) when estimated from baseline noise.
• Demonstrated direct measurement of ambient methane in atmospheric air, showing good agreement with reference values.

Conclusions:

• The developed 1.65 µm CRDS spectrometer with optical feedback offers high sensitivity for methane isotope analysis.
• The instrument is capable of directly measuring ambient methane concentrations and 13CH4 ratios in real-time.
• Further improvements are needed to overcome residual background oscillations for optimal ppb-level detection.