Performance Evaluation of a New, Tunable-Diode Laser Trace-Gas Analyzer for Isotope Ratios of Carbon Dioxide

Abstract:

Newly available interband cascade lasers (ICLs) have enabled the development of a family of tunable-diode laser trace-gas analyzers that do not require liquid nitrogen to cool the laser. The lasers are available in the 3000 to 6000 nm range, providing access to the strong mid-infrared absorption lines for important gases such as methane, nitrous oxide, and carbon dioxide. These ICLs are fabricated with distributed feedback to improve their stability and spectroscopic quality. A recently released trace-gas analyzer for carbon dioxide isotopes (TGA200A, Campbell Scientific, Inc.) was evaluated for short- and long-term precision using Allan variance. Accuracy and linearity of CO₂ mole fraction was assessed with a set of seven NOAA standard reference gases ranging from 298.35 to 971.48 ppm. Dilution of high-concentration CO₂ with CO₂-free air demonstrated the linearity of isotope ratio measurements beyond 1000 ppm CO₂. Two analyzer variants were tested: one for CO₂, δ¹³C and δ¹⁸O; and the other for CO₂ and δ¹³C at enhanced precision.