Development and Metrological Characterization of a Tunable Diode Laser Absorption Spectroscopy (TDLAS) Spectrometer for Simultaneous Absolute Measurement of Carbon Dioxide and Water Vapor

Simultaneous detection of two analytes, carbon dioxide (CO₂)and water vapor (H₂O), has been realized using tunable diode laser absorption spectroscopy (TDLAS) with a single distributed feedback diode laser at 2.7 μm. The dynamic range of the spectrometer is extended from the low parts per million to the percentage range using two gas cells, a single-pass cell with 0.77 m, and a Herriott-type multipass cell with 76 m path length. Absolute measurements were carried out, i.e., amount fractions of the analytes were calculated based on previously determined spectral line parameters, without the need for an instrument calibration using gas standards. A thorough metrological characterization of the spectrometer is presented. We discuss traceability of all parameters used for amount fraction determination and provide a comprehensive uncertainty assessment. Relative expanded uncertainties (k = 2, 95% confidence level) of the measured amount fractions are shown to be in the 2-3% range for both analytes. Minimum detectable amount fractions are 0.16 μmol/mol for CO₂ and 1.1 μmol/mol for H₂0 for 76 m path length and 5 s averaging time. This corresponds to normalized detection limits of 27 μmol/mol m Hz^_1/2 for CO₂ and 221 μmol/mol m Hz^−1/2 for H₂O. Precision of the spectrometer, determined using Allan variance analysis, is 3.3 nmol/mol for CO₂ and 21 nmol/mol for H₂O. The spectrometer has been validated using reference gas mixtures with known CO₂ and H₂O amount fractions. An application example of the absolute TDLAS spectrometer as a reference instrument to validate other sensors is also presented.