Laser-spectrometric gas analysis: CO₂–TDLAS at 2 µm

Abstract

Employing direct absorption spectroscopy and using a spectrometer comprising a single-pass and a multipass white cell, we probed the R(12) line of carbon dioxide (CO2) in the combination band around 2 µm. Gravimetric gas standards containing CO2, between 300 and 60 000 µmol mol−1 (0.03% to 6%), in N2 were quantified by means of the TILSAM method. The spectrometric results were compared with the gravimetric reference values. We describe our implementation of the 'Guide to the Expression of Uncertainty in Measurements' to infrared laser-spectrometric gas analysis. Data quality objectives are addressed by uncertainty and traceability flags. Uncertainty budgets are presented to show the quality of the results and to demonstrate software-assisted uncertainty assessment. The relative standard uncertainties of the spectrometrically measured CO2 amount fractions at, e.g., ambient levels of 360 µmol mol−1 and at exhaled breath gas levels of 50 mmol mol−1 were 1.4% and 0.7%, respectively. Our detection limit was 2.2 µmol mol−1. The reproducibility of individual results was in the ±1% range. Furthermore, we measured collisional broadening coefficients of the R(12) line of CO2 at 4987.31 cm−1. The relative standard uncertainties of the measured self-, nitrogen-, oxygen- and air-broadening coefficients were in the ±1.7% range.