Validation of spectroscopic gas analyzer accuracy using gravimetric standard gas mixtures: impact of background gas composition on CO₂ quantitation by cavity ring-down spectroscopy

Abstract. The effect of background gas composition on the measurement of CO₂ levels was investigated by wavelength-scanned cavity ring-down spectrometry (WS-CRDS) employing a spectral line centered at the R(1) of the (3 0⁰ 1)_III  ←  (0 0 0) band. For this purpose, eight cylinders with various gas compositions were gravimetrically and volumetrically prepared within 2σ = 0.1 %, and these gas mixtures were introduced into the WS-CRDS analyzer calibrated against standards of ambient air composition. Depending on the gas composition, deviations between CRDS-determined and gravimetrically (or volumetrically) assigned CO₂ concentrations ranged from −9.77 to 5.36 µmol mol⁻¹, e.g., excess N₂ exhibited a negative deviation, whereas excess Ar showed a positive one. The total pressure broadening coefficients (TPBCs) obtained from the composition of N₂, O₂, and Ar thoroughly corrected the deviations up to −0.5 to 0.6 µmol mol⁻¹, while these values were −0.43 to 1.43 µmol mol⁻¹ considering PBCs induced by only N₂. The use of TPBC enhanced deviations to be corrected to ∼ 0.15 %.

Furthermore, the above correction linearly shifted CRDS responses for a large extent of TPBCs ranging from 0.065 to 0.081 cm⁻¹ atm⁻¹. Thus, accurate measurements using optical intensity-based techniques such as WS-CRDS require TPBC-based instrument calibration or use standards prepared in the same background composition of ambient air.