Validation of spectroscopic gas analyzer accuracy using gravimetric standard gas mixtures: impact of background gas composition on CO 2 quantitation by cavity ring-down spectroscopy
Effect of background gas composition on the measurement of CO 2 levels was investigated by wavelength-scanned cavity ring-down spectrometry (WS-CRDS) employing a spectral line centered at the R(1) of the (3 0° 1) III  ← (0 0 0) band. For this purpose, eight cylinders with various gas compositions were gravimetrically and manometrically 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 manometrically) assigned CO 2 concentrations ranged from −9.77 to 5.36 μmol/mol, e.g., excess N 2 exhibited a negative deviation, whereas excess Ar showed a positive one. The total pressure broadening coefficients (TBPCs) obtained from the composition of N 2 , O 2 and Ar thoroughly corrected the deviations up to −0.5–0.6 μmol/mol, while these values were −0.43–1.43 μmol/mol considering PBCs induced by only N 2 . The use of TBPCs enhanced deviations to be corrected to ~ 0.15 %. Furthermore, the above correction linearly shifted CRDS responses for a wide 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 TBPC-based instrument calibration.