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Measuring 13C-enriched CO2 in air with a cavity ring-down spectroscopy gas analyser : evaluation and calibration

Dane Dickinson (UGent) , Samuel Bodé (UGent) and Pascal Boeckx (UGent)
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Abstract
Rationale: Cavity ring-down spectroscopy (CRDS) is becoming increasingly popular for C-13-CO2 analysis of air. However, little is known about the effect of high C-13 abundances on the performance of CRDS. Overlap between (CO2)-C-12 and (CO2)-C-13 spectral lines may adversely affect isotopic-CO2 CRDS measurements of C-13-enriched samples. Resolving this issue is important so that CRDS analysers can be used in CO2 flux studies involving C-13-labelled tracers. Methods: We tested a Picarro G2131-i CRDS isotopic-CO2 gas analyser with specialty gravimetric standards of widely varying C-13 abundance (from natural to 20.1 atom%) and CO2 mole fraction (xCO(2): <0.1 to 2116ppm) in synthetic air. The presence of spectroscopic interference between (CO2)-C-12 and (CO2)-C-13 bands was assessed by analysing errors in measurements of the standards. A multi-component calibration strategy was adopted, incorporating isotope ratio and mole fraction data to ensure accuracy and consistency in corrected values of C-13-CO2, x(12)CO(2), and x(13)CO(2). Results: CRDS measurements of x(13)CO(2) were found to be accurate throughout the tested range (<0.005 to 100ppm). On the other hand, spectral cross-talk in x(12)CO(2) measurements of standards containing elevated levels of (CO2)-C-13 led to inaccuracy in x(12)CO(2), total-xCO(2) (x(12)CO(2)+x(13)CO(2)), and C-13-CO2 data. An empirical relationship for x(12)CO(2) measurements that incorporated the C-13/C-12 isotope ratio (i.e. (CO2)-C-13/(CO2)-C-12, RCO2) as a secondary (non-linear) variable was found to compensate for the perturbations, and enabled accurate instrument calibration for all CO2 compositions covered by our standard gases. Conclusions: (13)C-enrichement in CO2 leads to minor errors in CRDS measurements of x(12)CO(2). We propose an empirical correction for measurements of C-13-enriched CO2 in air by CRDS instruments such as the Picarro G2131-i.
Keywords
ATMOSPHERIC (CO2)-C-13/(CO2)-C-12 MEASUREMENT, CARBON-DIOXIDE, ISOTOPE RATIO, HALF-WIDTHS, DIFFERENT TEMPERATURES, STABLE-ISOTOPES, AMBIENT AIR, SPECTROMETER, C-13, DELTA-C-13

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Chicago
Dickinson, Dane, Samuel Bodé, and Pascal Boeckx. 2017. “Measuring 13C-enriched CO2 in Air with a Cavity Ring-down Spectroscopy Gas Analyser : Evaluation and Calibration.” Rapid Communications in Mass Spectrometry 31 (22): 1892–1902.
APA
Dickinson, D., Bodé, S., & Boeckx, P. (2017). Measuring 13C-enriched CO2 in air with a cavity ring-down spectroscopy gas analyser : evaluation and calibration. RAPID COMMUNICATIONS IN MASS SPECTROMETRY, 31(22), 1892–1902.
Vancouver
1.
Dickinson D, Bodé S, Boeckx P. Measuring 13C-enriched CO2 in air with a cavity ring-down spectroscopy gas analyser : evaluation and calibration. RAPID COMMUNICATIONS IN MASS SPECTROMETRY. 2017;31(22):1892–902.
MLA
Dickinson, Dane, Samuel Bodé, and Pascal Boeckx. “Measuring 13C-enriched CO2 in Air with a Cavity Ring-down Spectroscopy Gas Analyser : Evaluation and Calibration.” RAPID COMMUNICATIONS IN MASS SPECTROMETRY 31.22 (2017): 1892–1902. Print.
@article{8531205,
  abstract     = {Rationale: Cavity ring-down spectroscopy (CRDS) is becoming increasingly popular for C-13-CO2 analysis of air. However, little is known about the effect of high C-13 abundances on the performance of CRDS. Overlap between (CO2)-C-12 and (CO2)-C-13 spectral lines may adversely affect isotopic-CO2 CRDS measurements of C-13-enriched samples. Resolving this issue is important so that CRDS analysers can be used in CO2 flux studies involving C-13-labelled tracers. 
Methods: We tested a Picarro G2131-i CRDS isotopic-CO2 gas analyser with specialty gravimetric standards of widely varying C-13 abundance (from natural to 20.1 atom\%) and CO2 mole fraction (xCO(2): {\textlangle}0.1 to 2116ppm) in synthetic air. The presence of spectroscopic interference between (CO2)-C-12 and (CO2)-C-13 bands was assessed by analysing errors in measurements of the standards. A multi-component calibration strategy was adopted, incorporating isotope ratio and mole fraction data to ensure accuracy and consistency in corrected values of C-13-CO2, x(12)CO(2), and x(13)CO(2). 
Results: CRDS measurements of x(13)CO(2) were found to be accurate throughout the tested range ({\textlangle}0.005 to 100ppm). On the other hand, spectral cross-talk in x(12)CO(2) measurements of standards containing elevated levels of (CO2)-C-13 led to inaccuracy in x(12)CO(2), total-xCO(2) (x(12)CO(2)+x(13)CO(2)), and C-13-CO2 data. An empirical relationship for x(12)CO(2) measurements that incorporated the C-13/C-12 isotope ratio (i.e. (CO2)-C-13/(CO2)-C-12, RCO2) as a secondary (non-linear) variable was found to compensate for the perturbations, and enabled accurate instrument calibration for all CO2 compositions covered by our standard gases. 
Conclusions: (13)C-enrichement in CO2 leads to minor errors in CRDS measurements of x(12)CO(2). We propose an empirical correction for measurements of C-13-enriched CO2 in air by CRDS instruments such as the Picarro G2131-i.},
  author       = {Dickinson, Dane and Bod{\'e}, Samuel and Boeckx, Pascal},
  issn         = {0951-4198},
  journal      = {RAPID COMMUNICATIONS IN MASS SPECTROMETRY},
  keyword      = {ATMOSPHERIC (CO2)-C-13/(CO2)-C-12 MEASUREMENT,CARBON-DIOXIDE,ISOTOPE RATIO,HALF-WIDTHS,DIFFERENT TEMPERATURES,STABLE-ISOTOPES,AMBIENT AIR,SPECTROMETER,C-13,DELTA-C-13},
  language     = {eng},
  number       = {22},
  pages        = {1892--1902},
  title        = {Measuring 13C-enriched CO2 in air with a cavity ring-down spectroscopy gas analyser : evaluation and calibration},
  url          = {http://dx.doi.org/10.1002/rcm.7969},
  volume       = {31},
  year         = {2017},
}

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