Calibration strategies for FTIR and other IRIS instruments for accurate δ 13 C and δ 18 O measurements of CO 2 in air

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1 Calibration strategies for FTIR and other IRIS instruments for accurate δ 13 C and δ 18 O measurements of CO 2 in air E. Flores 19th WMO/IAEA Meeting on Carbon Dioxide, Other Greenhouse Gases, and Related Measurement Techniques (GGMT-2017) August 2017 Empa Dübendorf Switzerland

2 Accurate measurements of the isotopic composition of CO 2 in air FTIR Delta Ray What standards are needed to calibrate the instruments? What measurement uncertainties can be achieved? 2

3 Standards used for calibration Ref 1 Ref 2 x(co 2 ) δ 13 C δ 18 O µmol/mol VPDB VPDB x(co 2 ) δ 13 C δ 18 O µmol/mol VPDB VPDB Ref 1 Ref 2 x(co 2 ) δ 13 C δ 18 O µmol/mol VPDB VPDB x(co 2 ) δ 13 C δ 18 O µmol/mol VPDB VPDB References PURE CO 2 δ 13 C = and δ 18 O = δ low δ high δ 13 C = and δ 18 O =

4 Measuring CO 2 isotopes by Infrared Spectroscopy Simplified view of absorption spectroscopy of CO 2 in air samples Typical transmittance spectrum ~ 99% of 12 C 16 O 16 O ~ 1% of 13 C 16 O 16 O I 0 (ν) I 0 (ν)e ( α(ν)l) L δδ 13 CC = δδ 18 OO = RR RR VVVVVVVV CCCC2 RR RR VVVVVVVV CCCC2 Conversion to delta scale Requires standards of known isotopic composition Measurement provides mole fractions RR 13 = xx 636 RR 18 = xx is 13 C 16 O 16 O 626 is 12 C 16 O 16 O 628 is 12 C 18 O 16 O 4

5 1111 Which RR VVVVVVVV CCCC and RR VVVVVVVV CCCC22 to use?? δδ 13 CC = δδ 18 OO = RR RR VVVVVVVV CCCC22 RR RR VVVVVVVV CCCC RR VVVVVVVV CCCC2, RR VVVVVVVV CCCC2 and RR CCCC2 VVVVVVVV values were taken from Brand et al for VPDB-CO 2 (RR VVVVVVVV CCCCCC = , RR VVVVVVVV CCCCCC 1111 = and RR VVVVVVVV CCCCCC = ) since those correspond to parameters used by the MPI-Jena (λλ=0.528). δ 13 C and δ 18 O are the delta values expressed in per mil, which in this case were measured by the MPI-Jena (realization of the VPDB-CO 2 scale). Brand Willi, A.; Assonov Sergey, S.; Coplen Tyler, B. In Pure Appl. Chem., 2010; Vol. 82, pp

6 Calibration strategy Ref 1 Principle : independent two points calibrations of each isotopologue, using standards of same δ 13 C but different mole fraction to bracket the target sample x(co 2 ) δ 13 C δ 18 O µmol/mol VPDB VPDB RR 13 = xx 636 RR 18 = xx Ref 2 x 626 x 636 x 628 µmol/mol µmol/mol µmol/mol x(co 2 ) δ 13 C δ 18 O µmol/mol VPDB VPDB x(co 2 ) δ 13 C δ 18 O µmol/mol VPDB VPDB ?? x 626 x 636 x 628 µmol/mol µmol/mol µmol/mol

7 Calibration strategy Ref 1 Principle : independent two points calibrations of each isotopologue, using standards of same δ 13 C but different mole fraction to bracket the target sample FTIR Transmitance Measured Fitted Residual RR 13 = xx 636 RR 18 = xx Ref 2 xx 636 xx 628 x 626 x 636 x 628 µmol/mol µmol/mol µmol/mol Wavenumber/cm -1 FTIR xx 636 xx 628 Transmitance Measured Fitted x 626 x 636 x 628 µmol/mol µmol/mol µmol/mol Residual Wavenumber/cm

8 Calibration strategy Ref 1 Principle : independent two points calibrations of each isotopologue, using standards of same δ 13 C but different mole fraction to bracket the target sample FTIR Transmitance Measured Fitted Residual RR 13 = xx 636 RR 18 = xx xx 636 xx Ref 2 x 626 x 636 x 628 µmol/mol µmol/mol µmol/mol Wavenumber/cm -1 / (ppm) sample Std 2 Std Measured FTIR Fitted x / (µmol mol -1 ) 0.85 xx 636 xx 628 Transmitance x 626 x 636 x 628 µmol/mol µmol/mol µmol/mol Residual Wavenumber/cm

9 Calibration strategy Principle : independent two points calibrations of each isotopologue, using standards of same δ 13 C but different mole fraction to bracket the target sample xx FTIR 626 xx 636 xx 628 Unknown RR 13 = xx 636 RR 18 = xx x 626 x 636 x 628 µmol/mol µmol/mol µmol/mol / (ppm) sample Std 2 x(co 2 ) δ 13 C δ 18 O µmol/mol VPDB CO2 VPDB CO Std 1 x 626 / (µmol mol -1 ) 9

10 Delta Ray Delta Ray uses a tunable diode laser that scans over a small spectral region in which the 12 CO 2 and 13 CO 2 isotopologues have absorption lines, fits the two corresponding peaks, determines their areas and calculate the ratio between both to provide δ-values. Output are delta values. Calibrated with pure CO 2 references, anchored to VPDB- CO 2. CO 2 in air Pure CO 2 10

11 Delta Ray Calibration Strategy Get Ready procedure Original idea : dilute the reference standards (pure CO 2 ) to match the sample δ low 2 References PURE CO 2 Carrier gas CO 2 free air δ high δ ref x(co 2 ) δ sample Bias > uncertainty The first measurements demonstrated the existence of a bias between Delta Ray instrument measured values and the reference values for these samples traceable to the VPDB-CO 2 (j-ras06) scale. When measuring samples with reference values of δ 13 C = and δ 18 O = the Get ready procedure calibrated instrument indicated δ 13 C = and δ 18 O =

12 Delta Ray Calibration Strategy Get Ready procedure Original idea : dilute the reference standards (pure CO 2 ) to match the sample 2 References PURE CO 2 δ low δ high Carrier gas CO 2 free air δ ref x(co 2 ) δ sample Modified calibration : add 2 CO 2 in air references and perform a 2 point calibration of δ values Sample Air Ref µmol mol -1 δ 13 C = δ 18 O = Sample Air Ref µmol mol -1 δ 13 C = δ 18 O = δ 13 C Measured Std 1 sample δ 13 C Calibrated Std 2 12

13 Uncertainty for FTIR δ 13 C and δ 18 O measurements 1. Standard Approach 2. Sensitivity Study Individual isotopologue mole fraction: Varying the calibration input parameters (GUM) two-point calibration process, model equation for each isotopologue deduced from the two calibration standards reference values and FT-IR responses. parameter variation chosen according by its standard uncertainty x i u(x i ) to x i + u(x i ). contribution to the total standard uncertainty was considered equal to half the variation (ΔY) observed in the delta values. 13

14 Uncertainty for FTIR δ 13 C and δ 18 O measurements 626 calibration Final uncertainties on delta values, for the «air like» sample: 636 calibration δ 13 C u(δ 13 C) δ 18 O u(δ 18 O) VPDB CO RR 1111 = xx xx uu RR 1111 = RR 1111 uu xx xx uu RR 1111 ~ uu xx xx Main component is the certified CO 2 mole fraction Uncertainty on delta value negligible (only repeatability, no uncertainty on VPDB) 14

15 Uncertainty for FTIR δ 13 C and δ 18 O measurements 2. Sensitivity Study δ 13 C u(δ 13 C) δ 18 O u(δ 18 O) VPDB CO Confirmed that for δ 13 C the main uncertainty contributor comes from the uncertainty of the mole fraction of both CO 2 /air mixtures used to calibrate the FTIR the δ 18 O main uncertainty contributor is FT-IR response (93% of the index for the total uncertainty combining the measurements of cylinders 1, 2, and 3). 15

16 Delta Ray Uncertainty Short term stability for δ 13 C and δ 18 O measurements 10 seconds average measurement for a CO 2 /air at μmol mol 1 nominal mole fraction. 16

17 Validation by comparison with value assigned by IRMS FTIR measurements on 3 samples : IRIS (Delta Ray) measurements on 3 samples : (δ 13 C FTIR - δ 13 C VPDB ) / st 2 nd 3 th (δ 13 C IRIS -δ 13 C VPDB ) / st 2 nd 3 th Ref µmol mol -1 δ 13 C = δ 18 O = δ 13 C VPDB / Ref µmol mol -1 δ 13 C = δ 18 O = Sample Air Ref µmol mol -1 δ 13 C = δ 18 O = δ 13 C VPDB / Sample Air Ref µmol mol -1 δ 13 C = δ 18 O =

18 FTIR vs Delta Ray Conclusions: Calibration methods FTIR: two cylinders with the same delta values and different mole fractions, bracketing the mole fractions to be measured, δ 13 C and δ 18 O values can be measured with uncertainties of u δ13c_ft = 0.09 and u δ18o_ft = 1.03 Delta Ray: residual bias in the δ 13 C and δ 18 O remaining after manufacturer s calibration procedure removed by the application of an additional two point calibration (two CO 2 in air standards with known but differing isotopic composition). The resulting standard uncertainty of measurements achieved was u δ13c_drt = 0.18 and u δ18o_dr = CO 2 in air CO 2 in air Pure CO 2 18

19 Thank you! Viallon J. Moussay P. Wielgosz R.I., George C. Rhoderick (NIST) Paul Brewer (NPL) Willi A. Brand (MPI-Jena) Griffith D.W.T. The Gas Analysis Working Group (GAWG) of the Consultative Committee for Amount of Substance: Metrology in Chemistry and Biology (CCQM) for the useful discussions. 19