IAEA stable isotope reference materials: addressing the needs of atmospheric greenhouse gas monitoring. S.Assonov, M.Gröning and A. Fajgelj IAEA, Vienna 18th GGMT meeting, La Jolla, Sept 15, 2015
CO2, mmol /mol d 13 C of air-co 2 and air-ch 4 and requirements for Reference Materials (RMs) Alert (82N, 62 W, 200 masl) Alert (82N, 62 W, 200 masl) Mauna Loa (19N, (19N, 155W, 155W, 3397 masl) 3397 masl) South Pole (89S, 24 W, 242810 W, masl) 2810 masl) Data by NOAA. d 13 C(CO2 ),
δ-scales: artefact-based, relative scales δ 13 C = [ 13/12 R Sample / 13/12 R Std - 1)], expressed per mil, The standard defining the 13 C-scale is a historical artefact (V)PDB, it will be so for many more years. (Other isotope scales are also artefact-based.) The highest-level RM, NBS-19 (marble), has been used for scale-realization for many years. Now exhausted, the replacement will be released soon. 13 C/ 12 C is ~0.0112, natural variations can be in the range of few tens of per mil ( ) only, compatibility goal for d 13 C(air-CO2) is 0.01.
Why Ref. Materials from IAEA are unique? Scale-defining Ref. Materials: VSMOW-2/SLAP-2 for d 2 H & d 18 O-scales (all 2 H data, most of 18 O data) NBS-19/LSVEC for d 13 C-scale (all 13 C measurements) NBS-19 for d 18 O-scale ( 18 O data in carbonates & air-co 2 ) These are high-level standards used to calibrate the entire measurement scheme. Metrological aspects are similar to the prototype of kilogram (BIPM, Paris) and include scale maintenance (also introducing replacement RMs), recommendations on calibration schemes and uncertainty propagation. BIPM, Paris
History of d 13 C scale and scale-defining RMs: 1957: Material defining the scale Most of natural d 13 C values range from -50 to +5 PDB d 13 C= 0 (scale-definition) 1984: Replacement for this scale-defining RM (exhausted) NBS19 (VPDB scale) d 13 C= 1.95 ±0.00 (scale-realisation) 2006: 2 nd highest-level RM LSVEC d 13 C= -46.60 ±0.00 defines the scale-span for normalisation NBS19 (VPDB scale) d 13 C= 1.95 ±0.00 (scale-realisation) 2014-2015: LSVEC s suitability questioned, under investigation 2015 d 13 C of air-co 2 is ~ -9, d 13 C of air-methane ~ -47 Replacement of NBS19 IAEA-603, work in progress Q: Do NBS19-replacement and LSVEC fit for the purpose?
Two RMs normalisation of d 13 C data Measured d 13 C values LSVEC was introduced in 2006, the material - Li 2 CO 3 ; Intended use: low-end d 13 C material, in order to normalize all d 13 C- data with 2 RMs; d 13 C=-46.60, initially assumed of very low uncertainty (i.e. zero-uncertainty); Presently 2-RMs normalisation is IUPAC-recommended for all d 13 C data. RM NBS-19 Sample-A Sample-B RM LSVEC True (assigned) d 13 C values
Two RMs normalisation of d 13 C data LSVEC was introduced in 2006; the material - Li 2 CO 3 ; Intended use: low-end d 13 C material, in order to normalize all d 13 C- data with 2 RMs; d 13 C=-46.60, initially assumed of very low uncertainty (i.e. zero- uncertainty) Presently 2-RMs normalisation is IUPAC-recommended for all d 13 C data. LSVEC and uncertainty of d 13 C data: Consequence: Uncertainty of LSVEC effects all d 13 C-calibrations, both for air-co 2 and air-ch 4.
Role of IAEA as RMs producer: Maintain the scale-defining (highest level) RMs with lowest uncertainty; Guarantee their integrity over time; Introduce adequate replacements based on the most critical requirements; Promote the use of modern metrological concepts, including uncertainty propagation; Recommend correct calibration schemes, at least for the most critical applications (e.g. greenhouse gas observations). d 13 C of any (randomly selected) RM-vial and any aliquot should be equal (within uncertainty!) to the d 13 C assigned.
Steps in producing RMs: 1. Defining the intended use of RM, its assigned value, suitability etc; 2. Selecting candidate material; 3. Feasibility study (material properties, homogeneity, stability); 4. Material preparation, mixing, packing etc; 5. Homogeneity study (inside an ampoule, ampoule-to-ampoule); 6. Calibration and evaluation of the total uncertainty, including: homogeneity, long-term stability.
Work on IAEA-603, d 13 C-scale-defining RM (replacement for NBS19). Material selected (white marble), crushed, sieved, washed from fines; Manual inspection for impurities; Feasibility study => suitable material, Batches of ca. 2.7 kg each (15kg in total) Packing: 5000 ampoules (0.5g) prepared/sealed (other batches reserved). Full calibration: Homogeneity study (in progress) Calibration vs. remaining NBS19 (in progress). High quality data on limited number of vials, obtained at MPI-Jena (W.Brand) and USGS (T.Coplen). Value assignment and uncertainty evaluation (still to be done); Stability study (after several years).
Runs on IAEA-603 at IAEA Relative d 18 O, relative-d 46 R vs Ref CO 2 gas, Carbonate Line (25 y.old, by C. Brenninkmeijer) with 20 samples in a single preparation-sequence; Mass-spec MAT251 (30 y.old) Mass-spec MAT 253 (year 2014) 11.70 11.65 Runs on 2014-06-23: 1StDev(d 13 C)=0.008 1StDev(d 18 O)=0.017 11.60 11.55 11.50 5.50 5.55 5.60 5.65 relative-d 45 R vs Ref CO 2 gas, Relative d 13 C,
Study of LSVEC, the 2nd anchor of d 13 C-scale:
Relative (not-normalised) d 13 C, Total range of 0.3 LSVEC: material stored in vials vs. the bulk stored at NIST. Various vials, range of ~0.25. (results by IAEA, confirmed at USGS and MPI-Jena) Highest d 13 C: vials untouched for years, + variability inside vials. (Numerous data are not plotted) Bulk-1 at NIST Bulk-2 at NIST Bulk-3 at NIST Bulk-containers, range of ~0.20. (results by IAEA, confirmed at USGS and MPI-Jena)
LSVEC problem & uncertainty of d 13 C -calibrations. Intended use: to normalize all d 13 C- data with 2 RMs; d 13 C=-46.60, initially assumed with zero-uncertainty. Problems recognised only in 2014-15: d 13 C-scatter up to 0.25, in some cases higher; alteration during storage => uncontrolled bias to positive d 13 C; increased uncertainty => the 2-RMs data normalisation is effected. Consequences: Increased uncertainty of d 13 C-calibrations & data, including air-co 2 and air-ch 4 Effect on the long-term data compatibility?
Take-home messages: All calibration mixtures made by CCL are based on the highest-level RMs provided by IAEA. d 13 C-scale-defining RM must be of low uncertainty. IAEA will soon release IAEA-603 (replacement for NBS19), this RM is of low uncertainty. The 2 nd anchor of the 13 C-scale, LSVEC is found to have problems, namely drift of its value and thus increased uncertainty. Consequently, the 2-RMs data normalization approach and hence all the calibrations for air-co 2 and air-ch 4 might have been affected. Needs for the LSVEC replacement. IAEA has started working on it, but resources are limited. Next, let s have a look in metrology.
Metrology: Science to make measurements Available schemes
Metrology and stable isotopes: Secondary level RMs
Calibration hierarchy Metrological traceability Uncertainty Simplified metrological concept for stable isotope data: RMs-hierarchy, traceability chain, related increase in the uncertainty. International scales: VPDB-scale (d 13 C, d 18 O-scales) highest-level, scale-defining RM (NBS-19) Secondary RMs Other carbonates, NIST-CO 2 gases etc Tertiary RMs CO 2 -in-air, other fir-for-purpose RMs Samples Each measurement step introduces an uncertainty; Each lower hierarchical level bears a higher uncertainty; Full uncertainty-budget can be based on the traceability chain only; it should include the uncertainty of all RMs in use and their measurements; Uncertainties of any correction(s) applied should also be included.
Conclusive remarks: IAEA maintains the d 13 C-scale by providing the highest-level RMs and introducing adequate replacements. All calibrations by CCL as well as all isotope measurement results are based on these highest-level RMs. IAEA is close to finalize work on d 13 C scale-defining RM IAEA-603 (replacement for NBS19), this RM is of low uncertainty. LSVEC, the second anchor on the d 13 C-scale, is found to have drifting d 13 C-value and increased uncertainty. The 2-RMs normalization scheme and all calibration schemes appears to be affected to some degree. IAEA works on its replacement and scale continuity. As RMs producer, IAEA focuses on correct use of modern metrological approach(es); there is a space for combined coordinated efforts by IAEA with isotope experts, WMO and BIPM. All network laboratories and CCL are advised to document their calibration scheme(s) and their practical realization (e.g. standard operating procedures under ISO 17025), with proper evaluation of the total uncertainly. These are prerequisite steps aimed to achieve the long-term data compatibility goals set by WMO-GAW.