Jan Kaiser School of Environmental Sciences …...University of East Anglia Norwich United Kingdom...
Transcript of Jan Kaiser School of Environmental Sciences …...University of East Anglia Norwich United Kingdom...
8 September 2009
How accurately do we know13C/12C, 18C/16O and 17O/16O ratios in CO2and their corresponding δ values?
Jan Kaiser
School of Environmental SciencesUniversity of East AngliaNorwichUnited Kingdom
Recommended inter-laboratory comparability (1σ)
14th WMO/IAEA Meeting of Experts on Carbon Dioxide Concentration and Related Tracers Measurement Techniques, 10-13 September 2007 in Helsinki, Finland
CO2 ±0.1 ppm (±0.05 ppm in southern hemisphere)13δ(CO2) ±0.01 ‰18δ(CO2) ±0.05 ‰
How do we measure carbon and oxygen isotope ratios with mass spectrometers?
1//
r44
r45
444545 −=
IIII
Iδ
Measure ion current ratios of sample relative to reference (NBS 19-CO2 as stand-in for VPDB-CO2)m/z 44 12C16O2
+
m/z 45 13C16O2+ 12C17O16O+
m/z 46 12C18O16O+ 13C17O16O+ 12C17O2+
We assume that all minor isotopologues have the same relative ionisation efficiency. Then, the ion current delta is the same as the isotopologue delta.
1//
r44
r45
44454545 −==
RRRR
I δδ
"Conventional" isobaric correction using isotopologue and isotope ratios R
r17
r13
r45 2 RRR +=
2r
17r
17r
13r
18r
46 22 RRRRR ++=
RRR 171345 2+=21717131846 22 RRRRR ++=
528.01817 RAR = 528.0r
18r
17 RAR =
Solve for 3 unknowns 13R, 17R, 18R
1//
r44
r45
444545 −=
RRRRδ
"New" isobaric correction using δ values (Kaiser & Röckmann 2008)
δδδ 17r
1713r
1345r
45 2 RRR +=
)2()(22 217172r
1717131713r
17r
1318r
1846r
46 δδδδδδδδ +++++= RRRRR
111 18
1813
17
1717
13
1313 −=−=−=
rrr RR
RR
RR δδδ
Solve for 3 unknowns 13δ, 18δ, 17δ(including possible 17O isotope excess aka oxygen isotope anomalies)
1)1)(1( 528.0181717 −++= δΔδ
"New" isobaric correction using δ values (Kaiser & Röckmann 2008)
1)1)(1( 528.0181717 −++= δΔδ
)(2 17454513 δδδδ −+= C
[ ]217174517464618 3)1()42()22()2( δδδδδδδ CCCCD +++−−−++=
r18
r17
r13
r13
r17
2 RRRD
RRC ==
How to measure C (and D)?
⇒ no isotope ratios required for δ calculation
See also Poster P10
C from δ measurements only
MOW/VCONBS1917
/wCOOw45
/wCOO45
/wCONBS1945
/VSMOWOw17
O/wO17
CO/VPDBCONBS1913
COVPDB13
VSMOW17
2
222216
222216
22
2 1)1)(1(2/)1(
S
RR
−−−−
−−−
−−
−−−
−
++
+=
δδδ
δδδδ
MOW/VCONBS1917
/wCOVSMOW45
/wCOOH45
/wCONBS1945normalised
O/VSMOWH17
CO/VPDBCONBS1913
COVPDB13
VSMOW17
2
2216
2
2162
22
2 1)1(
2/)1(
S
RR
−−−
−
−−
−−−
−
+
+=
δδδ
δδδ
derived from Li et al. 1988
derived from Assonov & Brenninkmeijer 2003
Values for the ratio 17R/13R for VPDB-CO2
103 C Reference
33.8±0.1 Craig (1957)
35.7±0.7 Santrock et al. (1985)34.6±0.2 Li et al. (1988)
35.16±0.05 Assonov & Brenninkmeijer (2003)34.17±0.04 Valkiers et al. (2007)
orange: derived from δ measurements only
Influence of C on the computed 13δ value
-0.10
-0.08
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
-100 -80 -60 -40 -20 0 20 4045δ /‰
Δ13δ /‰
CraigValkiersLiSantrockAssonov
Influence of 17Δ on the computed 13δ value
...528.01)1)(1( 1817528.0181717 ++≈−++= δΔδΔδ
Δδδ
Δδδ
δδδ
171845
171845
174513
0703.00371.00703.122528.0)21(
2)21(
−−≈
−×−+≈
−+=
CCCCC
17Δ(atm. O2) = –0.25 ‰ (Barkan & Luz 2000)17Δ(trop. CO2) = +0.65 ‰ (Hoag et al. 2005)
This could lead to systematic offsets in 13δbetween –0.05 and 0.02 ‰, depending on CO2 source.
Influence of the relative ionisation efficiency of 12C17O16O versus 13C16O2
Assume 13δ = –10 ‰, 18δ = 40 ‰ and 17Δ = 0 ‰.
Relative difference inionisation efficiency
45δ / ‰
0 –7.9680.001 –7.9660.01 –7.9490.1 –7.772
How good is the assumption that relative ionisation efficiencies are the same?
14th WMO/IAEA CO2 Experts Meeting (2007)accuracycloseness of agreement between a test result and the accepted reference value (VIM 2nd edition,1993).
measurement accuracy (2.13)closeness of agreement between a measured quantity value and a true quantity value of a measurand
measurement trueness (2.14)closeness of agreement between the average of an infinite number of replicate measured quantity values and a reference quantity value (5.18)
VIM 3rd edition (2008)
VIM 3rd edition (2008)
reference quantity valuereference value
quantity value used as a basis for comparison with values of quantities of the same kind
NOTE 1 A reference quantity value can be a true quantity value of a measurand, in which case it is unknown, or a conventional quantity value, in which case it is known.
Can secondary reference materials improve this situation?
Scale normalisation withL-SVEC (Li2CO3) 13δ = –46.6 ‰
(Coplen et al. 2006)
I think this is a conventional reference value.Not a true one.Nevertheless, it does improve our trueness.But not necessarily the accuracy.
13R(VPDB-CO2)
106 13R
Original (±1σ) References
11237.2 Craig (1957); Nier (1950)
IAEA Recommendation
13R(VPDB-CO2)
106 13R
Original (±1σ)
106 13R
Re-evaluated (Kaiser, 2008)
References
11237.2±30 11232.3±39 Craig (1957); Nier (1950)
13R(VPDB-CO2)
106 13R
Original (±1σ)
106 13R
Re-evaluated (Kaiser, 2008)
References
11237.2±30 11232.3±39 Craig (1957); Nier (1950)
11090.1±6 Nørgaard et al. (1999)
11101.4±9 11086.4±3 Russe et al. (2004); Coplen et al. (2006)
11179.7±28 11182.4±16 Chang & Li (1990)
11137.6±1.6 11137.6±1.6 Valkiers et al. (2007)
Average of last 4 values: (11124±45) × 10–6
18R(VPDB-CO2)
106 18R
Original (±1σ) References
2088.35 Baertschi et al. (1976); O’Neil et al. (1975)
IAEA Recommendation
18R(VPDB-CO2)
106 18R
Original (±1σ)
106 18R
Re-evaluated (Kaiser, 2008)
References
2088.35±0.80 2089.32±1.5 Baertschi et al. (1976); O’Neil et al. (1975)
18δVPDB-CO2/VSMOW = (42.0±0.7) ‰ (with 18δSLAP/VSMOW = –56.18 ‰)
Re-evaluated (Kaiser, 2008)
18δVPDB-CO2/VSMOW = 41.47 ‰ (with 18δSLAP/VSMOW = –55.5 ‰)Recommendation (IAEA, 1995)
18R(VPDB-CO2)
106 18R
Original (±1σ)
106 18R
Re-evaluated (Kaiser, 2008)
References
2088.35±0.80 2089.32±1.5 Baertschi et al. (1976); O’Neil et al. (1975)
2079.00±2.5 2078.71±2.5 Craig (1957); Nier (1950)
2107.69±2.4 Valkiers & De Bièvre (1993)
2087.54±1.6 Nørgaard et al. (1999)
2088.24±0.48 2088.24±0.48 Valkiers et al. (2007)
Average of rows 3, 5 & 6: (2088.37±0.90) × 10–6
17R(VPDB-CO2)
106 17R
Original (±1σ) References
395.11±0.94 Assonov & Brenninkmeijer (1993); Craig (1957)
IAEA Recommendation
17R(VPDB-CO2)
106 18R
Original (±1σ)
106 18R
Re-evaluated (Kaiser, 2008)
References
(395.11±0.94) (395.11±1.66) Assonov & Brenninkmeijer (1993); Craig (1957)
17R(VPDB-CO2)
106 18R
Original (±1σ)
106 18R
Re-evaluated (Kaiser, 2008)
References
(395.11±0.94) (395.11±1.66) Assonov & Brenninkmeijer (1993); Craig (1957)
379.95±0.5 380.18±0.2 Craig (1957); Nier (1950)
394.67±2 Valkiers & De Bièvre (1993)
(389.88±0.2) Nørgaard et al. (1999);Assonov & Brenninkmeijer (1993)
380.58±0.48 380.58±0.48 Valkiers et al. (2007)
Why should we care about R?
Nice to know …To make isotope measurements SI traceable[but: precision of R always worse than δ ] To cross-check with data-reduction coefficients from other molecules (O2, CO, N2O, SO2)[B, C, D, E, F, (G, H, I) – see poster P10]To callow calculation of isotopologue mixing ratios
Calculation of isotopologue mixing ratios
)1)(1()CO()OC( 181713
22
1612
RRRxx
+++=
x(CO2) = 390 μmol/mol18R = 2089.3 × 10–6
C = 0.0351613R = 11237.2 × 10–6 x(12C16O2) = 383.76 μmol/mol13R = 11086.4 × 10–6 x(12C16O2) = 383.82 μmol/mol
Δx = 0.06 μmol/mol
Is this important? You decide …
Conclusions
Agreed isobaric correction and normalisation schemes improve reproducibility (and maybe trueness) of δ values.
Several minor issues remain for the accuracy of δ valuesconfirmation of C (and D)17Δ value of tropospheric CO2
relative ionisation efficiency of 12C17O16OMajor issues remain for the accuracy of R values
13R(VPDB-CO2): converging?18R(VPDB-CO2): converged17R(VPDB-CO2): diverging [but can be linked to 13R]