RAPID COMMUNICATIONS IN MASS SPECTROMETRY
Rapid Commun. Mass Spectrom. 2004; 18: 1830–1838
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rcm.1560
The effect of N2O on the isotopic composition
of air-CO2 samples
Prosenjit Ghosh{ and Willi A. Brand*
Max-Planck-Institute for Biogeochemistry, P.O. Box 100164, 07701 Jena, Germany
Received 20 July 2003; Revised 13 June 2004; Accepted 21 June 2004
The ionisation efficiencies of N2O vs. CO2 as well as their ratios were measured in detail introducing clean N2O and CO2 into the electron impact ion source of an isotope ratio mass spectrometer.
Changes in the ionisation efficiency ratio (IER) were found for different electron energy settings
and compared with the ratios of literature ionisation cross-section values for pure N2O and CO2.
To establish the influence of mixtures of N2O and CO2 in a mass spectrometer, artificial air mixtures
were prepared by mixing different amounts of N2O and CO2 from well-calibrated spike cylinders
with CO2-free air. The mixing ratios varied from 8–512 ppb for N2O and from 328–744 ppm for CO2.
With these mixtures the effects of varying N2O concentrations on apparent CO2 isotope ratios in air
samples were determined. After applying a mass balance correction the d13C results were consistent
within small error margins. The data seemed almost independent from a particular choice for the
IER of N2O vs. CO2 in the correction algorithm. For d18O a small effect of the ionisation efficiency
ratio of N2O vs. CO2 was found. Several sets of calculations were made varying the IER between
0.88 and 0.62. The dependence of d18O was the smallest with an adopted IER of 0.68–0.72 in the
mass balance correction equation for isotopic analysis of CO2 in air. For high-precision measurements of the CO2 stable isotope ratios in air samples a careful assessment of the mass spectrometer
performance is necessary. Different ion sources, even different ion source settings, alter the IER of
N2O vs. CO2 which is used in the N2O correction algorithm. Preferably, the specific mass spectrometric behaviour should be established with clean N2O/CO2 mixtures or with air mixtures covering
a larger range of N2O concentrations. Copyright # 2004 John Wiley & Sons, Ltd.
Carbon and oxygen isotope ratios of atmospheric and soil
CO2 are measured in order to study and understand variations in the global carbon cycle, in particular the respective
impact on the global climate system. The procedure of
extracting carbon dioxide from air samples usually requires
a cryogenic separation where N2O is condensed along with
CO2. The presence of N2O (which is isobaric with CO2) causes
an alteration of the desired isotopic ratio as obtained using a
mass spectrometer. This alteration can be corrected for by
subtracting the weighted N2O contribution from the measured raw values using a mass balance relation. The need
for such an adjustment of the 13C/12C and 18O/16O isotopic
ratios of CO2 in air samples was already discussed in 1963
in a paper by Craig and Keeling.1 The derivation formulated
later for a N2O correction was based on equations governed
by the isotopic ratio of molecules in air (CO2þN2O) that contribute to the mass 44, 45 and 46 ion beams.2 The difference in
isotopic composition of CO2 due to the presence or absence of
N2O can then be calculated from Eqn. (1):2
*Correspondence to: W. A. Brand, Max-Planck-Institute for Biogeochemistry, P.O. Box 100164, 07701 Jena, Germany.
E-mail: [email protected]
{
Present address: California Institute of Technology, Mail code
170-25, 1200 E California Blvd., Pasadena, CA 91125, USA.
Contract/grant sponsor: European Union; contract/grant number: EVR1-CT-2001-40015.
13 ¼ ðN=C Þ E
18 ¼ ðO=O2 Þ E
ð1Þ
where r represents the N2O/CO2 molar ratio in the samples
and E is the ionisation efficiency ratio (referred to as IER in
the following text). The expressions dN/C and dO/O2 represent correction factors or properties obtained using raw isotopic ratios from VPDB and N2-air (dN/C ¼ 345.4% and
dO/O2 ¼ 500%, see also Eqn. (3)).
The effect of N2O on air CO2 isotopic measurements was
verified later2 with measurements of an original mixture of
N2O and CO2 followed by isotopic analysis after circulating
the gas through copper pellets at 6508C in order to remove all
N2O from the sample. The difference in isotopic composition
between the two sets of measurements was found to follow
the theoretical relationship described above. In the correction
procedure by Mook and van der Hoek in 1983,2 the IER of
N2O vs. CO2 was determined in independent experiments by
comparing the mass 44 ion beams from pure N2O and CO2 at
equal inlet pressure using an isotope ratio mass spectrometer.
The average IER of N2O vs. CO2 reported from this
experiment was 0.73. The overall additive corrections
established from the observations were þ0.23% for carbon
and þ0.33% for oxygen. The established correction factor
was determined based on the CO2 and N2O concentrations
(301 ppb of N2O and 337 ppm of CO2) available for air
Copyright # 2004 John Wiley & Sons, Ltd.
Effect of N2O on the isotopic composition of air-CO2 samples
samples collected between Sept. 1976 and May 1980 from the
Mauna Loa Station.3 The correction algorithm was later
confirmed in a separate experiment for atmospheric air
samples from an east Pacific coast station (San Diego, LJO)
and from the south pole (SPO).4 The shift in the 13C/12C ratio
due to the presence of N2O was consistent with the calculated
result within a precision of 0.016%.4 However, it was not
possible to verify the effect on d18O due to modification of the
18
O/16O signature during conversion of N2O to N2 over hot
copper. The described protocol of data reduction is still in use
today in the major laboratories around the world with only
minor modifications.5,6
The IER between N2O and CO2 is an important component
of Eqn. (1). The IER factor applied for estimating a drift due to
the presence of N2O has not been verified, however, for
different experimental setups and with different ionisation
parameters. The sensitivity of this parameter on data reduction is unspecified. The purpose of the present study is to
determine the IER of the pure N2O and CO2 gases as a function
of the ionisation conditions and to test the variability of this
ratio for samples of air with varying CO2 and N2O concentrations. Using our results a refinement of the established
correction procedure for measuring the isotopic ratio of CO2
in air samples becomes available and thus a higher degree of
precision of CO2-in-air measurements may be possible.
The ionisation efficiency ratio of a mixture of N2O and CO2
in a mass spectrometer depends on the respective electron
impact ionisation cross sections, on fragment ion formation
and additional ionisation processes which mainly are a
function of pressure. The ionisation cross section varies with
electron energy.7,8 Using literature EI cross section values for
a particular electron energy should give a first estimate of the
IER between N2O and CO2. However, literature data
available for the absolute EI cross sections for N2O and CO2
scatter in a wide range (e.g. from 2 1016 to 4 1016 cm2 for
CO2 at the same electron energy of 100 eV, see Fig. 6 in Ref. 8)
and cannot be used directly in the correction equation. We
therefore have made an independent attempt to determine
the relative ionisation efficiencies of pure N2O and CO2 on a
single instrument and compared the results with available
literature data.7,8
For CO2-in-air samples, the relevant molar ratio of N2O vs.
CO2 is only about 1 104. We therefore also tested the effect
of the N2O contribution on d13C and d18O of CO2 in artificial
air. For this test we prepared a number of glass flasks with
identical pressure by mixing variable amounts of CO2 and
N2O into synthetic air (O2, N2 and Ar).
EXPERIMENTAL
A first set of experiments was carried out in order to determine the relative ratio of the m/z 44 peaks comprising
12 16 16 þ
C O O and 14N14N16Oþ on a large radius (41 cm dispersion) l0 kV isotope ratio mass spectrometer (Finnigan MAT
252). Clean high-purity CO2 and N2O gases (purity 99.996%)
were administered to the reference side of the dual inlet system and introduced from here separately into the MAT 252
mass spectrometer. The ion beam intensity at m/z 44 was monitored with varying electron energy (56–147 eV). Other mass
spectrometric parameters (electron trap current, accelerating
Copyright # 2004 John Wiley & Sons, Ltd.
1831
voltage, extraction voltage, focusing potentials, etc.) were
kept constant throughout measurements. Measurements
were made at three separate inlet pressures (26.6, 60,
67 mbar). Results obtained are discussed below.
In a second series of experiments we prepared artificial
mixtures of air with varying contents of CO2 and N2O. The
isotopic composition of the CO2 added in the mixture was
established independently with repeated measurements
giving a consistent precision of 0.02% for d13C and 0.03%
for d18O. The mixing was done in an all-metal vacuum line
originally designed for mixing CO2 from a carbonate reaction
into CO2-free air (ARAMIS, Acid Reaction and Air Mixing
System) presently in operation at the Max-Planck-Institute
for Biogeochemistry (MPI-BGC) (Fig. 1). The design has a
provision for the introduction of isotopically known CO2 into
a 4.5 L mixing chamber where mixing with other gases, in
particular major air constituents, is carried out. From here,
the gas mixture can be transferred to the final containers in a
highly controllable fashion. To ensure an optimal procedural
repeatability the unit is under full computer control including transmission and equilibration times. For this experiment
the same cylinders as before with clean CO2 and N2O were
used. The pressure in the CO2 cylinder was 550 psi allowing
us to avoid the risk of differential partitioning of isotopes of
CO2 at high pressure between solid and liquid phases.9
The experiment was performed in two separate steps: In
the first step, only CO2 was mixed with the synthetic air and
the product mixture was transferred into four glass flasks of
5 L volume each. A total of seven batches with different
amounts of CO2 were prepared. In the second step, both CO2
and N2O were admitted in varying amounts using essentially
the same procedure:
From a 30 mL injection line volume, clean CO2 was flushed
stepwise into the ARAMIS mixing volume (see Fig. 1) using
high-purity synthetic air comprising 21% O2, 1% Ar and 78%
N2. When applicable, an appropriate N2O/air mixture
without CO2 was subsequently added in a similar way from
its injection loop. Absence of traces of CO2 and N2O was
carefully checked using a high-precision chromatographic
setup.10 The sequence of events for generating one batch
comprising four flasks of identical composition was:
– 60 s flushing of CO2 from the injection point A through the
–
–
–
–
30 mL line volume to vent using a flow rate of 70 mL/min,
keeping the manual valves V1 and V2 open (shown in
Fig. 1).
40–60 s expansion of the CO2 from the 30 mL line volume to
the mixing chamber.
10 min equilibration within the mixing chamber and pipe
work.
60 s flushing of the pre-fabricated N2O (2%) þ air mixture
through a loop (size varied between 10 and 5 mL) followed
by 40–60 s transfer of the mixture from the loop to the mixing chamber using the synthetic air at a transfer rate of 4–
5 L/min until a pressure of 1050 mbar was reached. The
mixture was then kept equilibrating for 30 min.
Admission of the mixture to the four evacuated 5 L flasks
through a mass flow controller (MKS model
1179AX53CS18V) using an initial flow rate of 2 L/min
(up to the final 200 mbar pressure).
Rapid Commun. Mass Spectrom. 2004; 18: 1830–1838
1832
P. Ghosh and W. A. Brand
Figure 1. Experimental arrangement for mixing of CO2 from a CO2 spike cylinder with synthetic air doped with different
concentrations of N2O. See text for further explanation.
– Addition of the final amount of synthetic air at 4 L/min
until a pressure of 1.45 bar in the flasks was reached.
– Final equilibration for 2 h, then the flask stopcocks were
closed.
Mixing ratio and isotopic analyses were performed using
established in-house quantitative gas chromatography
(GC)10 and isotope ratio11 procedures. In the latter it has
carefully been checked that the trapping efficiency for CO2
from air is close to 100%. For N2O, the boiling point under
ambient pressure conditions is 89.58C (CO2: 78.58C). In
spite of this difference it is assumed that the simultaneous
trapping of N2O from air samples at 1968C occurs in a
quantitative fashion, too.
RESULTS AND DISCUSSION
Determination of the m/z 44 beam intensity
using pure N2O and CO2 gases
The results of the first experiment, determination of the ionisation efficiency ratio (IER) of N2O vs. CO2, are presented in
Table 1 and plotted in Fig. 2. The curves 1 and 2 in the figure
represent cubic fits of our (two series of measurement) observations of the ion beam intensity at m/z 44 as a function of electron energy. The threshold electron energies required for the
production of singly charged N2Oþ. and COþ.
2 ions are 12.89
and 13.77 eV, respectively7,12 (outside of the observation
range). In spite of the lower ionisation potential for N2O,
the EI cross section for CO2 prevails at all energies measured.
Copyright # 2004 John Wiley & Sons, Ltd.
This may be due to the fact that the formation of NOþ and Nþ
2
fragments occurs at comparatively low energies (appearance
potentials 16.2 and 17.5 eV, respectively7) with a total contribution of up to 50% of the m/z 44 ion current, whereas the Oþ
and COþ fragments dwell around 10% of the total CO2 ion
current. The m/z 44 ion currents from both gases increase steadily with electron energy from 56 to about 80 eV. The
response reaches a maximum at 75 eV and subsequently
starts to decrease with a further increase in electron energy
up to the instrumental limit of 147 eV. Our measurements
resemble published absolute electron impact ionisation
cross-section values of N2O and CO2 at different electron
energy, as described in a number of different experiments.7,8
In Fig. 3 the same data are plotted as IER for three different
inlet pressure. For comparison, the ratios of the literature EI
cross sections of N2O vs. CO2 as reported by Iga et al.7 and
Orient et al.8 are also given in the figure. Reasonable
agreement in the shape of the curves is found over the
measured electron energy range; however, the ratio of
the literature data is 4–7% lower than our observation at
100 eV.
The agreement between our values and the literature crosssection ratios is satisfactory considering the fact that our
measurements were done with the same setup and at almost
the same time, whereas the other experiments were
performed almost 10 years apart with different experimental
arrangements and with other uncertainties discussed in the
respective papers. The curves for different inlet pressures in
Fig. 3 exhibit similar features but no consistent dependence
Rapid Commun. Mass Spectrom. 2004; 18: 1830–1838
Effect of N2O on the isotopic composition of air-CO2 samples
1833
Table 1. Ionisation efficiency ratios (IERs) of N2O vs. CO2
Bellow pressure 26.1 mbar
Bellow pressure 60.1 mbar
Bellow pressure 67.2 mbar
Electr. energy
CO2
N2O
N2O/CO2
CO2
N2O
N2O/CO2
CO2
N2O
N2O/CO2
56
58
59
60.9
63
64
65
68
69.6
70.1
57
60
75.1
78.1
80
85.2
87.2
90.1
95.1
97.1
99.6
101
105
107.5
109.1
110.3
115.2
117.1
120.1
125
127.2
130.1
132
135
137.1
139.1
140.3
142.2
144
145.9
147
1355
1400
1423
1457
1486
1497
1507
1527
1535
1539
1377
1439
1549
1547
1540
1513
1495
1464
1393
1359
1312
1282
1210
1167
1141
1119
1041
1019
974
907
878
844
822
790
768
750
738
721
705
689
681
1018
1047
1062
1084
1102
1110
1117
1131
1137
1138
1032
1074
1138
1135
1130
1110
1098
1075
1021
996
963
941
884
854
833
817
757
735
700
650
629
603
588
565
550
535
528
515
503
492
486
0.751
0.748
0.746
0.744
0.742
0.742
0.741
0.741
0.741
0.740
0.749
0.746
0.735
0.733
0.734
0.734
0.734
0.734
0.733
0.733
0.734
0.734
0.730
0.732
0.730
0.731
0.728
0.721
0.719
0.717
0.716
0.715
0.715
0.716
0.715
0.713
0.715
0.714
0.714
0.714
0.715
4281
4416
4472
4567
4658
4691
4720
4789
4815
4822
4345
4516
4862
4856
4843
4765
4716
4627
4404
4301
4164
4073
3824
3692
3608
3549
3291
3196
3052
2834
2745
2635
2568
2472
2407
2349
2314
2260
2218
2168
2144
3101
3173
3209
3268
3329
3355
3377
3428
3448
3455
3135
3231
3463
3458
3447
3391
3348
3284
3134
3063
2965
2904
2715
2618
2546
2506
2322
2255
2144
1986
1923
1844
1797
1726
1680
1640
1614
1578
1542
1511
1493
0.724
0.718
0.718
0.716
0.715
0.715
0.715
0.716
0.716
0.717
0.721
0.715
0.712
0.712
0.712
0.712
0.710
0.710
0.712
0.712
0.712
0.713
0.710
0.709
0.706
0.706
0.706
0.705
0.702
0.701
0.701
0.700
0.700
0.698
0.698
0.698
0.698
0.698
0.695
0.697
0.696
4391
4528
4595
4702
4801
4840
4876
4958
4993
5004
4451
4634
5082
5076
5060
4979
4924
4828
4597
4488
4340
4244
3980
3838
3752
3685
3414
3313
3155
2932
2839
2725
2653
2550
2478
2419
2379
2324
2274
2225
2198
3313
3392
3429
3496
3569
3597
3623
3686
3712
3720
3346
3447
3701
3679
3668
3608
3557
3487
3326
3252
3144
3077
2884
2764
2693
2649
2455
2378
2264
2094
2026
1941
1890
1816
1767
1722
1696
1656
1621
1585
1566
0.755
0.749
0.746
0.744
0.743
0.743
0.743
0.743
0.743
0.743
0.752
0.744
0.728
0.725
0.725
0.725
0.722
0.722
0.724
0.725
0.725
0.725
0.725
0.720
0.718
0.719
0.719
0.718
0.717
0.714
0.714
0.712
0.713
0.712
0.713
0.712
0.713
0.713
0.713
0.713
0.712
upon inlet pressure was observed. Variations in the ratios at
different inlet pressures probably reflect errors associated
with the measurement. Consistent features observed in the
ratio curves include a rapid drop in N2Oþ./COþ.
2 from 56 to
62 eV. The region between 62 and 110 eV, which coincides
with the range usually selected in isotope ratio mass
spectrometers, exhibits a plateau. At about 110 eV, a further
drop in the ratio is visible. The features suggest that a number
of processes are involved when energetic electrons collide
with the neutral molecules and subsequently form a range of
products. The processes of interest are ionisation into several
ionic states which subsequently relax through ro-vibrational
redistribution, fragmentation to form product ions as well as
charge transfer, e.g. from doubly charged to singly charged
ions, when the electron energy is high enough. We interpret
the observed rapid drop in the N2Oþ./COþ.
2 ratio with
increasing production of NOþ and Nþ
2 fragment ions whereas
the plateau marked by the energy window (62–110 eV) may
Copyright # 2004 John Wiley & Sons, Ltd.
þ
reflect a steady state for the Nþ
and the Oþ/COþ
2 /NO
fragmentation channels over a larger energy window.7 Such
variation in IER was not considered in detail earlier when the
contribution of N2O to the isotopic analysis of CO2 had to be
corrected for. We suggest that, because the ionisation
efficiency of N2O vs. CO2 is electron energy dependent and
the ratio varies over a considerable range (0.756–0.70), the
exact relationship should be determined individually for
every instrument (and indeed for every major change in
ionisation parameters on a particular instrument). In our
routine measurements of the isotopic composition of CO2 in
air samples we keep the electron energy constant at 70.5 eV.
For this value the observed IER of N2O vs. CO2 varies from
0.74 to 0.715. This applies to clean gases and is entirely
consistent with the finding of Mook and Jongsma.4 For
mixtures of gases, as occurs for the roughly 0.1% N2O in CO2
from air samples during isotopic measurement, the situation
may be different.
Rapid Commun. Mass Spectrom. 2004; 18: 1830–1838
1834
P. Ghosh and W. A. Brand
Figure 2. Ionisation efficiency of N2O and CO2 clean gases as a function of electron energy (1000 mV
: 3.3 nA). Curve 1 presents the observation with clean CO2 at the inlet pressure of 67.2 mbar whereas
curve 2 is drawn for N2O at same inlet pressure.
Artificial mixtures of air with variable contents
of CO2 and N2O
Determination of the isotopic composition of the CO2 in
the spike cylinder
For reasons discussed in the previous paragraph we have also
studied the behaviour of the mass balance correction term for
different amounts of N2O and CO2 in air. Table 2 shows
results from the first series of experiment involving air mix-
tures. The CO2 concentration in the different batches (composed of a number of flasks at 1.5 bar pressure) was varied
in a wide range between 544 and 361 ppm, approximately
covering present-day concentrations in air samples of different origin. The N2O concentration was below the detection
limit in the GC analysis. In spite of the variations in CO2 concentration the isotopic ratios proved reproducible between
batches within a standard deviation of 0.017% for carbon
and 0.030% in case of oxygen, illustrating the high level of
Figure 3. Ratio plot of the data in Fig. 2, together with literature efficiency data7,8 plotted as N2O/
CO2 ratio.
Copyright # 2004 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2004; 18: 1830–1838
Effect of N2O on the isotopic composition of air-CO2 samples
1835
Table 2. Variable amounts of CO2 were added from a cylinder and mixed with CO2-free synthetic air. The product mixture was
transferred into a number of glass flasks connected to the multiport (Fig. 1). The concentrations and isotopic ratios of CO2 were
measured separately using GC techniques and a Finnigan MAT 252 mass spectrometer
USN no.
CO2 conc. (ppm)
N2O conc. (ppm)
d13CPDB (%)
d18OPDB (%)
Batch no
20030495
20030496
20030497
20030498
20030499
20030500
20030501
20030508
20030509
20030510
20030511
20030516
20030513
20030514
20030515
20030517
20030518
20030519
20030520
543.5
543.5
543.5
543.5
543.5
543.5
544.4
459.4
458.6
457.5
458.4
361.6
363.0
361.9
362.3
391.4
389.6
390.4
392.2
0.001
0.001
0.001
0.001
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
38.585
38.595
38.592
38.556
38.593
38.585
38.552
38.570
38.602
38.606
38.604
38.609
38.596
38.590
38.580
38.5879
38.6226
38.6086
38.5838
28.64
28.63
28.64
28.60
28.66
28.61
28.59
28.64
28.65
28.64
28.64
28.60
28.54
28.61
28.58
28.59
28.63
28.58
28.59
3
3
3
3
3
3
3
4
4
4
4
5
5
5
5
6
6
6
6
38.59
0.017
28.61
0.030
Average
SD
trapping efficiency for CO2 in air samples during analysis. In
total, 19 flasks were analysed in five batches, where CO2-air
mixing was performed separately in five different experiments. Samples were analysed and results were evaluated
using the routine data reduction algorithm.
Since in this phase of the experiment the amount of N2O in
the samples was very small, applying the N2O mass balance
correction obviously was insignificant for the isotopic results.
The experiment was performed for two reasons: (1) to
establish a precise isotopic ratio for the tank CO2 with respect
to VPDB, and thus provide a high-precision reference for the
second phase of the experiment, and (2) to ensure that the
d18O and d13C variations of the CO2-in-air mixture show no
significant correlation with various experimental parameters, in particular for making sure that isotopic ratios are
independent of the concentration of CO2 in individual flask
samples and that the process of transferring CO2 from the
cylinder to the final flasks is free of isotopic fractionation. The
data in Table 2, although measured randomly with intermittent variation of temperature and other experimental
conditions, exhibit a well-defined mean of 38.59 0.017%
for carbon. For oxygen the values are 28.61 and 0.030%,
respectively.
Effect of the IER N2Oþ./COþ.
2 on the isotopic
composition of CO2
Again five batches comprising 19 sample flasks with different
amounts of N2O and CO2 in synthetic air were prepared, this
time varying the N2O content from 8–512 ppb and CO2
between 328–744 ppm. The respective ranges were selected
to cover a large part of the compositions observable in natural
air (atmospheric air, air from chamber experiments, mine air,
soil air, air included in ice cores, etc.). The purpose of this
exercise was to check the ionisation efficiency ratio factor
adopted to correct the raw data obtained from the mass specCopyright # 2004 John Wiley & Sons, Ltd.
trometric air-CO2 analyses. We also wanted to examine the
effect of the ionisation efficiency parameters for low and
high N2O concentrations. Our interpretation is based on the
assumption that if the IER adopted for this purpose is correct
then it should not exhibit any correlation between the measured isotopic compositions (both carbon and oxygen) and
the N2O/CO2 abundance ratios. The experiments were performed in an automatic sequence along with standard reference gas following the routine measurement protocol
established at BGC IsoLab.11 The electron energy was set to
70.5 eV. A subset of the isotopic data obtained from the analyses of the flasks containing different concentrations of N2O
and CO2 is listed in Table 3.
To establish a robust IER value we have analysed the data
using several values for the ionisation efficiency (0.88, 0.78,
0.72, 0.68 and 0.62) in our data reduction algorithm. The
algorithm used for correction is similar to Mook and van der
Hoek.2 In our correction algorithm we follow a mass balance
approach where the N2O contribution to the m/z 44–46 ion
currents is corrected starting from:
13 Cobserved ¼ 13 Ctrue þ E ½N=C 13 Ctrue ð2Þ
In this equation r is a close approximation (error: 106) for
the molar fraction of (neutral) N2O. E is the ionisation
efficiency ratio of N2O vs. CO2. E r 6.0 104 for current
atmospheric values (E ¼ 0.7).
When 13 Cobserved is plotted as a function of r the slope
gives the product of E and the difference of the apparent N2O
and the CO2 d values on the VPDB scale.
For applying the necessary corrections for both isotopes,
Eqn. (2) can be rewritten as:
h
i
C
þEC
C
13 Ctrue ¼ 13 Cobserved CO2 CCO N2 O þ 345:4 E CNCO2 O
2
2
h
i
ð3Þ
C
þEC
C
18 Otrue ¼ 18 Oobserved CO2 CCO N2 O þ 500 E CNCO2 O
2
2
Rapid Commun. Mass Spectrom. 2004; 18: 1830–1838
1836
P. Ghosh and W. A. Brand
Table 3. Concentrations of CO2 and N2O in glass flasks along with their carbon and oxygen isotopic compositions. The samples
were prepared from artificial CO2-free air by mixing in an aliquot of CO2 from a tank with previously determined isotopic
composition and another aliquot of N2O from a separate flask. Carbon and oxygen isotopic compositions were calculated
adopting several different values for the ionisation efficiency ratio (IER) between 0.62 and 0.88 in the data reduction algorithms
(only three sets are shown)
Sample ID
ppm
CO2
ppm
N2O
N2O/
CO2
d13CVPDB
0.78
d18OVPDB(gas)
0.78
d13CVPDB
0.88
d18OVPDB(gas)
0.88
d13CVPDB
0.68
d18OVPDB(gas)
0.68
USN 20030558
USN 20030559
USN 20030560
USN 20030561
USN 20030562
USN 20030614
USN 20030615
USN 20030616
USN 20030617
USN 20030618
USN 20030672
USN 20030673
USN 20030674
USN 20030675
USN 20030677
USN 20030678
USN 20030679
USN 20030680
USN 20030681
USN 20030708
USN 20030709
USN 20030710
USN 20030711
USN 20030696
USN 20030697
USN 20030694
USN 20030695
503
502
502
502
502
420
420
419
420
420
395
395
396
395
744
744
744
744
743
328
328
328
328
345
346
345
345
0.009
0.008
0.008
0.009
0.009
0.215
0.145
0.145
0.144
0.141
0.059
0.059
0.059
0.059
0.038
0.038
0.038
0.038
0.038
0.339
0.339
0.338
0.338
0.512
0.502
0.511
0.512
0.00001703
0.00001681
0.00001679
0.00001816
0.00001816
0.00051256
0.00034511
0.00034593
0.00034285
0.00033667
0.00014965
0.00014920
0.00014884
0.00014939
0.00005143
0.00005090
0.00005100
0.00005124
0.00005141
0.00103328
0.00103260
0.00103193
0.00103120
0.00148359
0.00145099
0.00147800
0.00148258
38.56
38.59
38.60
38.59
38.60
38.55
38.58
38.60
38.59
38.58
38.57
38.59
38.59
38.59
38.59
38.59
38.62
38.61
38.58
38.59
38.60
38.60
38.61
38.57
38.51
38.55
38.56
28.57
28.59
28.60
28.63
28.59
28.54
28.59
28.61
28.60
28.58
28.54
28.60
28.61
28.57
28.61
28.61
28.64
28.66
28.59
28.50
28.56
28.55
28.59
28.55
28.45
28.50
28.56
38.59
38.62
38.63
38.62
38.63
38.56
38.60
38.62
38.60
38.60
38.60
38.62
38.61
38.62
38.62
38.62
38.65
38.64
38.61
38.58
38.60
38.59
38.60
38.55
38.49
38.53
38.54
28.61
28.63
28.64
28.68
28.63
28.56
28.62
28.63
28.63
28.60
28.57
28.63
28.64
28.61
28.65
28.65
28.68
28.70
28.63
28.49
28.55
28.54
28.58
28.52
28.42
28.47
28.53
38.535
38.563
38.571
38.565
38.569
38.540
38.561
38.582
38.569
38.566
38.547
38.569
38.565
38.566
38.560
38.564
38.594
38.587
38.550
38.593
38.610
38.606
38.617
38.589
38.532
38.572
38.585
28.531
28.551
28.559
28.592
28.548
28.526
28.568
28.583
28.576
28.550
28.501
28.560
28.573
28.536
28.574
28.567
28.603
28.623
28.545
28.506
28.568
28.555
28.600
28.582
28.475
28.532
28.593
38.58
0.02
28.58
0.05
38.60
0.04
28.60
0.07
38.57
0.02
28.56
0.03
Average value
SD
where CCO2 and CN2 O denote the concentrations of CO2 and
N2O in the sample, respectively. The coefficients (–345.4)
and (–500) correspond to dN/C and dO/O2 in Eqn. (1).
They represent the apparent d13C and d18O values that
N2O would exhibit versus a CO2 standard gas, assuming
the ratios of international standards (VPDB and air-N2) for
d13C and identical oxygen values for d18O. Errors in those
values have a very small effect on the correction and are
considered negligible.
The expression in brackets is close to one (¼1 þ E 0.00085
for atmospheric levels). For d values at ambient CO2 levels
(d13C 8%) and E ¼ 0.7, the deviation from one amounts to
0.005%.
Figures 4 and 5 show d18O- and d13C results obtained for
the different assumed IER values plotted against N2O/CO2 in
the samples.
A significant dependence of the evaluated values adopting
an IER of 0.88 is found (correlation coefficient R2 ¼ 0.8). The
correlation becomes less significant for an IER value of 0.78
(R2 ¼ 0.5). It almost vanishes for 0.68 (R2 ¼ 0.03), but jumps
back to R2 ¼ 0.18 for an IER of 0.62. For d13C the effect is even
more pronounced. This is shown graphically together with
further statistical data in Fig. 6. The data for d13C follow the
trend of the d18O data: they also exhibit a valley around 0.7
and the minimum of the precision data coincides with the
Copyright # 2004 John Wiley & Sons, Ltd.
other data. From this observation we conclude that the IER
which has the smallest effect on the measured d13C and d18O
values is between 0.68 and 0.72. This ratio is slightly lower
than but close to the IER value (0.715–0.74, see Fig. 3)
obtained from introducing the pure gases into the mass
spectrometer. The difference does not appear significant
based on the data in Figs. 4 and 5. The effect itself is very small
and the influence on the extracted isotopic results negligible.
On the other hand, the improvement in the correlation
coefficient of d18O alone is very pronounced, supporting that
0.68 to 0.72 is very close to the optimal value for the IER in our
instruments when measuring CO2-in-air mixtures. The slope
of the data is closest to zero for the IER ¼ 0.68 and 0.72 data
sets. When the IER is smaller (0.68) the slope for both isotope
ratios becomes negative. It attains higher positive values for
larger values of IER.
CONCLUSIONS
We have shown that the IER applied in the N2O correction
equation varies with electron energy inside a mass spectrometer. The electron energy used to obtain a maximum in
ion beam intensity depends on the selected focusing conditions and varies from one instrument to another. The role of
the IER becomes more serious when the N2O contents of
Rapid Commun. Mass Spectrom. 2004; 18: 1830–1838
Effect of N2O on the isotopic composition of air-CO2 samples
1837
Figure 4. Isotope ratio results from the mixing experiment. d18O of CO2 was calculated using different
values for the IER N2O/CO2. CO2 is from the same cylinder and should exhibit a value independent of the
N2O admixture. This is observed when values of 0.68 and 0.72 were used for the IER in the mass balance
correction (fat lines). See Fig. 6 for further statistical details.
the samples are higher than atmospheric values. A thorough
evaluation of the IER for a particular instrument used for
measurement of isotopic compositions of atmospheric CO2
is recommended. This can enhance the possibility of obtaining high-precision isotope data irrespective of a varying concentration of N2O and/or CO2 in the samples. We observed
an overall improvement in precision by 0.02% for d18C and
0.04% for d18O when adopting an ionisation efficiency ratio
(IER) of 0.68 in the data reduction algorithm. On the other
hand, the effect on the final data is rather small, in particular
when reference is consequently made to a well-established
air standard that is extracted and measured along with samples using identical procedures.11
For the relationship between the CO2-in-air scale and
VPDB, based on a calcite mineral (NBS19), the situation is
different. The mineral has no N2O, any change in air
composition (r) or in the value for the IER according to
Eqn. (3) will alter true d13C and d18O values in a directly
Figure 5. d13C values as in Fig. 4. The fat solid line is the regression for IER ¼ 0.68 as well as 0.72,
indicating that both values can correct the experimental data equally well.
Copyright # 2004 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2004; 18: 1830–1838
1838
P. Ghosh and W. A. Brand
Figure 6. d13C and d18O data as corrected in Figs. 4 and 5. The five sets of evaluations of the data
suggest an IER value of 0.7 as the best fit.
proportional fashion. Establishing a valid IER for isotopic
measurement of air –CO2 samples is an important factor for
improving measurement precision and thereby increasing
the possibility of detecting the very small annual changes in
isotopic composition of atmospheric CO2 in the future.
Careful N2O correction will have two major effects for CO2in-air isotope measurements made in a variety of laboratories: Firstly, inter-laboratory comparability of the measurements can be enhanced by unifying the correction and taking
the individual mass spectrometric effects into account. To do
this, the mixing ratios of N2O as well as CO2 must be
measured for every sample and the IER must be established
for each mass spectrometer and relevant change in ion source
setting, preferably from gas mixtures rather than from the
clean CO2 and N2O gases. Second, the relation of the air-CO2
isotope scale and VPDB, defined through a calcite material
(NBS19), can be improved using a high-precision N2O
correction. The remaining error in IER of about 3% constitutes
an uncertainty of 0.007% for N2O corrected d13C data for
CO2 in air when reported on the VPDB scale. If reporting is
consistently made on the basis of a (yet to be agreed upon)
CO2-in-air reference material, even this small remaining error
will tend to cancel out.
Acknowledgements
We are indebted to Armin Jordan for determining the trace
gas mixing ratios and to Michael Rothe for high-precision
Copyright # 2004 John Wiley & Sons, Ltd.
isotope ratio analysis. The reviewer’s comments have been
very helpful and have led to a considerable improvement in
the presentation of the data. This work was made possible
through a postdoctoral fellowship to P. Ghosh in the framework of the TACOS project, funded by the European Union
under contract number EVR1-CT-2001-40015.
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