N2O influence on isotopic measurements of atmospheric CO2

RAPID COMMUNICATIONS IN MASS SPECTROMETRY

Rapid Commun. Mass Spectrom. 2004; 18: 1839–1846

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rcm.1559

N2O influence on isotopic measurements

of atmospheric CO2

Carmina Sirignano*, Rolf E. M. Neubert and Harro A. J. MeijerCentre for Isotope Research (CIO), Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Received 4 May 2004; Revised 18 June 2004; Accepted 21 June 2004

In spite of extensive efforts, even the most experienced laboratories dealing with isotopic

measurements of atmospheric CO2 still suffer from poor inter-laboratory consistency. One of the

complicating factors of these isotope measurements is the presence of N2O, giving rise to mass

overlap in the isotope ratio mass spectrometer (IRMS). The aim of the experiment reported here

has been twofold: first, the re-establishment of the correction for ‘mechanical’ interference of

N2O in the IRMS, along with its variability and drift, and the best way to quantitatively determine

the correction factors. Second, an investigation into secondary effects, i.e. the influence of N2O

admitted with the CO2 sample on the ‘‘cross contamination’’1 between sample and (pure CO2)

working gas. To make the suspected effects better detectable, isotopically enriched CO2 gas with

different concentrations of N2O has been measured for the first time. No evidence of secondary

effects was observed, from which we conclude that N2O is not a major player in the inter-laboratory

consistency problems. Still, we also found that the determination of the ‘mechanical’ N2O correc-

tion needs to be very carefully determined for each individual IRMS, and should be periodically

re-determined. We show that the determination of the correction should be performed using

CO2/N2O mixtures with concentration ratios around that of the atmosphere, as the extrapolation

from pure gas end member behaviour will give erroneous results due to non-linearities. For our

IRMS, a VG SIRA series II, we find a correction of 0.23% for d45CO2 and 0.30% for d46CO2 of atmo-

spheric samples, (with 0.85% mixing ratio). This implies that the relative ionisation efficiency (E)

value associated with this machine is 0.75. Copyright # 2004 John Wiley & Sons, Ltd.

The spatial and temporal variations of the sources and sinks

of atmospheric CO2 can be distinguished and quantified

applying measurements of stable isotopic ratios of atmo-

spheric CO2,2–7 especially the ratio of the stable carbon

isotopes 12C and 13C. Since the trends in this ratio caused by

anthropogenic emissions are small compared with the natur-

al variations, very high precision and accuracy are needed

in these measurements, as well as long-time stability and

traceability. For d13C an inter-comparability of 0.01%between different laboratories or measurement networks is

demanded, to permit the integration of data from all collec-

tion sites into one global dataset as input for global carbon

cycle models.8 Slightly relaxed requirements apply to d18O.

d13C and d18O are defined in the usual way and given on

the VPDB-CO2 scale,9 where not stated otherwise.

Sources of uncertainty can be found throughout the process

from sampling to data processing. Air samples from remote

places are normally filled into glass flasks for laboratory

analysis manually or applying dedicated devices.5,10–12

Concentrations and isotopic compositions of the sampled

gases might be changed during storage and travel times by

effects such as water interaction13 or leakage and permeation

through the flask valves.14 In the laboratories, different

extraction devices are used to quantitatively freeze out the

CO2 for mass spectrometric isotope analysis.15 Finally,

the calibration of reference gases (derived from calcium

carbonate material), IRMS effects, like reservoir bleed effect

and cross-contamination,1 and the ion correction algorithms

employed9 may vary between laboratories or between mass

spectrometer types. Corrections have to be applied due to:

(i) conversion of the measured d45CO2 to d13C, correcting

for the 12C17O16O contribution to m/z 45;

(ii) conversion of the measured d46CO2 to d18O, correcting

for the 13C17O16O and 13C17O2 contributions to m/z 46;

and

(iii) cryogenic co-extraction of N2O with CO2 (both have

isotopomers with m/z 44, 45, 46).

Although all steps have been studied quite extensively,

doubts arise every time an inter-comparison exercise is

conducted, often resulting in a consistency among different

laboratories that is poor compared not only with the precision

requirements, but also the respective laboratories’ internal

precision.

At present, the majority of the groups that perform high

accuracy isotopic measurements on atmospheric CO2 rely on

their suite of ‘‘whole air standards’’, from which they extract

CO2 in the same way they extract CO2 from their samples.

Variability or drifts between the ‘‘whole air CO2’’ and pure

Copyright # 2004 John Wiley & Sons, Ltd.

*Correspondence to: C. Sirignano, Centre for Isotope Research(CIO), Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Gro-ningen, The Netherlands.E-mail: [email protected]

CO2 reference materials is quite commonly observed.

Whereas this complicates the calibration of the isotopic

measurement, and—thus—the comparability of one’s pro-

gram with that of other groups, it is generally agreed upon to

be the best way of keeping one’s own measurement series

internally consistent.

Differential drift, i.e. the difference between isotopic

measurements of whole air CO2 and pure CO2, is commonly

attributed to variability in the cryogenic extraction process.

Variability in completeness of extraction, wall exchange

effects, etc., are the type of effects that are usually referred to.

Other possible sources of differential drift, such as N2O

effects, are generally thought to be less of a problem.

The N2O experiments we report on here result from the

unique situation that emerged connected to the isotopic

analysis of atmospheric CO2 from the sampling network of

the Carbon Dioxide Research Group12 of the Scripps

Institution for Oceanography of the University of California,

San Diego (SIO). Our group had started these isotopic

analyses on CO2 already in 1977.16 From 1993 onwards, the

measurements were carried out at SIO itself, with a full year

of overlap between the two laboratories, as well as intensive

exchange of whole air standards and pure CO2 gases. The

unique fact is that from 1977 onwards, until the present day,

the CO2 extraction has been performed at the SIO laboratory

with the same cryogenic device and procedure. The extracted

CO2 samples were transferred, first to CIO, and later ‘‘in-

house’’, in so-called flame-off tubes (Pyrex tubes that were

closed by melting the glass together).

The results of the detailed CIO-SIO inter-comparison

showed a systematic offset in the d13C values of whole air

samples measured at the CIO of (�0.102� 0.005)% compared

with those measured at SIO (i.e. the CIO values being more

negative), while the offset in pure CO2 was found to be

significantly smaller at (�0.049� 0.005)% (determined using

the GS-19 and GS-20 reference materials, which are pure CO2

gases close to atmospheric isotopic CO2 composition; H. A. J.

Meijer et al., in preparation). Obviously, in this case, the

cryogenic extraction procedure cannot be blamed for this

difference.

Therefore, an alternative explanation has been sought.

As the only difference between the two classes of samples is

the presence of N2O, we decided to study the effects of N2O in

detail. Our idea was that there might be an N2O influence

beyond the ‘‘mechanical’’ addition of isotopomers with

masses 44, 45 and 46. For example, the traces of N2O might

influence the ionisation parameters or the cross contamina-

tion1 of the mass spectrometers in different ways. Therefore,

in order to enlarge the suspected effects, isotopically enriched

CO2 gas, mixed with different portions of N2O, has been

applied along with CO2-N2O mixtures of natural isotopic

composition.

Furthermore, we investigated in detail the usual calibra-

tion and correction procedures for N2O with emphasis on the

variability and the possibility of systematic errors.

THEORETICAL BACKGROUND

For mass spectrometric stable isotope analysis of carbon

dioxide from atmospheric samples, CO2 has to be separated

from the other air constituents. The most widely used proce-

dure for extracting CO2 is by quantitatively freezing out the

CO2 fraction in a cold trap at liquid nitrogen temperature

(�1968C), combined with a separation step from water

vapour, kept at �808C. Unfortunately, nitrous oxide (N2O,

an atmospheric greenhouse gas with concentrations around

320 ppb) has very similar physical properties and thus cannot

be separated cryogenically from CO2. Moreover, the masses

of the N2O isotopomers equal those of CO2 with the main

mass numbers 44, 45 and 46 amu. Thus atmospheric N2O

interferes with the mass spectrometric isotope analysis of

CO2, turning the apparent d13C and d18O values of CO2

too negative by (0.21� 0.01)% and (0.30� 0.01)%,17–20

respectively.

Until now any alternative procedure of removing the N2O

fraction proved to be inadequate because of insufficient

precision (e.g. separation by gas chromatography21) or

because it destroys the oxygen isotope ratio (reducing

N2O in a copper oven19). A correction for the N2O contri-

bution after measurements of its fragments in the same

time (following Friedli and Siegenthaler18 or Mook and

Jongsma19), or after N2O concentration measurements, is still

the most common practice. This, however, depends strongly

on the mass spectrometer type and even on the actual

operating conditions.

As reported by Friedli and Siegenthaler,18 the observed

isotopic ratios R45m for masses 45:44 and R46

m for 46:44 of the

CO2 and N2O mixtures can be expressed as:

R�m ¼ R�

C þ �ER�N

1 þ �E� ð1Þ

Here � ¼ ½N2O�=½CO2� is the concentration ratio of the two

species, which can also be expressed as the ratio of the

respective partial pressures � ¼ pN2O=pCO2, further on re-

ported as mol N2O/mol CO2 (sometimes omitted) or in %units. E, termed ‘ionisation efficiency ratio’,20 is the ratio of

the ionisation yields of N2O and CO2. With ionisation yield

we mean the ion current of m/z¼ 44 (I44) produced by the

CO2 or N2O gas, with a certain partial pressure p.

E ¼I44ðN2OÞ

pN2O

I44ðCO2ÞpCO2

ð2Þ

where the subscripts c, n and m refer to CO2, N2O and mea-

sured, respectively, and the symbol * stands for either 45 or

46.

The observed raw delta value measured on a mass

spectrometer when a CO2/N2O mixture has been admitted

can be expressed as:

��m ¼ R�m

R�reference

� 1

¼ �E

1 þ �E� R�

n � R�reference

R�reference

þ 1

1 þ �E� R�

c � R�reference

R�reference

ð3Þ

Writing the delta values that one would obtain measuring

N2O as if it were CO2, i.e. versus a CO2 standard, as:

��n ¼ R�n

R�reference

� 1 ð4Þ

Copyright # 2004 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2004; 18: 1839–1846

1840 C. Sirignano, R. E. M. Neubert and H. A. J. Meijer

and substituting in Eqn. (3), the relation between the mea-

sured and the corrected delta values can be given as:

��m ¼ �E

1 þ �E��n þ 1

1 þ �E��c ð5Þ

Finally, from the last equation the corrected value for CO2

samples, contaminated by co-extracted N2O, can be derived:

��c ¼ ��m þ Eð��m � ��nÞ� ð6Þ

This equation shows that the deviation introduced by the

N2O contribution is with a good approximation linearly

dependent on the mixing and ionisation efficiency ratios, the

variation introduced into the brackets by dm being negligible

as compared with dn and the measurement uncertainties.

Craig and Keeling22 were in 1963 the first to correct for the

N2O contribution in the isotopic analysis of atmospheric

carbon dioxide, suggesting a correction of þ0.31% for d13C

and þ0.44% for d18O. Later on Mook and van der Hoek,20

for samples of natural isotopic composition, approximated

Eqn. (6) to:

��c ¼ ��m � E��n� ð7Þ

and suggested a correction as follows:

�13C � �13Cm ¼ ð343 � 6Þ�E ð% Þ�18O � �18Om ¼ ð497 � 6Þ�E ð% Þ ð8Þ

They also took into account the different ionisation yield of

the two gas species, that they determined by comparing the

behaviour of the pure gases, stating that the efficiency ratio

depends neither on the inlet pressure, nor on the concentration ratio.

The literature values of E given for different mass spectro-

meters range from 0.68 to 0.75,18–20,23 leading to reported

corrections that vary from 0.20 to 0.22% for d13C and from

0.29 to 0.32% for d18O with r¼ 0.85%.

EXPERIMENTAL

Three series of four different artificial mixtures each, i.e.

CO2 with N2O traces, were prepared. Series AN and BN

were prepared using CO2 with a natural isotopic composition

and the series BE using CO2 enriched both in 13C and in 18O.

Series AN and series BN/BE were prepared and measured in

different time periods with an 18 month time lag. Table 1

gives a summary of the sample descriptions. Doubly labelled

CO2 was used in order to highlight possible secondary N2O

effects, such as an enhanced cross contamination of the

machine working standard due to N2O admitted to the

source with the sample. The enriched gas was prepared at

the CIO. For the oxygen signature, enriched water, usually

employed as tracer in biological applications, was brought

into isotopic equilibrium with the CO2 sample. For the carbon

signature, CO2 was developed from highly enriched sodium

carbonate (98% Na213CO3). The CO2 gas, enriched or not, was

transferred into 250 mL stainless steel cylinders, and sealed

with two manually operated Nupro valves series H (Nupro

Company-Willoughby, OH, USA). The same cylinders

were used later also to contain the final samples. After their

filling with CO2, the cylinders of each series were connected

to a common manifold (pumped to vacuum beforehand) and

left open and connected for about a week to allow the gas to be

isotopically the same in all the cylinders of a certain batch.

This appeared to be sufficient for the natural series only.

Repeated measurements of all four cylinders of series BN

showed a standard deviation of 0.015% for delta 45 and

0.011% for delta 46 as measured with respect to the machine

reference gas. No differences in the composition of the gas

could be found in the different containers. Therefore, just

one cylinder was used as a test cylinder to directly compare

the pure CO2 gas with the N2O-spiked ones. The enriched

samples, however, stayed significantly different from each

other even after 1 week of diffusive exchange. Thus they

had to be characterised individually by repeated indepen-

dent measurements before the addition of N2O. Furthermore,

the oxygen measurements of the enriched samples in parti-

cular were influenced by daily variations in the machine

behaviour. This could be confirmed by the variations in the

measurements of an independent enriched CO2 gas standard

(GS-45), routinely measured in three flasks at the end of every

measurement day. This part of the test was conducted in early

summer, when, according to our experience, the increase

in temperature and thus the variability of the laboratory

Table 1. The isotopic characteristics of the pure gases (all values are given in%with respect to our machine reference gas) and

mixing ratios of the artificial mixtures used

Samplename

r assigned (molN2O/mol CO2)

r calculated*(mol N2O/mol CO2)

d45CO2 raw(w.r.t. mach.

Ref.) pure gases SD

d46CO2 raw(w.r.t. mach.

Ref.) pure gases SD

CO2 Series AN AN0 0 0 7.11 0.02 �4.22 0.02AN1 (4.21� 0.04)10�4 4.58 10�4

AN2 (8.83� 0.09)10�4 9.17 10�4

AN3 (1.68� 0.02)10�3 1.82 10�3

N2O �349.16 0.35 �491.60 0.30CO2 Series BN BN0 0 0 6.78 0.02 �0.37 0.05

BN1 (3.59� 0.04) 10�4 3.08 10�4

BN2 (7.25� 0.07)10�4 6.70 10�4

BN3 (1.45� 0.01)10�3 1.37 10�3

CO2 Series BE BE0 0 0 446.09 0.02 510.77 0.08BE1 (3.61� 0.04)10�4 3.38 10�4 445.39 0.02 508.12 0.14BE2 (7.23� 0.07)10�4 6.58 10�4 445.89 0.04 509.86 0.28BE3 (1.44� 0.01)10�4 1.35 10�3 446.01 0.04 509.98 0.23

* Calculated from the ion beam of m/z 30, according to Eqn. (9).

N2O influence on isotopic measurements of atmospheric CO2 1841


conditions influence the measurement performance. We sus-

pect that traces of water vapour, entering the inlet system

with every sample change, affect the cross contamination of

the mass spectrometer. Enriched samples with high precision

demands are the most susceptible to this.

The addition of N2O was done by freezing N2O gas, kept at

a known pressure in a calibrated volume, into the sample

cylinders cooled down to liquid nitrogen temperature. The

mixing ratios assigned volumetrically are accurate to��1%,

taking into account the errors in the pressure, temperature

and volume determinations.

The N2O/CO2 mixing ratios (see Table 1) vary largely

around the atmospheric mixing ratio. We roughly aimed at

one ‘‘blank’’ sample, containing pure CO2, one sample at half

of the present atmospheric mixing ratio (0.4%), one sample at

the present atmospheric mixing ratio (0.8%), and one sample

with double the present atmospheric mixing ratio (1.6%).

The assigned mixing ratios were checked by inference

based on the mass spectrometric determination of the ion

current of m/z¼ 30 (NOþ) and after subtraction of the back-

ground signal representing fragments, mainly 12C18O,

produced from CO2 (ðI30ÞCO2):19

½N2O�½CO2�

� �SAMPLE

¼

ðI30ÞSAMPLE � ðI30ÞðCO2ÞSAMPLE

ðI30ÞSTANDARD � ðI30ÞðCO2ÞSTANDARD

½N2O�½CO2�

� �STANDARD

ð9Þ

The standard for this ‘mass 30’ N2O determination was

prepared volumetrically, in a similar way as described above

for the mixtures; it had a N2O/CO2 ratio of 0.0089� 0.0005,

i.e. ten times natural, and was measured along with the B

series, that means at a different time than series A. The ‘mass

30’ results are also reported in Table 1, but are not used in

further calculations. Comparison shows only fair agreement,

which implies that the ‘mass 30’ correction method appears to

be vulnerable for systematic errors.

The actual experiment consisted of two separate parts; first,

verifying the ionisation efficiency of the pure gases CO2 and

N2O, admitted into the mass spectrometer at different inlet

pressures, and second, measuring the artificial mixtures for

their isotopic composition and comparing the results with

those obtained applying the theoretical correction described

in the previous paragraph.

For the first part of the experiment, aliquots of pure gases

were expanded from the cylinders into small glass flasks and

then admitted into the VG SIRA series II IRMS, of which the

electron energy was 58 eV, as for the routine measurements.

For the second part, the test cylinders were directly con-

nected to the manifold of the machine. Each time one aliquot

of the gas mixtures, trapped in the volume between the two

sealing valves, was expanded into the inlet system. After-

wards the pure N2O gas, used for the spiking, was measured

for its delta values ‘‘as if it were CO2’’, with respect to the

regular machine reference gas.

RESULTS AND DISCUSSION

The mass spectrometric response to different inlet pressures

of the pure gases used for the mixtures is shown in Fig. 1(a).

All gases were admitted to the source through the sample

side capillary and the variation in pressure was obtained by

expanding different aliquots of the same gas, filling different

flasks with different pressures, as well as controlling the inlet

volume by adjusting the sample bellow. On the vertical axis

the major beam currents are given as registered on the

m/z¼ 44 Faraday cup of the detector. The gases admitted to

the source of the IRMS were: CO2, both enriched and natural

(higher ionisation efficiency), and pure N2O (lower ionisation

efficiency). The inlet pressure was varied from a few tenths of

a hPa up to 39.5 hPa, resulting in ion beams between 0.1 and

7 nA for CO2 with natural isotopic composition and 5.41 nA

for N2O. The inlet pressure of enriched CO2 was varied

from 0.8 up to 31.85 hPa, producing a current from 0.79 up

to 5.59 nA. As expected the enriched CO2 curve lies below

the natural CO2 one, due to the lower 44CO2 abundance that

accompanies the enrichment in 45CO2 and 46CO2. From mass

balance considerations, the ionisation yield of enriched CO2

(d45CO2¼ 446%, d46CO2¼ 509%) would be expected to be

just 99.3% of that of CO2 of natural isotopic composition.

This results also in a higher relative ionisation efficiency of

N2O when compared with enriched CO2 than when com-

pared with natural CO2. Shown in Fig. 1(b) are the E values,

computed by comparing the current of the N2Om/z¼ 44 with

that of CO2 at the same inlet pressure. The fit curves are

computed from the cubic splines fitting the observation in

Fig. 1(a). These results show a clear picture of the non-linear

response of the ionisation yield at different inlet pressures of

a gas. The assumption that the relative yield of the two species

is independent of the gas pressure in the ion source, com-

monly adopted to estimate E for a given mass spectrometer,

is clearly not even applicable in the case of pure gases. Con-

sidering a pressure domain between 15 and 32 hPa, E is esti-

mated to vary from 0.80 to 0.87, for the N2O yield compared

with that of enriched CO2, and from 0.78 to 0.83, when the

N2O yield is compared with that of natural CO2. Approxi-

mately at the centre of this range, 24 hPa for enriched CO2

and 23 hPa for natural CO2, are the inlet pressures that give

a m/z¼ 44 current of about 4 nA, on which reference and

sample beam are normally adjusted for the measurement.

At these inlet pressures N2O produces signals of 3.4 and

3.2 nA, that give E values of 0.85 and 0.80, respectively,

with a difference too large to be explained simply by isotope

mass balance considerations. Besides this, both these estima-

tions differ from the best guess of E (0.75) obtained from

measurements on the artificial mixtures as described below.

The plot in Fig. 1(c) compares the behaviour of pure

enriched CO2, the same as used for the mixture, pure N2O

and a mixture of the two in equal parts. The behaviour of the

mixture does not follow quantitatively the expected response

estimated from the behaviour of the pure gases (solid line in

the plot).

A summary of the measurements on the artificial mixtures

is given in the plots in Figs. 2–4. The averages of the mea-

surements (5 and 10 repetitions, respectively, on AN and BN)

on the ‘natural CO2’/N2O mixtures have been reported as

completely raw data in terms of delta 45 and delta 46 on the

machine working gas scale. The linear relationship expressed

by Eqn. (6) is completely fulfilled. Regression analysis

between the isotopic results and the assigned mixing ratios

shows values comparable to each other and to those reported



in the literature: for the coefficients we find E �45m � �45

n

� �of

�(268� 10)% and �(267.7� 6.0)%, and E �46m � �46

n

� �of �(378� 19)% and �(320� 20)%, for the AN and BN

series, respectively. If we assume an atmospheric N2O/CO2

mixing ratio of 0.85%, computed from the most recent

concentrations and trends given for both species by

Houghton et al.,24 then the corrections to be applied would

be 0.23 and 0.30% for d45CO2 and d46CO2, respectively. Some

d46CO2 values of the BN series have suffered from the

consequences of traces of water in the mass spectrometer

inlet. This is even more so for the values of the enriched series

(BE), measured during the same period. For obvious reasons

the samples of the BE series could not be measured with the

same accuracy as the natural series. However, the isotope

ratios show a linear dependence of the mixing ratio (r) as

described in Eqn. (6), with a coefficient E ��m � ��n� �

estimated

from a least-squares fit to be (583� 33)% for �45c � �45

m

� �and

(910� 90)% for E �46c � �46

m

� �. These coefficients depend on ��m,

so they increase as the CO2 gas gets enriched. As mentioned

above, in this case, the deviation from the CO2 gas present in

each cylinder before the addition of N2O had to be regarded,

instead of the absolute d-value. Comparing the theoretical

corrections with those measured, no peculiar effects or evident

deviations from the expectations can be shown, indicating that

no evident secondary effects due to N2O ‘contamination’ can

be proven.

As the conclusive step of this experiment, we assigned the

best estimate for the ionisation efficiency ratio E according to

Figure 1. Mass spectrometric response to different inlet pressures of the pure gases used for the mixtures. (a) m/z¼ 44

currents versus the pressure registered by the inlet transducer of the VG SIRA series II mass spectrometer. (b) Relative ionisation

efficiency (E) of N2O compared with CO2 natural and enriched, calculated from pure gases. Symbols represent E values

computed as the ratio of the N2O and CO2m/z¼ 44 at the same inlet pressure. The lines are computed from the cubic spline fits in

(a). (c) Representation of the behaviour of a 1:1 mixture compared with that of its pure gas constituents. The solid line in the plot is

the calculated mean response of those of the pure gases.



the measurements conducted on the mixtures belonging to

the AN, BN and BE series. d46CO2 values have been excluded

from the best estimation of the E parameter, because some of

them, belonging to the BN and BE series, suffered from the

consequences of traces of water in the inlet of the mass

spectrometer. The deviation of the true d45CO2 values from

those computed applying Eqn. (6) is:

��45CO2 ¼ �45c � �45

m þ E �45m � �45

n

� ��

� �ð10Þ

Dd45CO2 must be equal to zero when the correct parameters

are applied. As ‘true value’ for (d45c) we took the original CO2

composition of the samples before the N2O spiking. This is a

single value per series for the natural mixtures AN and BN, as

measured on the pure CO2 samples along with the mixtures.

For the enriched series individual ‘true values’ were assigned

to each different mixture, according to the initial isotopic

composition of the pure CO2 gas. The results of this exercise

are reported in Figs. 5(a) (series AN), 5(b) (series BN), and 5(c)

(series BE), respectively, in which we plotted the results for E

values from 0.6 to 0.9, in steps of 0.02. The closer the arbitrary

E values get to the best E value, the more the corrected values

approach the true pure CO2 value and the deviations

Dd45CO2 tend to be constant at 0. A summary of this simu-

lation can be obtained by plotting versus E the slope or the

correlation coefficient of the regression line through the

different data sets obtained by each value of E (shown in

Fig. 6). The definition of the best E value to be assigned to the

VG SIRA series II instrument is given by the intercept of the

slope or the correlation coefficient functions with the x-axis,

associated to a horizontal line (slope¼ 0) fitting perfectly

uncorrelated values.

As shown in Fig. 6, the best value for E is 0.749� 0.003,

showing no correlation at all between the final CO2 isotopic

values with changing CO2/N2O ratios. The estimation

derived from the analysis of the mixtures containing enriched

CO2 is just slightly higher (0.750) than the estimations based

Figure 2. Isotope measurements on series AN N2O/CO2

mixtures. Mass ratios 45/44 (left axis) and 46/44 (right axis)

are expressed as d45CO2 and d46CO2, respectively. [N2O]/

[CO2] is the partial pressure ratio of N2O to CO2. The symbols

represent the average of repeated measurements, their

standard error is given as error bars (�1s/Hn). Least-squares

regression leads to: dm45CO2¼ (7.11� 0.01) � (268�

10)[N2O]/[CO2]; dm46CO2¼ (�4.22� 0.01) � (378� 19)[N2O]/

[CO2]. The present [N2O]/[CO2] mixing ratio is indicated in the

figure as a vertical line; it is computed from the most recent

(2001) concentrations and trends given for both species by

Houghton et al.24

Figure 3. Isotope measurements on series BN N2O/CO2

mixtures. Least-squares regression leads to: dm45CO2¼

(6.79� 0.01) � (267.7� 6.0)[N2O]/[CO2]; dm46CO2¼ (�0.36�

0.01) � (320� 16)[N2O]/[CO2]. For further description, see

Fig. 2.

Figure 4. Isotope measurements on series BE N2O/CO2

mixtures. The apparent change in isotopic composition

(d45CO2, left axis, and d46CO2, right axis) is shown as the

deviation from the individual CO2 gas composition present in

each cylinder before the addition of N2O. The error bars give

the standard errors of repeated measurements after N2O

spiking. Least-squares regression leads to: (dc45CO2�

dm45CO2)¼ (583�33) [N2O]/[CO2]; (dc

46CO2�dm46CO2)¼ (910�

90) [N2O]/[CO2].



on natural mixtures, as is expected from mass balance

considerations. This testifies to the absence of any provable

secondary effect due to the N2O admitted to the source of a

mass spectrometer together with the sample.

CONCLUSIONS

In order to achieve very accurate atmospheric CO2 isotopic

determinations, a very accurate N2O correction has to be

applied. We did not find any secondary effects produced

by N2O different from the barometric additions on m/z¼ 44,

m/z¼ 45, m/z¼ 46, as shown by the behaviour of enriched

CO2 samples spiked with N2O. However, evidence of non-

linear response to the inlet pressure of the pure gases and a

different behaviour of these compared with mixtures of

CO2 with traces of N2O suggest a careful determination of

the relative ionisation efficiency used as parameter in the

N2O correction algorithm. In particular it is recommended

to estimate the N2O correction to be applied for each machine

from mixtures of well-known mixing ratios in the range of the

measured samples, and using the same settings of the IRMS

source as for the actual measurements. This procedure is pre-

ferable over the application of the theoretical algorithms with

the E parameter being derived from pure gas measurements.

For our VG SIRA series II, we find as best value for the ionisa-

tion efficiency ratio E¼0.75. This results in a correction of

0.23% for d45CO2 and of 0.30% for d46CO2, for atmospheric

samples with 0.85% N2O/CO2 mixing ratio. Regarding our

results, we conclude that it is unlikely that N2O effects cause

the difference between pure gas and whole air CO2 results

between CIO and SIO. The N2O corrections made for

d45CO2 were almost the same (0.217 and 0.210% for CIO

and SIO, respectively, for 1992–1994 air with mixing ratios

around 0.87%15). To explain the difference of over 0.05%in d45CO2, we would have to assume drastic errors in these

values, and thus in the E determination of the IRMS systems

involved. Furthermore, as our results show that secondary

effects of N2O are absent, the differential drifts between

pure CO2 and atmospheric CO2 that many groups observe

cannot be blamed on N2O either. However, the CIO-SIO

comparison shows that, besides the cryogenic extraction,

there must be other sources of error and deviation, yet to be

identified.

Figure 5. Differences between the true d45CO2 values and

those computed applying Eqn. (6), using different E values.

Reported are the results for E ranging from 0.60 to 0.90, in

steps of 0.02: (a) simulation on series AN; (b) simulation on

series BN; and (c) simulation on series BE.

Figure 6. Correlation coefficients (on the right axis) and

slopes (on the left axis) associated with the regression lines

fitting the computations shown in Figs. 5(a)–5(c). The data

are plotted against E, in steps of 0.01, with a higher resolution

than in the previous plots, but for the same simulated range

of E.



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N2O influence on isotopic measurements of atmospheric CO2

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