Introduction
Carbon monoxide (CO) has an average mole fraction of only ∼ 100 nmol mol-1
(parts per billion or ppb) in the atmosphere, but it has
a large yearly turnover of about 2700 Tg (Brenninkmeijer et
al., 1999) because of its fast reaction rate with the hydroxyl radical
(OH⚫). It is produced by numerous sources at the earth's surface
and in the atmosphere. About one-third of the atmospheric CO originates from
methane oxidation while fossil fuel combustion, biomass burning, and
oxidation of non-methane hydrocarbons (NMHC) are other important sources
(Brenninkmeijer et al., 1999). The strong latitudinal
gradient is a result of the main sources being in the Northern Hemisphere,
and the seasonal cycle of CO is largely driven by the seasonality of the
OH⚫ (Röckmann et al., 2002). The reaction
CO + OH⚫ is the main sink not only for CO but also for
OH⚫, with CO consuming approximately 60 % of OH⚫ in
the atmosphere (Crutzen and Zimmermann, 1991). An increase in the
CO mole fraction will therefore cause a decrease in the oxidation efficiency
of the atmosphere, resulting in a build-up of other gases, such as the
long-lived greenhouse gas methane, which are primarily removed by
OH⚫. Consequently, CO is established as an important indirect
greenhouse gas in the recent Intergovernmental Panel on Climate Change (IPCC) assessment report
(Hartmann et al., 2014). In addition, under high-NOx (mono-nitrogen oxides) conditions
the oxidation of CO leads to the production of ozone, contributing to the
build-up of photochemical smog (Westberg et al.,
1971).
The stable isotopes of carbon and oxygen in CO, 12C, 13C,
16O,17O, and 18O, are naturally abundant at levels of
98.89 %, 1.11 % (for carbon), 99.76 %, 0.04 %, and 0.20 % (for
oxygen) (Röckmann and Brenninkmeijer, 1998), respectively.
Delta (δ) values are defined as relative isotopic enrichments of a
sample to a reference.
δ=RSampleRReference-1
The isotope ratio, R, is 13C / 12C in the case of carbon (δ13C) and 18O / 16O for oxygen (δ18O). For CO,
δ values for 13C and 18O are usually reported against the
international reference materials V-PDB (Vienna Pee Dee Belemnite) and
V-SMOW (Vienna Standard Mean Ocean Water), respectively. Since stable isotope
variations in nature are small, these δ values are expressed in per
mill (‰).
Precise measurements of CO mole fraction and isotopic composition are useful
in constraining individual source and sink processes. The combination of the
δ13C and δ18O values gives a distinct isotopic
signature for each individual CO source. CO from methane (CH4)
oxidation is the most 13C-depleted source, with δ13C
values around -50 ‰ (Brenninkmeijer and
Röckmann, 1997). δ13C values for CO from fossil fuel
combustion, biomass burning, and NMHC oxidation
range between -27 and -32 ‰
(Manning et al., 1997; Stevens and Wagner, 1989). For
CO sources that have a range overlap in δ13C values (vehicle
emissions range ∼ -36 to -20 ‰, biomass
burning range ∼ -25 to -21 ‰, and NMHC oxidation range ∼ -37 to -27 ‰), δ18O proves to be a better tracer. Carbon monoxide from vehicle
emissions has the highest δ18O values, 24 ‰
(Popa et al., 2014; Tsunogai et al.,
2003) compared to 7–10 ‰ for biomass burning
(Röckmann
et al., 1998; Tarasova et al., 2007). For NMHC oxidation no direct
measurements are available and a δ18O value of 0 ‰ was indirectly derived from isotope budget
considerations (Brenninkmeijer and Röckmann, 1997).
Originally, CO isotope analysis was carried out with offline extraction
systems, which require large amounts of air (Brenninkmeijer,
1993; Stevens and Krout, 1972), but in recent years, continuous-flow
techniques have been developed to accommodate smaller sample volumes
(Tsunogai et al., 2002; Wang and Mak,
2010). The method of Brenninkmeijer (1993) required sample sizes of the order of
100 L, whereas the method of Wang and Mak (2010) was optimized for a volume of 0.1 L. There are two conceptually different techniques allowing for isotopic
analysis of δ13C and δ18O of carbon monoxide. One
uses the principle of conversion to carbon dioxide (CO2) and subsequent
isotope analysis of CO2 (Brenninkmeijer, 1993; Stevens
and Krout, 1972; Wang and Mak, 2010), and the other uses isotope measurement
on CO directly (Tsunogai et al., 2002). The
technique of converting CO into CO2 has the advantage as high-precision
mass spectrometry is often based on CO2, which allows for the possibility
of using standardized techniques and isotope calibration scales
(Brenninkmeijer et al., 1999). CO is converted to CO2
using an oxidizing agent, and if the isotopic composition of this oxidizing
agent is constant, its effect on the isotopic composition of the CO2 product can be taken into account. The need to correct for the additional
O atom is the weakness of this method.
In the direct method, CO+ ion currents at masses 28, 29, and 30 are
monitored simultaneously (Tsunogai et al.,
2002). Since CO is not converted to CO2, there is no introduction of an
additional oxygen atom that needs to be calibrated.
Current continuous-flow isotope-ratio mass spectrometry (CF-IRMS) techniques
move towards faster methods, smaller sample sizes, and most importantly more
precise and reproducible results. Rapid methods allow for multiple measurements
that can be combined to improve the error of the mean of measurements for
one sample. A method that requires a smaller sample volume for a single run provides the opportunity not only to measure samples multiple times but also
to measure small air samples, e.g. from ice cores (Wang and Mak,
2010) or firn air
(Petrenko
et al., 2013; Wang et al., 2012), expanding the range of possible
applications.
This paper presents a method to measure the mole fractions, δ13C and δ18O of CO, in an air sample in less than 20 min. The
method uses 100 mL STP (standard temperature and pressure) sample gas. The minimum pressure
required in a 1 L glass flask, to perform a single run with a stable flow rate of 20 mL min-1, is 1550 mbar abs. The blank from the oxidant (Schütze reagent)
that is used to convert CO to CO2 is strongly reduced by continuously
flushing the reagent with high-purity He, which leads to highly reproducible
results.
Conceptually, the key difference to the system by Wang et al. (2010) is that
our extraction system always operates under a flow of He; it is thus a
conceptually different realization of the same idea of extraction and
preparation of CO. The system is particularly well-suited for routine
operation and automated analysis of many samples. In terms of precision, the
overall results are similar to the system described by Wang et al. (2010),
and the main improvement is the strong reduction of the system blank.
Experimental
Method and instrumentation
A diagram of the analytical system is shown in Fig. 1. The system consists
of an automated multiple sample inlet system, the CO extraction and
conversion set-up, a gas chromatograph (GC) for purification of the CO2,
an open-split system, and an IRMS. The system is at all times flushed with
ultra-high-purity helium (He) with BIP®
technology (BIP is Built In Purification; specification number: He-26507;
assay: 99.997 %), provided by Air Products. Fused silica capillaries are
used for connecting components unless specified differently. There are two
membrane vacuum pumps (P1 and P2) attached to the set-up: one to
evacuate the multi-sample inlet line (P1) and the other one at the
exhaust of the extraction (P2), conversion, and collection unit. A
single, automated, measurement is performed in 18 min.
A diagram of the continuous-flow isotope-ratio mass spectrometry
system for measuring 13C and 18O of CO. S1 to S8
represent the sample flasks; the connections to the flask can be changed to
accommodate other types of cylinders or cans. The multi-sample inlet system
can be evacuated by a pump (P1). V1, V2, V3, V4, and
V5 are Valco valves. The sample is admitted to the analytical system via
a mass flow controller (MFC) and a 6-port valve (V3) and either sample
gas or He carrier gas are processed through the system by a vacuum pump
(P2) at the outlet of the extraction system. T1, T2, T3,
and T4 are the cryogenic trap filled with glass beads, the glass tube
with Schütze regent, the collection trap to extract the CO2 from CO,
and the focus trap, respectively. Final separation of the CO-derived CO2
is achieved on a gas chromatographic column (GC: Poraplot-Q, 25 m × 0.25 m).
Multi-sample inlet system
Sample flasks are connected to the automated multi-sampling unit for
analysis. This unit allows for automated measurements of the reference gas and
up to eight sample flasks. Sample flasks are connected to a 8-port 16-position dead-end flow path selector (V1: VICI; product number: SD8MWE). The
3-port switching valve (V2: VICI; product number: 3UWE), after the
sample multi-port, provides the option to select either the reference gas
cylinder or one of the samples. The air to be analysed is then directed via
a mass flow controller (MKS; model 1179, 100 sccm) to a 6-port 2-position valve (V3: VICI; product number: 6UWM) from where the gas can be either
injected to the extraction system (position 1) or evacuated (position
2). A 1/16 inch Restek Silcosteel® coated stainless steel
tubing is used for connecting the individual components
(Tsunogai et al.,
2000). This multi-sampling unit is controlled by LabView software, which
specifically controls V1, V2, the flow rate of the mass flow
controller, the sample injection time, the sample flush time, and the number
of times each sample is measured. The sample is injected into the system at
a flow rate of 20 mL min-1 for 5 min. A higher flow rate and longer
injection time can be used for measuring samples with lower mole fractions.
The LabView program also records the values from the pressure sensor before
the mass flow controller and gives a start signal to the ISODAT program to
start its acquisition. The interface that is essential for the communication
between the valves and the PC is National Instruments USB NI-6008 unit.
When starting an automated measurement sequence, first the eight samples are
connected to the inlet system (V1) and V2 is set in the direction
of the samples. Then V3 is set to evacuation position and the
membrane pump valve is opened, allowing the air from the point of the sample
connection to V3 to be evacuated. Following this procedure the lines
connected to all the flasks are evacuated by switching V1 and tested
for leaks. After the leak test, the sample bottle/can/cylinder valves are
opened. From this point onwards the method is fully automated. The final
pressure in the sample admission part of the system prior to the
introduction of the sample is ∼ 1 mbar.
To avoid contamination with remaining air when switching between samples via
the multi-sample inlet system, V1 is first set to the
close position between two sample ports and the system is evacuated for 60 s.
Afterwards the multi-sample inlet system is flushed with the new sample air
for 55 s at a flow rate of 20 mL min-1 before it is injected via
V3.
Extraction and conversion set-up
By switching the injection valve (V3), the sample is injected into the
extraction system and directed through a chemical trap containing
Ascarite™ (CO2 absorbent, 8–20 mesh, Aldrich) and magnesium perchlorate (Sigma-Aldrich), removing CO2 and H2O, respectively.
The subsequent cryogenic trap (T1-3 mm ID, 6 mm OD, 62 cm length,
glass), containing glass beads (US mesh 40–60), removes CO2, N2O,
and other condensable gases at liquid Nitrogen temperature (-196 ∘C).
CO is then selectively oxidized to CO2 using the Schütze reagent
(Schütze, 1949; Smiley, 1965) in T2. The
oxidation reactor (T2) consists of a 10 cm length 6 mm OD glass tube
and it is filled with 3 g of Schütze reagent. In order to reduce the
Schütze blank, the oxidation tube is located in the loop position of
a 6-port 2-position valve (V4: VICI; product number: C6UWM) and it is
continuously flushed with He (flow rate ∼ 8 mL min-1)
when not in use. The flow is directed through the Schütze oxidant only
during the sample injection and flushing period.
Synthesis of Schütze reagent
In 12.5 mL of water (Sigma-Aldrich, product number 270733:
CHROMASOLV® for High Performance Liquid Chromatography (HPLC) graded water, filtered
through a 0.2 µm filter) 2.5 g of diiodine pentoxide (I2O5, 99.9 %, Aldrich) was
dissolved to obtain a solution of iodic acid
(colourless); 20 g of silica gel (Grade 40, 6–12 mesh, Sigma-Aldrich) was
added to this solution. The mixture, covered with a watch glass, was dried
in the oven for 1.5 h at 145 ∘C. Immediately out of the oven, 5 mL
of concentrated sulfuric acid (H2SO4) was added to the mixture and
the covered beaker was left overnight in the laboratory hood. This allows
the H2SO4 to coat the mixture well, dehydrating it. The mixture
was then placed in the Schütze reagent reactor (Fig. 2). The Schütze
reagent reactor was continuously flushed by a slow nitrogen gas stream and
heated at 220 ∘C for 6 h or until the mixture was dry. The
air exiting the reactor was passed through a molecular sieve and a large
beaker of water. This was done to ensure that the H2SO4 vapour
carried out with the N2 was removed before the N2 was released to
the laboratory. The N2 flow rate was adjusted so that there was a slow
release of bubbles visible in the beaker. The active chemical
(I2O5) was formed only when all the water was removed. The
Schütze reagent is white, but sometimes it may have a bright yellow tint
when iodosyl salts are formed (Schmeisser and
Brändle, 1963). When the reagent turns brown (Formation of iodine: 5CO + I2O5→ 5CO2+ I2) with use, it must be
replaced with a new batch. When the Schütze reagent is replaced, not
only the capillaries but also the reagent is exposed to the atmosphere
causing a build-up of CO2. Once the connections are properly tested for
leaks, the reagent needs to be flushed well with helium, for 3 days to a
week, until the CO2 blank is back to normal low levels (see Sect. 3.1).
A diagram of the apparatus used for preparing the Schütze
reagent. It is mounted in an oven and flushed with a slow continuous flow of
nitrogen gas in order to dehydrate the iodine pentoxide-coated silica gel
efficiently, while the temperature distribution is homogeneous.
Collection, focus, and separation
The CO-derived CO2 is trapped in the collection trap, T3 (1/16 inch stainless steel tubing), using liquid nitrogen while the other gases
are removed via the vacuum pump. The CO2 sample is then transferred to
a focus trap (T4), when the 6-port 2-position valve (V5: VICI;
product number: 6UWM) is switched to the inject
position. In T4, the
320/430 µm fused silica capillary, used throughout the system,
continues through 1/16 inch stainless steel tubing (tubing is used only to
protect the capillary). The liquid nitrogen level of the cold traps is
controlled by a liquid nitrogen refiller (NORHOF 900 series LN2 micro-dosing
system) to improve the reproducibility of the peak areas. When T1, T3, and T4
are in the down position the traps acquire the temperature of liquid
nitrogen (-196 ∘C), and when the traps are in the up position they
warm up to room temperature (25 ∘C).
The sample is separated from other residual components, on a Poraplot-Q (25 m × 0.25 mm) gas chromatography column (at 50 ∘C). It is then dried via
a Nafion dryer. Finally, the sample is transferred into a Thermo-Finnigan
Delta V Plus IRMS through a custom-made
(Röckmann et al., 2003)
open-split interface. The ISODAT program controls the components from the
cryogenic trap to the open split. All the valves and the traps are air
actuated and controlled by solenoids linked to the interface with IRMS.
In routine operation, the entire system is flushed for 425 s between
runs, the Schütze reagent is introduced into the main gas stream 425 s before injection of sample air, the sample processing takes 300 s, followed by another 300 s of flushing before the sample is
transferred from trap T3 to T4. The cryogenic trap that removes
the remaining traces of CO2 and N2O is warmed to room temperature
for 302 s in between runs to remove the eluted gases and is cooled
again for 123 s before the next sample is admitted.
Data processing and calibration
In order to monitor the performance of the CO isotope system, we prepared a
reference air (Ref) sample with a known mole fraction and isotopic
composition (see Sect. 2.2.2). This reference air is dry ambient air in a
30 L aluminum cylinder (Luxfer with Rotarex Ceodeux brass valve, used with
a Scott Specialty Gases type 51–14C pressure regulator) pressurized up to
130 bar at the Max Plank Institute for Biogeochemistry (MPI-BGC) in Jena,
Germany, in 2009.
Ref is run multiple times and an evaluation of these runs helps determine
the reproducibility and accuracy of the system. Ref is analysed before and
after every sample run to enable calibration and to detect variations in
system sensitivity.
CO2 derived from CO in a sample is analysed by the mass spectrometer.
CO is quantitatively converted to CO2 using Schütze reagent
(Brenninkmeijer, 1993). Therefore the quantity (in moles) of CO is equal to
the quantity of CO2 derived from the CO in the air sample.
Carbon monoxide mole fractions are calculated using a 1-point calibration,
according to
cS=area allSarea allR⋅fRfS⋅tRtS⋅cR,
where cS is the mole fraction of the sample, area allS is the
area of the sample peak, area allR is the area of the Ref peak,
fRfS is the ratio of the reference flow rate and sample
flow rate, tRtS is the ratio of the reference injection
time and sample injection time, and cR is the mole fraction of the
reference air cylinder. For typical ambient air samples
fRfS=tRtS=1.
The ISODAT software reports the δ values of each peak in the
chromatogram (both sample and reference air) vs. the laboratory working
gas, δS vs. WG, and δR vs. WG. In
our data reduction procedure, we first use these values to calculate the isotopic composition of the sample vs. the reference, δS vs. R, according to
δS vs. R=δS vs. WG-δR vs. WG1+δR vs.
WG.
For δR vs. WG we use the average of the reference δ
values before and after the sample run. Then, the δ value of the
sample is converted to the international reference scales via
δS vs. V=δS vs. R+δR vs. V+δS vs. R⋅δR vs. V.
δR vs. V is the δ value of the reference air cylinder
vs. the international standard, V-PDB or V-SMOW.
In δ18O data evaluation, δS vs. V is the δ value of the sample vs. the international standard, V-SMOW for
CO2. CO2 is generated when the CO from the sample is oxidized by
the Schütze reagent.
CO+I2O5→CO2
Therefore, a correction has to be made to get the, δS vs. V for
CO (Brenninkmeijer, 1993).
δ18OS vs. V:CO=2δ18OS vs. V:CO2-δ18OSchütze
Reagent=2δ18OS vs.
V:CO2-2δ18OR vs. V:CO2-δ18OR vs. V:CO
In Eq. (), δ18OS vs. V : CO is the
δ value of the sample vs. the international standard for CO, δ18OS vs. V : CO2 is
the δ value of the sample vs. the international standard for
CO2 and δ18OSchütze Reagent is
the O from the Schütze reagent, which is derived using the δ
value of the reference vs. the international standard for CO2
(δ18OR vs. V : CO2) and
the δ value of the reference vs. the international standard for
CO (δ18OR vs. V : CO).
Mole fraction calibration
The mole fraction in Ref air cylinder was determined to be 185.4 nmol mol-1 at MPI-BGC, by using a Trace Analytical
reduction gas analyser
and it is linked to the WMO X2004 calibration scale.
The Ref air cylinder has been regularly measured (since March 2013) against
other gas cylinders, for isotope calibration (see Sect. 2.2.2) and for
checking the system stability. No significant drift in CO mole fraction
relative to other gases has been observed since the measurements described
here have begun.
Isotope calibration
Our measurements of δ13C and δ18O of CO are
ultimately referenced to a calibration gas (Cal) obtained from Carl
Brenninkmeijer, Max Plank Institute for Chemistry, Mainz. The Cal gas is a
mixture of CO (269 × 103 nmol mol-1) in nitrogen with originally
assigned δ values of
δ13CCal vs. VPDB=-44.3‰andδ18OCal vs. VPDB-CO2=δ18ODiCalCO vs. VPDB-CO2=11.43‰(Brenninkmeijer,
1993).δ18ODiCalCO vs.
V-SMOW=53.45‰
An independent calibration of the Cal cylinder was published in 1997
(Brenninkmeijer and Röckmann, 1997), which confirmed the
originally assigned values after a long period of storage. The estimated
maximum uncertainty for 13CCal vs. VPDB was given as -44.30 ± 0.2 ‰. The uncertainty
for 18OCal vs. VPDB-CO2 was given as 11.43 ± 0.3 ‰.
The Cal gas was diluted to a suitable mole fraction (130 nmol mol-1)
with CO-free zero air (checked with a Peak Performer
1 reduction gas
analyser). This diluted calibration gas is referred to as DiCal and it is
assumed that DiCal has the same isotopic composition as Cal. Then, Ref and DiCal were measured 10 times vs. the lab CO2 working gas
using the new measurement system, and the averages were used for calibration
of the isotopic composition of CO in Ref relative to the isotopic
composition of CO in DiCal.
For δ18O, the correction for the oxygen atom from the
Schütze reagent is done in the same manner as Eq. ().
δ18OR vs. V:CO=2δ18OR vs. V:CO2-δ18OSchütze Reagent
The δ18O of the Schütze reagent oxygen is derived by δ18OSchütze
Reagent=2δ18ODiCal vs. V:CO2-δ18OCal vs. V:CO, where δ18ODiCal vs. V:CO2 is the
measured δ18O of CO2 from DiCal CO and δ18OCal vs. V:CO is the known δ18O of Cal CO.
Following this calibration, the values of the reference gas (Ref) against
the international standards were determined as δ13C (Ref,
V-PDB) = -29.61 ± 0.1 ‰ and δ18O
(Ref, V-SMOW) = 8.45 ± 0.2 ‰. In the absence of
international standards for the isotopic composition of CO, we note that
there may be additional systematic errors (e.g. temporal changes of the
primary calibration cylinder or dilution artefacts), which may introduce an
additional unspecified uncertainty to these values.
The ISODAT software assumes mass-dependent fractionation (MDF) when
calculating the δ values. However, atmospheric CO possesses mass-independent oxygen isotope
anomalies with Δ17O values (Δ17O ≡δ17O–0.52 ⚫δ18O) between 2.5 and 7.5 ‰
(Röckmann and Brenninkmeijer, 1998;
Röckmann, 1998). Both 13C16O16O and
12C17O16O contribute to the ion signal at m/z = 45. This
means that, when assuming MDF, the contribution of 17O to the ion beam
at mass 45 is underestimated, leading to an overestimation in the δ13C. Röckmann and Brenninkmeijer (1998) calculated this
overestimation (error) of δ13C to be 0.08–0.25 ‰ for a Δ17O range of 2.5–7.5 ‰. Since the current method does not resolve the
contribution from 17O, we report the δ13C and δ18O values calculated assuming MDF.
Results and discussion
Blanks
A blank run is performed using the same method as a sample run but without
the injection of reference or sample gas. The continuous He flow collects
the background of the system for the usual injection time of 5 min. The
peak area of this system blank is ∼ 0.1 Vs, which is 2.2 %
of the average reference gas (Ref) peak area (the system blank ranges
between 8–23 pmol in a 0.7 nmol Ref sample). The majority of the blank
signal originates from CO2 that is released by the Schütze reagent.
It is an accumulation of CO2 formed by the system CO blank or CO2
from the reagent itself, which are released in later measurements.
When a blank run is done excluding the Schütze reagent trap, the peak
has an area of ∼ 0.019 Vs. The blank including the background
CO2 released from the Schütze reagent is used as the system blank.
When the system was first built, the system blank was 10 % of the sample
peak. The simple modification of adding a 6-port Valco valve to continuously
flush the Schütze reagent with He at a flow rate of 8 mL min-1 reduced the
blank to 1–3 %. This blank affects both the sample and the reference air
in a typical measurement sequence and is not considered when calculating the
mole fractions and δ values of a sample.
Removal efficiency of CO2 and N2O
When the air sample is injected into the extraction system, CO2 and
nitrous oxide (N2O) must be completely removed. CO2 in the sample
must be removed as CO is converted to CO2 for isotope analysis.
N2O shares the same molecular mass as CO2 and interferes with the
CO2 peak derived from CO on the chromatogram. CO2 is largely and
efficiently removed by the Ascarite trap, and remaining traces are together
with N2O condensed in the cryogenic trap (T1) with glass beads.
The glass beads increase the surface area for condensation. T1 is
warmed and evacuated at the end of each run. Periodically, checks are done to
confirm that CO2 and N2O traps (Ascarite trap and T1 trap
respectively) work efficiently. This is done by bypassing the Schütze
reagent so the CO is not converted to CO2. The result of such runs
should be the same as a blank run without the Schütze reagent trap, such
as
a blank run that does not show a CO2 peak in the chromatogram. If the
result shows a CO2 peak on the chromatogram then the Ascarite trap
needs to be changed.
Peak area and isotopic composition of the reference gas, and an
aliquot of the reference gas that was spiked with 2000 nmol mol-1
N2O.
Area all (Vs)
δ13C (‰)
δ18O (‰)
Ref
4.43 ± 0.03
6.6 ± 0.1
-4.7 ± 0.2
Ref + 2000 nmol mol-1 N2O
4.45 ± 0.03
6.6 ± 0.1
-4.6 ± 0.1
N2O has the same nominal isotopocule masses as CO2, but with much
smaller molecular isotope ratios 45R and 46R. Therefore, a small
amount of N2O seeping through the cryogenic trap can be detected in the
resulting δ values. Table 1 shows a comparison of average values of
10 runs from the reference gas (Ref) and 10 runs from a can with
approximately 2000 nmol mol-1 N2O (15 µL of N2O was
added to a 2.5 L steel can and was filled with reference gas, which already
contained atmospheric levels of N2O). The results show that there is no
evidence of N2O leaking from the cryogenic trap (T1) even at a
high mole fraction. Figure 3 shows a case where the N2O peak appears on
a chromatograph because trap T1 is not used to remove N2O. The
retention time of N2O is ∼ 355 s, about 25 s longer than
for CO2 at ∼ 330 s. Due to the different isotope
ratios of N2O (see above), the isotope ratios show an inverted peak;
thus, an N2O interference is easy to recognize.
Repeatability
The peak area of 100 mL aliquots of Ref is ∼ 4.6 Vs with a
standard deviation of ∼ 0.03 Vs, which corresponds to a
relative repeatability of 0.7 % for the mole fraction on a single sample.
δ13C has a repeatability of 0.1 ‰. δ18O has a repeatability of 0.2 ‰. System
reproducibility is tested on a daily basis and often with overnight runs.
When the system stays idle, at least five runs should be performed to regain
its normal repeatability. Ref gas is measured often (several times per day)
and all the samples are measured relative to the Ref; it is the
repeatability on a short term (hours to days) that is the most important.
Linearity
Ideally, the δ value of a sample measured on an isotope instrument
vs. a certain reference should be independent of the amount of sample
that was injected into the instrument. In reality, isotope systems often
show a dependence of the isotope results on the total amount of sample
injected, which is commonly called a non-linearity. The non-linearity of
our system was calibrated by injecting varying amounts of sample (for sample
linearity) and He (for blank linearity) into the system.
A visual comparison of the system blank peak (top), the sample peak
(middle), and the N2O peak (bottom). During a normal measurement,
N2O is removed from the sample to avoid isobaric interference with the
CO-derived CO2 masses. As a further precaution N2O and CO2 are
separated on the gas chromatograph where N2O peaks at ∼ 355 s
compared to CO2 peaking at ∼ 330 s. The mass ratio 45/44 and
46/44 traces show inverted compared to the CO2 and are easily
recognizable. The absence of a N2O signal shows that N2O is
quantitatively trapped in the cryogenic trap and does not reach the IRMS. The
mass traces (44, 45 and 46) shown are direct output from the ISODAT software.
For the ratios 45/44 and 46/44, a value of 100 mV was arbitrarily added to
the signals in order to avoid artificial noise arising from the ratio of two
small numbers.
Blank linearity
A blank linearity test was performed to characterize the effect of the
CO2 released by the Schütze reagent. No sample was injected in
these blank experiments. Injection time on the x axis of Fig. 4 depicts
the period, in seconds (s), for which the He flow was directed through the complete
system (including the Schütze reagent). These injection times are 100,
300, 600, 900, and 1200 s. Each injection time test was repeated four
times. The peak area (area all) as a function of the injection times is
shown in Fig. 4a. The injection time for a usual measurement is 300 s.
The peak area for the blank for this injection time is ∼ 0.1 Vs corresponding to about ∼ 4 nmol mol-1. Figure 4b
shows the dependence of δ13C and δ18O of the blank
peak on the injection time. Since the various injection times lead to
different peak areas, Fig. 4b implicitly shows the dependence of δ13C and δ18O on peak areas from Fig. 4a. The blank
areas increase roughly linearly in size with injection time, indicating
constant accumulation of a trace contamination of CO2. The δ
values do not show a significant trend. The uncertainties of the average
δ values for all the runs are ± 1.3 ‰ and
±5.1 ‰ for δ13CV-PDB (-23.2 ‰) and δ18OV-SMOW (55.2 ‰) for these low peak areas.
Dependence of the peak area all (a) and δ13C and
δ18O (b) on the injection time for blank runs, i.e.
when the sample inlet valve was actually not opened. The peak area of the
blank increases approximately linearly with injection time, and the
dependence of the isotope values on injection time is relatively small. Note
that the scatter in the isotopic composition measurements is so large because
of the very small peak areas of these blank experiments.
Sample linearity
The amount of sample was varied by changing the mole fraction of a sample
with initial high mole fraction, by dilution with CO-free air. For the
dilution test, 8 mL of high mole fraction (∼ 269 µmol mol-1) CO sample was injected into a 1 L glass flask, which was then
filled with zero air to 1.8 bar. A sequence of 59 measurements was made
while the flask air pressure kept constant at 1.8 bar by refilling with
zero air after every run, which results in an extended dilution series.
Figure 5 shows the δ13C and δ18O values as a
function of area all (Vs). The δ13C values are constant down to
about 4 Vs and then start deviating systematically for the low peak areas
(i.e. they become non-linear). The δ18O values are relatively
constant (with a small trend) for areas above 1.5 Vs (60 nmol mol-1).
The average δ13C and δ18O values in the peak area
range between 4 and 15 Vs where samples usually measured are
-44.2 ± 0.1 ‰ and 54.1 ± 0.2 ‰, respectively. The small trend at higher peak areas
visible in Fig. 5 in particular for δ18O is not further
investigated. Measurements over many months indicate that the area below,
where non-linearity is observed, depends on the state of the filament in the
IRMS. Therefore, the non-linearity is checked regularly.
Dilution test: δ13CV-PDB (black) and δ18OV-SMOW (red) plotted against peak area (area all in Vs). The
δ values show a strong non-linearity at peak areas below 4 Vs
(shaded region). Values between 4 and 15 Vs are used to calculate the
repeatability of the system.
Application example: CO emissions from vehicles
As an application example, Fig. 6 shows δ13C and δ18O values of air samples collected at the Islisberg highway tunnel in
Switzerland that were presented and discussed in detail in
Popa et al. (2014). Air samples from the entrance
and the exit of the tunnel were collected in 1 L glass flasks under
∼ 1.8 bar pressure and analysed on the analytical system
described here for δ13C and δ18O. The different
colour markers in Fig. 6 represent the samples from the entrance and the exit
of the tunnel. The exit samples contain air that has accumulated emissions
of vehicles passing through the tunnel; these samples have very high (2–10 ppm) CO mole fractions, and their isotopic values represent the isotopic
signature for CO emitted by vehicles. The isotopic composition of the
vehicle emissions based on these samples was δ13C = -25.6 ± 0.2 ‰ and δ18O = 24.1 ± 0.2 ‰, respectively (Popa et
al., 2014). The air collected near the entrance is much closer to background
air, but since the collection was actually in the tunnel, it is also
influenced in varying proportions by the emissions of vehicles on the
highway. These entrance data thus fall in between the CO isotopic signature
of fossil fuel combustion and the isotopic composition of background
atmospheric CO.
δ13CV-PDB (‰) vs.
δ18OV-SMOW (‰) plot of Islisberg highway tunnel
samples. Data from Popa et al. (2014). The δ values of the samples
collected at the entrance and exit are depicted by blue and red markers,
respectively.