RAPID COMMUNICATIONS IN MASS SPECTROMETRY
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
Extraction of CO2 from air samples for isotopic analysis
and limits to ultra high precision d 18O determination in
CO2 gas
Roland A. Werner, Michael Rothe and Willi A. Brand*
Max-Planck-Institut für Biogeochemie, PO Box 100164, 07701 Jena, Germany
Received 18 June 2001; Revised 1 August 2001; Accepted 17 August 2001
The determination of d 18O values in CO2 at a precision level of 0.02% (d -notation) has always been
a challenging, if not impossible, analytical task. Here, we demonstrate that beyond the usually
assumed major cause of uncertainty ± water contamination ± there are other, hitherto underestimated
sources of contamination and processes which can alter the oxygen isotope composition of CO2.
Active surfaces in the preparation line with which CO2 comes into contact, as well as traces of air in the
sample, can alter the apparent d 18O value both temporarily and permanently. We investigated the effects
of different surface materials including electropolished stainless steel, Duran1 glass, gold and quartz, the
latter both untreated and silanized. CO2 frozen with liquid nitrogen showed a transient alteration of the
18
O/16O ratio on all surfaces tested. The time to recover from the alteration as well as the size of the
alteration varied with surface type.
Quartz that had been ultrasonically cleaned for several hours with high purity water (0.05 mS)
exhibited the smallest effect on the measured oxygen isotopic composition of CO2 before and after
freezing. However, quartz proved to be mechanically unstable with time when subjected to repeated large
temperature changes during operation. After several days of operation the gas released from the freezing
step contained progressively increasing trace amounts of O2 probably originating from inclusions within
the quartz, which precludes the use of quartz for cryogenically trapping CO2. Stainless steel or gold
proved to be suitable materials after proper pre-treatment.
To ensure a high trapping efficiency of CO2 from a flow of gas, a cold trap design was chosen
comprising a thin wall 1/4@ outer tube and a 1/8@ inner tube, made respectively from electropolished
stainless steel and gold. Due to a considerable 18O specific isotope effect during the release of CO2 from
the cold surface, the thawing time had to be as long as 20 min for high precision d 18O measurements.
The presence of traces of air in almost all CO2 gases that we analyzed was another major source of
error. Nitrogen and oxygen in the ion source of our mass spectrometer (MAT 252, Finnigan MAT, Bremen,
Germany) give rise to the production of NO2 at the hot tungsten filament. NO2‡ is isobaric with C16O18O‡
(m/z 46) and interferes with the d 18O measurement. Trace amounts of air are present in CO2 extracted
cryogenically from air at 196 °C. This air, trapped at the cold surface, cannot be pumped away quantitatively. The amount of air present depends on the surface structure and, hence, the alteration of the
measured d 18O value varies with the surface conditions.
For automated high precision measurement of the isotopic composition of CO2 of air samples stored
in glass flasks an extraction interface (`BGC-AirTrap') was developed which allows 18 analyses (including
standards) per day to be made. For our reference CO2-in-air, stored in high pressure cylinders, the long
term (>9 months) single sample precision was 0.012% for d 13C and 0.019% for d 18O. Copyright # 2001
John Wiley & Sons, Ltd.
The vast majority of stable isotope ratio measurements made
are performed on CO2 gas. CO2 is used exclusively to
determine d13C values from any precursor material. From
carbonate and water samples CO2 is used to measure d13C
and d18O after appropriate conversion. In water samples the
equilibration reaction
H2 18 O ‡ C16 O2
! H2 16 O ‡ C18 O16 O
…1†
serves to transport the isotopic signature (all oxygen isotopes
*Correspondence to: W. A. Brand, Max-Planck-Institut fuÈr Biogeochemie, PO Box 100164, 07701 Jena, Germany.
E-mail: [email protected]
DOI:10.1002/rcm.487
are exchanged) from the original H2O liquid to the CO2 gas
phase for determination with an isotope ratio mass spectrometer (IRMS).1 At room temperature a large equilibrium
isotope fractionation (1) causes an enrichment of the 18O in
CO2 of ‡41%2 compared to the liquid. It takes several hours
(5±8 h) to achieve complete equilibrium between CO2 gas
and liquid H2O (sealed vessels, room temperature, no
catalysis). The rate limiting factor in this reaction is the
surface area of the liquid. However, for trace amounts of
H2O adsorbed on the surfaces of a vacuum system, the
relative surface area is large. Hence, oxygen isotope
exchange proceeds rapidly, and can change the d18O value
of CO2 by up to several per mill in some cases. The size of the
Copyright # 2001 John Wiley & Sons, Ltd.
Ultra high precision d18O determination in CO2
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Figure 1. Schematic layout of the automated extraction system (‘BGC-Airtrap’) for high precision
measurement of CO2 isotope ratios in air. See text for further explanation.
change depends on the relative proportions of the CO2 and
H2O, surface temperature and residence time of the gas. Due
to the unavoidable presence of water on almost any surface,
this oxygen isotope exchange has plagued the analyst since
the inception of d18O measurements in the mid-1930s1,3,4 and
it is generally viewed as the single most relevant contributor
to imprecision in d18O measurements.
The isotopic composition of atmospheric trace gases, in
particular the greenhouse gas CO2, is used to quantify or
constrain sources and sinks of these gases.5±7 Annual global
changes of d13C in air-CO2 due to release of fossil fuel
amount to about 0.025% per year. d18O of air-CO2 is
influenced mainly by oxygen exchange with leaf water in
the presence of the enzyme carbonic anhydrase (CA) and by
exchange with soil water following autotrophic and heterotrophic respiration. As a result a northern to southern
hemisphere gradient of about 2% and seasonal cycles of
0.2% to 0.7% at high latitudes are observed. In order to
extract this information, a very high degree of precision and
accuracy in determining d13C and d18O values over a long
time window (exceeding decades) is mandatory. Few laboratories (NOAA-CMDL,8 CSIRO Atmospheric Research,9
Scripps Institution of Oceanography (SIO10), to name the
most prominent) have demonstrated the ability to measure
d13C from CO2 in air with a long term precision of 0.015%.
Comparable determination for d18O has proven much more
difficult to attain, with current limits at a level of 0.05%.11
Agreement between laboratories is much worse than `in-lab'
precision which, besides reference materials and preparation, is at least partly due to differences in the analytical
procedures. Among the major causes of error that have been
identified are
. the material and surface conditions of the sampling flasks
(including O-rings),
Copyright # 2001 John Wiley & Sons, Ltd.
. a possible small NO2‡ contribution on mass 46 produced in
the ion source of the mass spectrometer from traces of N2
and O2, and
. contamination of analytical lines or sampling equipment
with water giving rise to oxygen isotope exchange.12
The analytical precision for d18O achieved between
different laboratories to date is not acceptable, even after
considering and addressing these experimental challenges in
day to day analytical procedures and protocols. In this study,
we aim to find possible causes for inconsistencies and
sources of error in the analytical procedures and the
materials which are in common use for extracting CO2 from
air samples. This includes studying the basic aspects of
freezing CO2, both statically and from a flow of air, as well as
aspects of mass spectrometric behavior and strategies for
long term referencing.
EXPERIMENTAL
Setup
We have built an automated sampling line (`BGC-AirTrap'*)
for extracting CO2 (along with N2O) cryogenically and thus
separating it from other air constituents (Fig. 1). The device is
coupled to the inlet system and directly to the changeover
valve of a Finnigan MAT 252 isotope ratio mass spectrometer
(IRMS).
Our sampling containers are 1L glass flasks, each fitted
* The BGC-AirTrap system is derived from the Finnigan MAT
`Carbo¯o' unit, originally developed by one of us (WAB) for
measuring d13C and d18O in carbonates. We have replaced the
autosampler and common acid bath, modi®ed the trap design
and rearranged the plumbing. The original software (`Isodat') is
used to drive our system.
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
2154 R. A. Werner, M. Rothe and W. A. Brand
with two valves (Glass Expansion, Melbourne, Australia)
and sealed with Teflon1 PFA (perfluoroalkoxyalkane) Orings. They are usually filled with dry air at a pressure of 2
bar. In addition to high precision isotope ratio measurements
on CO2, the air in the flasks is subject to determination of the
mixing ratios of CO2, N2O, CH4, CO, SF6 and H2. A high
precision measurement of the O2/N2 ratio is also performed
which is used to monitor the decline in atmospheric oxygen
accompanying the long term increase in CO2 due to fossil
fuel and biomass burning as well as other anthropogenic
activities. The amount of sample consumed by a single
analysis hence must be minimized in order to leave enough
material for the determination of the remaining parameters.
Using 1/2@ Cajon Ultratorr1 adaptors the sample flasks
are connected to a 16-port Valco multiport valve that is
operated by a 32 position driver. Consequently, there is a
`closed' position in between two adjacent flasks that we use
to pump out the lines before admitting the next sample.
Initially we used a 0.5 mm i.d. stainless steel capillary with a
crimp at the entry to adjust the flow through the trapping
unit. However, at a later stage, we replaced the capillary and
crimp with a mass flow controller (10±100 mL/min; MKS
Instruments, Andover, USA) acting as a variable conductance. This enabled us to maintain constant flow conditions
independent of the sample flask pressure. At the end of the
trapping unit the gas passes through a 0.5 mm i.d. capillary.
The relation between the two flow restrictions (adjusted by
the crimp or the mass flow controller, respectively) determines the pressure during trapping, which must be lower
than 500 mbar in order to avoid trapping of significant
amounts of oxygen or argon. In our system, the flow rate
during sampling ( freezing) is adjusted to 60 mL/min and
the pressure in the trapping line is constant at 330 mbar. The
effects associated with variations of these parameters are
described and illustrated later in this study (Fig. 9).
First the sample gas passes through a water trap (T1) held
at 70 °C (dry ice/ethanol). Any residual moisture is
removed here as a precaution, even though, in the field,
flask samples are chemically dried with Mg(ClO4)2 to avoid
alteration of d18OCO2 inside the flasks. Trap 2 (T2) is kept at
196 °C. The traps are designed from an outer 1/4@ tube and
an inner 1/8@ tube. The 1/8@ tube internally protrudes
through to the bottom of the 1/4@ cold fingers in order to
force the gas through the coldest portion of the cold trap.
Thus, the design parallels that of a chemical washing flask.
Using 0.5% CO2 in helium, the trapping efficiency of this trap
design was measured at 99.5%. However, when switching to
air with 0.037% CO2, we observed a decline in trapping
efficiency to 94.5%, varying slightly with the level of liquid
nitrogen. The storage dewar is a 7 L large diameter (23 cm
i.d.) glass unit. We chose the large size to minimize the
decrease in the liquid nitrogen level during a typical analysis
sequence lasting about 20 h. In order to improve on trapping
efficiency, T2 initially was built from two connecting cold
fingers. Some of the experiments reported here were made
with this initial setup. We abandoned this design later for
several reasons including surface area during freezing as
well as total volume. In addition, equilibration time of CO2
gas isotopically altered upon release from the freezing step is
shorter in a single trap.
Copyright # 2001 John Wiley & Sons, Ltd.
All materials used in the trapping line are stainless steel
unless stated otherwise. The valves are all-metal valves with
gold seats (Finnigan MAT, Trapping Box1 design13,14).
Connections between 1/4@ and 1/8@ lines are mostly made
with PCTFE or Vespel1 reducing ferrules (Valco, Switzerland and Analyt, Germany). Quartz and Duran1 lines were
connected with Teflon reducing ferrules (Analyt).
The temperature reservoirs for the traps (Dewar vessels
with ethanol/dry ice and liquid nitrogen, respectively) are
operated by a 20 mm diameter compressed air (6 bar) piston.
The pistons are actuated through ordinary pilot valves and
I/O switches from the host computer. Advantages of the
design are low maintenance operation and economical use of
liquid nitrogen. The liquid nitrogen reservoir is filled at the
beginning of a sample sequence and refilled once 10 samples
have been processed. Liquid nitrogen is not boiled off
actively by the operation and thus consumption is minimized. The liquid nitrogen necessary for a complete 18
analysis run amounts to 4±5 L. Dry ice refilling is required
only once a day.
Trap 1 has two outlets, one to trap 2, the other one to a
membrane pump. This pump operates with a measured final
vacuum of 0.3 mbar in the system. It is used to evacuate the
air flask necks as well as the AirTrap lines during measurement in preparation for the next sample.
Through a larger (1/4@) line to the MAT 252 dual inlet
system, trap 2 can be pumped out using the roughing pump
or the turbo pump. During air extraction, the inlet system
roughing pump is used to maintain the sample flow through
the 0.5 mm i.d. outlet capillary. After sampling about 500 mL
of air (corresponding to about 180 bmL CO2) valve 2 is closed,
residual air is pumped away with the fore- and high vacuum
pumps of the inlet system and trap 2 is isolated by closing
valve 3. The CO2 (‡N2O) is allowed to thaw and enter the
ion source via the capillary directly connected to the
changeover valve for a defined amount of time (see Results
section) before the isotope ratio measurement on the IRMS is
started. Prior to sample/reference gas comparison, the
reference gas mass 44 reading is adjusted to that of the
sample side (`pressure adjust').
Operating conditions of the mass spectrometer
The Finnigan MAT 2521 IRMS was operated at a reduced
sensitivity by lowering the electron emission current from
1.5 to 0.7 mA and by fully opening the ion source window or
`VISC' (Variable Ion Source Conductance). The total sensitivity thus was only about 3000 molecules/ion. This is a
reduction by a factor of 3 compared to the maximum
sensitivity obtainable with this instrument. The reasoning
behind this measure was to
. minimize background contamination effects,
. enhance the sample flow through the capillaries (for
washing out the effects from initial gas/wall isotope
exchange or the isotope effect associated with gas freshly
entering a capillary), and
. minimize residual cross talk between the two gases to be
compared during changeover (Z-effect).15,16
The capillaries of the mass spectrometer and from the
BGC-AirTrap to the changeover valve are 0.1 mm i.d.
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
Ultra high precision d18O determination in CO2
stainless steel, length 100 cm (Upchurch Scientific, USA),
treated with a proprietary steam cleaning procedure. These
capillaries were used as supplied, avoiding treatment with a
torch or similar procedure that would possibly alter the
internal surface texture. With a crimp adjusted for equal
conductance on both sides we tested the capillaries by
applying different pressures. The results indicated a high
tolerance of the measured isotope ratios to changes in
sample flow. One of the tests of the mass spectrometer
included a CO2 gas overnight run from the sample and
reference bellows with capillaries crimped for equal conductance. In a sequence of 45 separate measurements an
external precision (expressed using the measured standard
deviation) of 0.007% for d13C and 0.010% for d18O was
observed with no additional drift correction or data
handling. This sequence of measurements required 20 h,
during which the major ion beam decreased from 22 to 8 nA.
In contrast, a single CO2 extraction and measurement with
the BGC-AirTrap takes more than 60 min and a complete
sequence of 18 extractions will take up to 20 h. Hence, it is
important that the reference gas remains isotopically
unaltered during this time period and that reference gas
consumption is low. This requirement was met by tightening
the capillary crimp on the reference gas side and working
with a higher reference gas pressure in the variable volume.
As a consequence, the capillaries are not operated according
to conventional rules, i.e. with a conductance adjusted to
yield equal ion currents for equal inlet system pressures on
both sides. An important result of this change in crimp
conductance is that the difference in isotope ratios measured
for an identical gas in both reservoirs starts to deviate from
zero for ion currents below 6 nA. This is illustrated in Fig. 2,
where the difference in the C and O isotope ratios of CO2
from both reservoirs has been measured as a function of the
major beam intensity, given in Volts (1 V  3.3 nA). The
deviations, expressed in Dd13C and Dd18O, are reproducible
and were fitted using a 3-parameter exponential function.
Above 1.5 V, residuals show an average deviation of <0.01%
within the limits of precision of these measurements. The fit
has an R2 of >0.98 which is considered excellent. The data in
Fig. 2 represent measurements over a period of 4 days. All
data we generate are adjusted using this experimentally
derived fit function. For most analyses this is of no
consequence because the size of the signal remains adjusted
through the use of a mass flow controller in the inlet line.
However, some measurements are made from soil air
containing comparatively high amounts of CO2 with an
isotopic signature differing considerably from that of airCO2. In such cases, signal heights are different from the
average and an adjustment is necessary.
2155
0.01% for d13C and 0.02% for d18O, over the required long
time period (decades) for background air monitoring.
The initial results were far from satisfying and did not
meet the initial moderate precision targets. We changed the
multi-loop trap design in the Finnigan CarboFlo unit and
took a number of other steps until the results of our
experiments led us to separate the trapping efficiency
problem from the freeze/release behavior of CO2 and to
study the latter in greater detail.
The freeze/release step is critical for precise d18O
determination
Figure 3 depicts an example from initial observations when
monitoring the changes in oxygen isotopic composition after
freezing CO2. In this experiment the sample gas entered the
trapping line from a 1L glass flask attached to the carousel
(the 16-port valve in Fig. 1). The flask was filled with pure
CO2 gas at a pressure of about 100 mb.
Aliquots of the gas were repeatedly admitted into the trap
2 reservoir. Before entering T2 the gas was allowed to
communicate through the multisampling valve and the inlet
capillary with the trap 1 compartment at all times. (In order
to exclude problems arising from a possible moisture
contamination of the sample gas, mass 18 was monitored
carefully and the measurements were done with and
without the first trap immersed in dry ice.) Each data point
in Fig. 3 corresponds to a complete cycle comprising
pumping of the trap 2 lines followed by equilibration of
sample gas from the trap 1 to the trap 2 compartment. The
RESULTS AND DISCUSSION
The freeze step
Our initial design was tested by directly measuring the
isotopic composition of CO2 from a tank containing dry air at
150 bar. We were aiming for a precision of 0.04 to 0.08% for
d13C and 0.1 to 0.15% for d18O. Further improvements were
planned by developing a proper standardization scheme for
maintaining the final experimental precision, hopefully
Copyright # 2001 John Wiley & Sons, Ltd.
Figure 2. Deviation of measured CO2 d-values on both sides
of the dual inlet system. The capillary on the reference gas side
is crimped to yield a mass 44 ion current signal of 6 nA for an
inlet pressure of 50 mb. The sample capillary is crimped for a
yield of 6 nA at a reservoir pressure of 20 mb. All
measurements are adjusted to correct for this imbalance.
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
2156 R. A. Werner, M. Rothe and W. A. Brand
Figure 3. Initial CO2 freeze tests: repeat aliquots of CO2 are sampled into the T2 volume and
analyzed, either with or without freezing the CO2 with liquid nitrogen prior to measurement. Freeze
and no-freeze runs were alternated in batches of 6 aliquots. The timing for both procedures is
identical. The d13C values for the same series have a precision of 0.014% with no apparent
difference between the freeze and no-freeze points.
equilibration time and measurement were identical for all
samples. For data points labeled `freeze', the processing
steps included freezing the sample gas (immersing T2 in
liquid nitrogen for 60 s) and releasing it again (thawing for
90 s). The `no freeze' points represent measurements with
the same overall timing; however, the liquid nitrogen dewar
was not actuated.
The data in Fig. 3 exhibit a clear difference in d18O between
the gas aliquots that were frozen and those that were not.
The `no freeze' data are rather consistent with a slight trend
towards the end and an overall precision of 0.031%. In
contrast, the `freeze' data scatter considerably within each
packet of six separate measurements. In addition the
subsequent packets do not fit the previous data, for
unknown reasons. The data set for d13C did not exhibit this
behavior (data not shown); there was no apparent difference,
or trend, and the overall external precision was 0.014%.
The `freeze' data in Fig. 3 are worse than most laboratories
would accept. This is because the frozen surface area is a
large part of the total area of the compartment and the
surface material and conditions have not yet been optimized
for the freezing of CO2. However, the experiment is very
simple and should show the effects of the freezing step
alone. No other effects arising for instance from further gas
transfer into the dual inlet for measurement are present.
From Fig. 3 it can be concluded that the presence of water on
the surfaces is not responsible for the observed phenomenon.
If water were present and could alter the CO2 isotopic
composition through oxygen isotope exchange, the `no
freeze' data should be affected more because the time for
isotope exchange is longer and the temperature is higher.
We have obtained similar patterns to those in Fig. 3 for
Copyright # 2001 John Wiley & Sons, Ltd.
several surface materials or combinations of materials. The
d13C values in general had acceptable precision with
standard deviations ranging from 0.006 to 0.014% with no
obvious difference between the `freeze' and `no freeze' runs.
d18O values were acceptable for the `no-freeze' runs (0.01 to
0.04%), but scattered for the `freeze' runs, sometimes as
severely as exhibited in Fig. 3.
CO2 released from various surface materials
In order to study the influence of the freeze/release process
on d18O in more detail and to observe a clearer relation
between the d18O scattering and the surface material and/or
treatment, we operated only one cold finger of trap 2 (in
these experiments, T2 had a twin trap design with two cold
fingers connected in series. Using a small storage dewar we
applied liquid nitrogen to one of them only). The internal 1/
8@ tube was taken out and replaced by a short connection
which did not reach into the cold zone during freezing.
Before installation all trap fingers were cleaned thoroughly
with deionized water (0.05 mS) in an ultrasonic bath followed
by rinsing with methanol (analytical grade) and drying.
Prior to the final cleaning steps the gold trap was also
subjected to HCl (conc.) in order to wash off non-gold
surface atoms.
Figures 4(a)±(f) exhibit some of the results of our release
tests (in total the number of test analyses exceeded 4000) for
the different materials we used for trap 2. These were
electropolished stainless steel, gold, Duran1, and quartz,
both clean and silanized. For better comparability, all d18O
scales have an identical range of 0.3%. The aliquoting
process was changed to an initial filling and measurement
(`pre-freeze') followed by a 80 s freezing step and a 90 s
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
Ultra high precision d18O determination in CO2
Copyright # 2001 John Wiley & Sons, Ltd.
2157
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
2158 R. A. Werner, M. Rothe and W. A. Brand
Figure 4. (a)–(e) Selected results for CO2 trapping sequences using various materials
and surfaces for the cold finger (T2). All d18O-scales have an identical range of 0.3%.
For further discussion see text. (f) Trapping sequence exhibiting initial changes of the
18
O/16O signal with a freshly installed clean quartz trap. Conditioning is complete after
about 30 freeze-release cycles. Please note that the changes upon freezing at the
beginning do not approach the original value with time. This effect should dominate
experiments involving sealed tubes.
thawing time before the next measurement (`freeze'). Three
measurements of 10 min duration each followed for studying the release process further as a function of time (`postfreeze'). Figures 4(a)±(f) all have the same timing, etc., so that
a maximum comparability is achieved. Obviously both the
sample and the reference gases had to be changed between
different experiments. Both were refilled from the same
sources without paying a great deal of attention to the
absolute isotope ratios. Hence, between the figures, the reported d-values vary in absolute terms. The major difference
between the runs is the material of the cold finger. Electropolished stainless steel (Fig. 4(a)) and gold (Fig. 4(b))
exhibit an almost identical freezing behavior of CO2. The
first measurements after freezing CO2 always have a
negative offset of the d18O value >0.1% from the `pre-freeze'
Copyright # 2001 John Wiley & Sons, Ltd.
values. This offset is specific for d18O values and was not
observed in the d13C record (not shown in the figures). We
interpret this finding in terms of a strong, temporary,
oxygen-specific isotope effect during release of CO2 from
the cold surface. The result is a temporary isotope shift that is
enhanced by the design of the twin trap and the choice of
material used in the trap.
The release of gas after the 80 s freezing step involves a
90 s thawing time followed by an additional time of about
100 s during peak centering and pressure adjust before the
actual collection of data is started. However, the CO2
pressure stabilized within 15 to 20 s after removal of the
liquid nitrogen reservoir.
The measurements following the freeze steps still show a
memory of the freeze step for both surfaces. Only the last
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
Ultra high precision d18O determination in CO2
two points of each cycle reflect the initial d18O composition.
The time elapsed for the mass spectrometric measurement of
a single data point in Fig. 4 is about 10 min. Hence, for
stainless steel and for gold, the cumulative time necessary
for obtaining the original pre-freeze isotopic signature is
longer than 20 min. Figure 4(c) shows the data for silanized
quartz. Silanization was made with 10% dichlorodimethylsilane (DCDMS) in toluene for 30 min followed by immediate washing with toluene and methanol. The observed data
pattern is similar to the previous ones with the sign of the
d18O offset reversed and with a different size of the d18O
offset. Most importantly, the data following the initial freeze
run do not exhibit the freeze memory indicating that the
release time for CO2 has shortened to less than 15 min.
Neither the offset nor the release time could be improved by
moderate heating, as demonstrated by the indicated point in
Fig. 4(c).
Figure 4(d) shows the data for Duran1 glass which is
similar to Pyrex1. The systematic d18O offset seen in the
previous figures is no longer present. Instead a larger scatter
of the freeze data is noticeable. The release time for obtaining the correct d18O values is shorter than for stainless steel
or gold and is similar to that observed for silanized quartz
(Fig. 4(c)).
Figure 4(e) represents the apparently best system. Untreated high purity quartz with a very clean surface
exhibited the smallest systematic offset and showed the best
potential for a fractionation-free measurement of CO2 in dry
air samples, although we still observed an average offset of
0.023% for d18O. Further sequence runs of this type have
shown that this offset can vary with time and that it seems to
be of random nature. However, with increasing number of
measurements, we observed a deterioration of the d18O
performance which is associated with an increasing con-
2159
tamination of the released CO2 with air (measured as O2‡).
Each freezing step adds a small amount of air contamination
which is associated with a shift in the measured d18O value.
We interpret this finding to indicate that quartz is unstable
with respect to frequent temperature cycling between
196 °C and ambient temperature, possibly resulting in
microcracks. Upon temperature change, these microfractures would release small amounts of air from inclusions. We
were not able to detect any leak when flushing the cold
finger with argon from outside and monitoring mass 40. This
effect is discussed in further detail below.
Initial surface conditioning
Figure 4(e) shows the first sequence we ran with quartz as a
cold trap material in order to demonstrate that some initial
conditioning takes place even for carefully cleaned untreated
quartz. With other surfaces, this effect was in general more
pronounced. As an example, the first sequence of d18O
measurements with a freshly mounted clean quartz finger
which exhibited worse behavior is shown in Fig. 4(f). In this
dataset, the initial effects on d18O are more clearly visible:
there is an initial 0.1% d18O offset between the `pre-freeze'
and the `freeze' runs that decreases with time. It takes about
30 runs or 6 h of repeated loading with fresh CO2 gas before
this effect starts to level off. Figure 4(f) is the clearest example
of this trend which we observed with all materials. We
attribute this initial behavior to a surface conditioning after
which the isotopic exchange still takes place but the
corresponding compartments have adjusted to each other.
Consequently, there will be a memory effect that causes a
change in the measured isotope ratio when sequentially
analyzed samples differ widely in isotopic composition.
This finding has a number of consequences. Obviously,
CO2 frozen into Pyrex tubes in particular will suffer from
Figure 5. Effect of air impurities in CO2 gas. The first six runs are made with a large impurity,
represented by an O2 m/z 32 signal of 1.63% of the m/z 44 CO2 signal. In run number 7, residual
gas was pumped away during freezing, resulting in a remaining O2 impurity of 0.08%.
Copyright # 2001 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
2160 R. A. Werner, M. Rothe and W. A. Brand
this effect. Since the degree of conditioning can vary from
tube to tube, the freeze/release step itself may alter the d18O
value of the gas by 0.1% or more and it will introduce a
scatter of the values of the same order of magnitude. We
suspect that the freeze/release step is the final obstacle to
routinely obtaining an analytical sample to sample precision
better than 0.05 to 0.08% in careful d18O measurements on
CO2 gas.
Traces of air alter d18O of CO2 upon freezing
The observation made with quartz led us to study the
contamination with air in further detail. Figure 5 (runs
number 1±6) shows a series of data points for d18O from CO2
doped with 8% air, that is 1.63% O2 as an impurity and about
6.4% N2. The second series (runs number 7 and up) was
obtained by including a pumping step of the frozen CO2
prior to thawing. After this step, there was 0.08% oxygen left
in the CO2 gas. Clearly, the d18O memory of the freezing step
has declined considerably and the values are different from
those measured with the relatively large air contamination
present. Thus, not only is air responsible for a delayed
thawing of the original CO2, it is also acting inside the ion
source to generate a different apparent 46/44 isotope ratio.
We interpret this finding in terms of production of traces of
NO2 from nitrogen and oxygen at the hot filament in the ion
source. NO2 gets ionized and the ions are collected by the m/z
46 Faraday cup. It does not require a large amount of NO2 to
generate the observed shift in d18O of about 0.2%. The NO2/
CO2 ratio in the ion source would have to be 0.8 ppm
(= 0.2% 2 0.2 at. %). On the other hand, nitrogen as well
as oxygen should be released from the trap faster than CO2
upon thawing. Why then is this phenomenon observed with
such a long delay or time constant?
In order to find an answer to this question we observed the
mass 32 signal (O2‡) during and following freezing of CO2
(Fig. 6). From an initial signal of 17 pA (520 mV above
background, this is about 5 times the background we see on
average in pure CO2) the O2‡ intensity drops to almost zero
during freezing. Please note that this is quantitative only in a
range where the oxygen content is than 1%. Upon removal
of the liquid nitrogen reservoir from the trap the m/z 32 level
increases rapidly for a short time (phase III). The size of this
peak can exceed the `initial' amount by a factor of 2 when the
oxygen is considerably higher than in the example given in
Fig. 6. The peak is followed by a rise of the O2 background
approaching the original level only very slowly.
Figure 6. O2 as a proxy for the behavior of an air impurity in CO2 gas during freezing and
thawing of trap 2. Due to the activity of surfaces, O2 is pumped together with CO2 by applying
liquid nitrogen to the cold finger. Phase I reflects transport of air from the crimp of the capillary
back into the reservoir. Phase II is the background of the impurity while CO2 and most of the
air impurity is frozen at 196°C. A small part of the loss is due to the low temperature (about 2
out of a total compartment volume of 15 mL are immersed in liquid nitrogen). Phase III is
generated by thawing CO2 which pushes and compresses the background gas towards the
crimp. Phase IV is ambiguous. We suspect that frozen air is released faster at the beginning
so that the first CO2 entering the capillary is enriched in air impurity. Phase V is the final
release and equilibration of the background air signal into the ion source. It takes a long time
giving rise to a change in background NO2 production. Please note that the pre-freeze level is
not achieved after more than 300 s following removal of the liquid nitrogen reservoir.
Copyright # 2001 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
Ultra high precision d18O determination in CO2
Possible mechanism for the surface activity of CO2
during freezing
The mechanism that underlies both the observed temporary
and permanent changes in d18O is not easy to interpret. CO2
can bind to surfaces in a number of ways.17,18 In general it
can bind via the oxygen as a single or bridged twin bond to
any surface atom that would accept an oxygen as a
convenient terminator, i.e. an electropositive receptor.
Alternatively, the C atom of the CO2 can bind to an oxygen
on the surface to form a CO32 ion which has three
equivalent oxygen positions. In the latter case an exchange
of the oxygen isotope signature is possible via rotation. This
does not require a lot of energy and could happen at rather
low temperatures, either in the frozen state or during
thawing. Even `clean' surfaces exhibit lots of opportunities
of this kind, hence a further clarification of the mechanism is
difficult to achieve with our experimental options. In
accordance with Freund and Roberts,17 we speculate that
alkali ions on or in the surface which mask themselves with a
more loosely bound oxygen forming a `kink' site provide a
docking position for CO2 molecules provided their kinetic
energy is small enough. Thus this can happen preferentially
at low temperatures with CO2 condensing. Upon heating,
the CO2 may leave in its original form or be altered by the
surface oxygen. Traces of oxygen in this context would serve
to enhance the number of kink sites and thus lead to a larger
temporary effect. This mechanism ± if active ± must be
accompanied by an oxygen isotope effect altering the isotopic composition of the gas on the surface considerably.
Subsequent isotope exchange with the gas phase obviously
can reverse the isotope effect very slowly. It is worth noting
that such surface carbonates are reported to be stable up to
480 K.19
2161
Extracting CO2 from gas mixtures
Measurement of d18O from CO2 in helium
Although quartz looked like the material of choice we had to
discard it as a suitable material for trapping CO2 due to the
observed instability versus large changes in temperature. We
therefore continued our experiments with a T2 single trap
design comprising an outer 1/4@ stainless steel tube and an
1/8@ inner gold tube. With this setup we studied the CO2
collection efficiency and the external precision of d18O and
d13C from gas streams. In the first series of gas flow
experiments a high pressure cylinder was connected to two
of the carousel ports. The cylinder was filled with He
containing 0.5% CO2. All lines of the trapping system were
cleaned by flushing dry gas from the high pressure cylinder
through the entire setup, followed by evacuation using the
turbo pump of the dual inlet system. For sampling the
carousel was actuated to the tank port for 60 s so that the gas
passed the entire trapping line with traps T1 and T2
immersed in their respective cold reservoirs. The total
amount of about 60 mL gas contained 300 bar-mL CO2. The
residual helium was pumped off stepwise before the CO2
was allowed to thaw for 20 min followed by the actual
isotope ratio measurement.
A typical sequence of these experiments is shown in Fig. 7.
Leaving out the first point the observed precision is 0.009%
for d13C and 0.013% for d18O. Prior to the measurement the
vacuum system of our MAT 252 had been subject to a routine
maintenance involving exchange of oil on the fore-vacuum
pumps and of the lubricant of the turbo pumps. The
reference gas was kept enclosed in the reference bellows
during that time. First results after the vacuum had reached a
5 10 8 mbar background pressure indicated a 0.2% offset
for both d13C and d18O. After 24 h of pumping the data
Figure 7. Extraction of CO2 from a high pressure helium cylinder containing 0.5% CO2. The trap
design is the final 1/4@ electropolished stainless steel plus internal 1/8@ glod tube. Trapping efficiency
for CO2 in helium was >99.5%.
Copyright # 2001 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
2162 R. A. Werner, M. Rothe and W. A. Brand
Table 1. Composition of high pressure
cylinder air
CO2
N2O
CH4
Ar
O2
N2
Linde air 20L
Linde TL0009
440 ppm
0.4 ppm
2.3 ppm
Ð
20.0%
Remainder
369 ppm
0.33 ppm
1.67 ppm
0.92%
20.94%
Remainder
shown in Fig. 9 were measured reaching the same result
within 0.01% as observed before vacuum system maintenance. Please note that the difference actually measured
(Dd-values) of the reference gas and the sample CO2 in
helium was 44% (d13C) and 14% (d18O), respectively, and
thus constitutes a worst case experiment. When acquiring
CO2 from air samples, we obviously strive to match the
oxygen and carbon isotopic composition of the reference and
the sample gas as closely as possible in order to minimize
systematic effects.
Measurement of d18O from CO2 in high pressure
cylinder air
A similar set of experiments was made with two cylinders of
dry synthetic air attached to the carousel ports 1 and 2
(`Linde air 20 L') and 5 and 6 (`Linde TL0009'), respectively.
The air composition in the two cylinders is given in Table 1.
Figure 8 shows a typical sequence result for d18O and d13C
of the two gases. The trapping time for extracting enough
CO2 from these gases was 600 s at a flow rate of 60 mL/min
Figure 8. Sequence of isotopic analyses of CO2 from two high pressure cylinders with
synthetic gas mixtures similar to air (see Table 1). A complete sequence lasts about
18 h. Table 2 holds the results of several such sequences measured over a period of
more than two weeks. The absolute d-values are based on reference CO2 gas only
and are not important in this experimental context.
Copyright # 2001 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
Ultra high precision d18O determination in CO2
Table 2. Repeated series of CO2 extraction/analysis from two
tanks of synthetic air. Each value represents the average of a
number of measurements, 8 and 9, respectively, for Linde 20L
and Linde TL0009 (see Fig. 8)
BGC-AirTrap: Two high pressure air tanks on carousel
Series results Linde 20L vs. Linde TL0009
Dd13C [%]
Average
stdev%
Dd18O [%]
44.081
44.100
44.088
44.084
44.090
44.079
44.089
44.098
44.101
44.090
44.093
44.089
44.104
12.567
12.582
12.587
12.559
12.566
12.551
12.559
12.584
12.580
12.550
12.572
12.560
12.595
44.091
0.008
12.570
0.014
9/2000
(i.e. 600 bmL). The absolute d-values on the scale are not
calibrated precisely on the respective scales. The working
reference gas in the mass spectrometer inlet was isotopically
close to the CO2 extracted from TL0009. The precision of the
data is very good with typical values of 0.02% for d18O and
0.01% for d13C. For the last two points the liquid nitrogen
inside the trap 2 dewar had declined to a level that was not
sufficient to avoid fractionation. The experiment was
repeated 12 times in order to test the measurement precision
for the difference between the two gases. The results are
given in Table 2, where the 6th data pair represents the
experiment depicted in Fig. 8.
The results in Table 2 indicate that the use of regulators
(Draeger Tescom, 7400 series) does not preclude measurements of the isotopic difference of air-CO2 in high pressure
cylinders with high precision. A calibration scheme based on
a working reference air delivered from a high pressure tank
therefore is feasible for our high precision d18O and d13C
work from air samples.
Not all air samples have the same CO2 content of about
380 ppm. The air samples for isotope ratio measurement in
our laboratory come from the free atmosphere as well as
from forest canopies or soils. Hence, we must be certain that
a change in flow rate through the trapping line or a change in
trapping time does not alter the measured isotope compositions. In the experiments shown in Fig. 9, we have
simultaneously varied the flow rates and the trapping time
to yield identical amounts of CO2 (about 200 bmL) in the
volume represented by T2. The precision and mean values
confirm that the results are independent of those two
variables.
Measurement of ¯ask samples
Air samples are collected in the field using an assembly
which includes a drying tube, membrane compressor,
Copyright # 2001 John Wiley & Sons, Ltd.
2163
exhaust pressure control and glass flasks. The referencing
scheme using high pressure cylinders and regulator valves
for samples in glass flasks is only valid when the measured
CO2 isotopic composition is independent of the sample gas
pressure or sample port. In order to test the validity of
this assumption we filled 13 sampling glass flasks (flask
volume = 1L) with working reference air (Westf. 10±2000) at
a pressure ranging from 1.6 to 2 bar and measured them
directly against the tank (Westf. 10±2000) attached to
position 1 and 2 of the BGC-AirTrap autosampler. Three
aliquots were analyzed from each flask in order to cover a
wide range in initial pressures.
The air was pumped through T2 for 10 min in order to
obtain sufficient amounts of CO2 in the trap 2 compartment
for analysis. With a controlled flow rate of 60 mL/min this
amounts to 600 bmL air per analysis. The inlet pressure at the
pressure regulator (TesCom 7400 series) was set to 1.8 bar
absolute pressure in order to resemble a typical glass
sampling flask attached to the same port (in our laboratory
we usually measure the mixing ratios of the trace gases prior
to isotopic determination). Figures 10(a) and 10(b) show the
results of this experiment. Full symbols are the raw values
and open symbols are the final values after a number of
routinely applied corrections including correction for N2O
contribution, capillary mismatch correction (see Fig. 2) and
offset correction by running reference air through the same
inlet line. The mass flow controller requires a pressure
differential of 100 mb and the pressure in the trap area
during sampling is 330 mb. Hence, exhaustion of a flask is
reached when the flask pressure drops to 430 mb. This
happened for all 3rd runs of the flasks giving rise to the
observed deviations in the uncorrected delta values. (When
the initial pressure in a 1000 mL flask is at 900 mb and the
mass flow controller maintains a flow of 60 bmL/min, the
threshold of 430 mb, required for constant flow, is reached
after roughly 8 min. During the remainder of the sampling
time, the flow rate will steadily decrease, resulting in a lower
than target CO2 ion current.)
The average measured difference between our working air
in flasks above 0.9 bar starting pressure and the working air
in the tank was 0.004% for d13C and 0.021% for d18O. These
values are identical (d13C) within the statistical limits of our
measurements or very slightly off (d18O) of these margins
(see Figs. 11(a) and 11(b)). Between 0.9 and 0.5 bar starting
pressure, the data are still acceptable but show a larger
scatter. Below 0.5 bar initial pressure, the offset level reaches
0.05% for d13C and 0.1% for d18O. Results obtained from two
flasks appear as outliers and are indicated by the dotted
circles around them in Fig. 11. We suspect that these two
flasks were affected by moisture although all flasks had been
washed prior to filling with dry air several times. We are
now continuously monitoring this behavior in order to
assess whether or not our present flask treatment procedures
are adequate given that we aim for extremely high precision.
Long term precision and principles of standardization
The annual decrease in the average isotopic composition of
atmosheric CO2 is small ( 0.025%/y for d13C) and the
seasonal cycle is 0.05% peak to peak at the South Pole
increasing to a maximum of about 0.8% at 60 ° North. To
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
2164 R. A. Werner, M. Rothe and W. A. Brand
Figure 9. A series of isotopic measurements of CO2 from two reference air cylinders as a
function of sampling flow. In order to obtain comparable signal size, the sampling
(= trapping) time was adjusted accordingly. Within measurement precision the results are
identical to the long term records given in Fig. 11. The typical sampling flow adopted for
routine measurements is 60 mL/min. The data suggest that within the flow range tested a
possible change of the isotopic composition of the extracted CO2 is small.
follow the subtle isotopic changes and deduce net changes in
global fluxes from these numbers requires utmost precision
of the analysis and stability of the scales over decades.
Hence, the analytical precision must be anchored to an
internationally accepted scale. In our laboratory we have
received a suite of six high pressure air cylinders from
CSIRO Atmospheric Research (CAR), Melbourne, Australia.
In collaboration with the International Atomic Energy
Agency, IAEA, this laboratory maintains an international
isotope scale for CO2 in air linked to VPDB. The procedure
used by CAR to report isotopic measurements on the VPDBCO2 scale is based on careful measurements of a reference
Copyright # 2001 John Wiley & Sons, Ltd.
CO2 tank versus NBS19,20 a calcite material which defines
the VPDB scale today.21,22 The reference tank we use for
positioning our measurements on the international scale has
the cylinder ID CA01656 and has the isotopic composition of
d13C = 8.078 0.017% and d18O = 0.847 0.033% on the
VPDB-CO2 scale. Both values are close to current atmospheric CO2 d-values. The other tanks of the suite serve as
control standards.
For every day referencing we have a working standard gas
(Westf. 10±2000) in a 50L tank filled to a pressure of 250 bar.
The constituents are O2 (20.94%), Ar (0.96%), CO2 (370 ppm),
CH4 (1.7 ppm), N2O (0.315 ppm), and N2 (rest). The bulk
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
Ultra high precision d18O determination in CO2
composition of air in this cylinder hence reflects that of
ambient air; however, the isotopic composition of the CO2 is
2.77% (d13C) and 14.67% (d18O), considerably different
from the ambient air value. This working standard is
permanently attached to positions 1 and 2 of the Valco
multiport valve. The primary reference gas CA01656 is
permanently attached to position 9. The remaining 13
positions of the carousel are used for sample flasks and for
occasional measurement of other control standards for
detecting a possible drift of the two tanks together. A routine
sequence starts with measurement of position 1 and finishes
with position 2 after a complete rotation. Thus, the working
reference is measured twice at the beginning of the run and
twice at the end. These four analyses form the basis for
2165
positioning the results of the 13 sample flasks and the quality
control position 9 onto the international isotope scales.23
Systematic absolute errors associated with the 17O correction, the filling or performance of the reference side gas, the
N2O mass balance correction, etc., tend to cancel as long as
the gas is close to atmospheric levels and as long as the
primary reference gas remains stable over time. Figures 11(a)
and 11(b) represent the performance of our measurement
system from the start of the routine flask analyses in October
2000 until June 2001. The average values in this figure differ
from those in the previous figures due to a recent scale
update which also affected our reference tank CA01656. For
assembling Fig. 11 we generated one point (Westf. 10-2000
vs. CA01656) per complete carousel which is a measurement
Figure 10. Extraction of CO2 gas from glass flasks. The flasks were filled with
reference air Westf. 10–2000 by flushing them with five times their volume in
series at elevated pressure (2 bar). The different starting pressures (2 bar to
1.6 bar) were generated by selectively pumping on the flasks attached to the
BGC-AirTrap carousel. Each flask was measured three times during three
complete, separate sequences. Filled symbols are the raw values, open symbols
the final results after correction for ion current size dependence (Fig. 2), N2O
contribution and reference CO2 offset. During the last sequence (‘run’) the
starting pressure was too low for the flow controller to maintain a constant yield.
The results indicate the validity of the correction procedure for low amounts of
CO2 in front of the capillary to the changeover valve (Fig. 2).
Copyright # 2001 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
2166 R. A. Werner, M. Rothe and W. A. Brand
Figure 11. Long term CO2 isotope analysis record (performance or QA chart) of two
reference air tanks. The isotope scales are anchored to the CSIRO air-CO2 scale
(CG99) by one of the tanks (CA01656). Deviations from the absolute values given in
Figs 8 and 9 arise from a scale adjustment from the CG92 scale to the CG99 scale.
Possible causes for the deviation to lower d-values between the beginning of
December 2000 and the end of January 2001 are discussed in the text. Leaving this
period out, the overall single sample precision is 0.008% for d13C and 0.013% for
d18O. This figure includes possible effects of the pressure regulator. Flask filling or
storage effects must be addressed separately.
of the difference between the primary reference and the
working reference gas. The statistical basis for this measurement is the same as for all sample flasks attached to the
carousel except for the filling procedure or flask personality
which needs to be addressed further. The results indicate a
change in isotopic composition between the end of November and the end of January. The difference in the respective
average values is 0.012% for d13C and 0.024% for d18O. The
exact causes of the shift are not clear to us. The factor of 2
between the deviations in d13C and d18O suggests a fractionation in the gas phase. We discovered that a tiny leak had
developed in the reference CO2 gas refill line which, when
fixed, ended the deviation at the end of January 2001. In spite
of the shift, the overall precision with 0.012% for d13C and
0.019% for d18O is very acceptable and, with the leak fixed,
Copyright # 2001 John Wiley & Sons, Ltd.
there is a chance to improve further towards values of 0.01%
and 0.015%, respectively, in the future. We will continue the
routine measurement of the primary reference as a QA
standard for at least another year in order to detect possible
drifts before switching to another QA standard and measure
the primary reference less frequently.
CONCLUSIONS
Extraction of CO2 from dry air samples for ultra high
precision d18O and d13C determination has been achieved by
careful trap design, particularly regarding the material and
surface treatment of the trap itself. Most materials we tested
for this application proved unable to deliver high precision
results. Although we have found that carefully cleaned high
Rapid Commun. Mass Spectrom. 2001; 15: 2152±2167
Ultra high precision d18O determination in CO2
purity quartz performed best in terms of precision, it
displayed limited stability following the temperature cycling
that is a crucial part of the extraction procedure and is
therefore unsuitable for use. Our final selection for construction of the traps is an outer finger made from stainless steel
with an internal electropolished surface finish and an
internal finger made from carefully purified gold.
We have shown that the d18O of CO2 gas is altered,
sometimes permanently, during the freezing step and that
this affects our ability to perform these measurements with
sub-0.05% precision. We suspect that the current unsatisfactory situation with d18OCO2 measurements in long term air
CO2 records is at least partly due to the effects described that
occur during the freezing step (unidentified sources of
moisture and insufficiently controlled flask conditioning and
sampling remain other sources of poor precision).
The idle time following freezing is a critical parameter for
high precision results. With our trap design the required
post-freeze delay before measurement is longer than 20 min.
Using our optimized experimental procedures, we have
maintained the isotopic analysis of CO2 extracted from
600 mL of air with a high level of precision for both d13C and
d18O of 0.01% and 0.02%, respectively. These values are
close to what we believe is ultimately achievable within one
laboratory.
The challenge for the future is two-fold:
. to assure that this level of precision can be maintained over
decades in a single laboratory independent of the particular experimental setup, and
. that the same level of precision can be reached throughout
a large number of collaborating scientific laboratories.
Acknowledgements
For fruitful discussions in our laboratory we are indebted to
Ray Langenfelds from CSIRO Atmospheric Research, Melbourne. With critical comments and suggestions, Jon Lloyd
(MPI Jena), Colin Allison (Melbourne) and Bernhard Mayer
(Calgary) have helped to improve the manuscript. C. B.
Copyright # 2001 John Wiley & Sons, Ltd.
2167
Douthitt (Dallas) has left his mark as an ever critical editor in
terms of language and readability.
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