Optimization of automated gas sample collection and IRMS analysis
Transcript of Optimization of automated gas sample collection and IRMS analysis
Optimization of automated gas sample collection and IRMS1
analysis of δ 13C of CO2 in air2
Matthias J. Zeeman1, Roland A. Werner1, Werner Eugster1, Rolf T. W. Siegwolf2, Günther Wehrle2,3
Joachim Mohn3, Nina Buchmann14
1Institute of Plant Sciences, ETH Zurich, Universitaetsstrasse 2, CH–8092 Zurich, Switzerland5
2Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Bachstrasse 1, CH–5232 Villingen, Switzerland6
3Laboratory for Air Pollution & Environmental Technology, Empa, Überlandstrasse 129, CH–8600 Dübendorf,7
Switzerland8
Abstract9
The application of 13C/12C in ecosystem–scale tracer models for CO2 in air requires accurate mea-10
surements of mixing ratios and stable isotope ratios of CO2. To increase measurement reliability and11
data intercomparability as well as to shorten analysis times, we have improved an existing field sampling12
setup with portable air sampling units and developed a laboratory setup for analysis of δ 13C of CO2 in13
air by isotope ratio mass spectrometry (IRMS). The changes consist of (a) optimization of sample and14
standard gas flow paths, (b) additional software configuration and (c) automation of liquid nitrogen refill-15
ing for the cryogenic trap. We achieved a precision better than 0.1� and an accuracy of 0.11±0.04�16
for δ 13C of CO2 in air and unattended operation of measurement sequences up to 12 hours.17
18
The interest in the global atmospheric car-19
bon cycle has intensified as a response to re-20
ported trends in global climate change. These21
trends are primarily related to atmospheric in-22
creases in greenhouse gas concentrations.1 On23
the global average, carbon dioxide (CO2) plays24
the most important role and thus ecosystem ori-25
ented research has particularly focused on CO2.26
The potential use of the stable isotope ratios of27
CO2 (e.g. 13C/12C, 18O/16O) in ecosystem–scale28
atmosphere–biosphere process studies has often29
been highlighted and is believed to be a power-30
ful tool for carbon cycle studies, in particular to31
disentangle ecosystem flux components.e.g. 2–7 It is32
commonly used to quantify mixing contributions33
from sources with differing isotopic composi-34
tions.8,9 However, this requires accurate measure-35
ments of both CO2 mixing ratios and isotopic com-36
position in order to be useable in ecosystem–scale37
tracer model approaches.4,10,11 On local (species38
to ecosystem) scales this can be quite a challenge;39
CO2 mixing ratios and isotopic composition in the40
air close to the vegetation are known to fluctu-41
ate strongly, i.e. on short time scales (seconds to42
hours), especially under less turbulent atmospheric43
conditions due to accumulation of CO2. Moreover,44
with conventional flask sampling the measurement45
strategy is mostly limited to discrete sampling, and46
typically these samples need to be transferred to a47
distant laboratory for analysis by an Isotope Ratio48
Mass Spectrometer, so the insight into ecosystem49
processes is hampered by technical and logistical50
constraints.51
In this paper, we aim to optimize and ex-52
tensively test air sampling and analysis of stable53
carbon and oxygen isotope ratios in atmospheric54
CO2 for stable isotope studies at the ecosystem55
level. The setup described here has been suc-56
cessfully used for grassland ecosystem studies in57
Switzerland and intercomparisons of stable iso-58
∗Correspondence to: MJ Zeeman, ETH Zurich, Institute of Plant Sciences, Universitaetsstrasse 2, CH–8092 Zurich, Switzer-land, Email: [email protected], Phone: +41 44 632 81 96, Fax: +41 44 632 11 53
1
tope ratio instrumentation (e.g. a comparison of59
a quantum cascade laser based absorption spec-60
trometer, a field-deployable Fourier transform in-61
frared spectrometer and an Isotope Ratio Mass62
Spectrometer).e.g. 12–14 The basic considerations63
for the chosen measurement approach haves been64
(a) the collection of samples at multiple locations65
for (b) sample measurements by laboratory based66
high precision Isotope Ratio Mass Spectrometer.67
The most important implication of this approach68
is that the conditions (e.g. temperature, pres-69
sure) might be different between location of sam-70
ple collection and the laboratory. Thus, gas sam-71
ples might be contaminated during the storage pe-72
riod between sampling and analysis, which is es-73
pecially likely if samples are collected at higher74
altitudes under reduced ambient pressure.15–18 For75
time series analysis, e.g. to understand diur-76
nal cycles or effects of weather events, samples77
or series of samples are repetitively collected at78
equally spaced time intervals. If a Keeling plot ap-79
proach (inverse [CO2] related to isotope δ–value)80
is used, the accuracy of y–axis intercepts is di-81
rectly related to the precision and accuracy of the82
measurements.cf. 4 Thus, the analysis must be as83
accurate and precise as possible, deviations should84
be on the order of 0.1� for δ 13C at most.85
To achieve our aims, we have substantially86
improved existing gas sampling equipment previ-87
ously described by Theis et al. 19 and developed a88
new Isotope Ratio Mass Spectrometer setup, pro-89
gramming and measurement routines for δ 13C of90
CO2 in air. An overview of this improved setup91
is shown in Figure 1 for both field and laboratory92
setup. An important part of these improvements93
was to optimize the automation of the operations94
during sampling and isotope ratio analysis to allow95
for accurate timings and increased reproducibility.96
Thus, our objectives were to 1) apply digital com-97
munication protocols between the sampling unit98
and the control computer to store status informa-99
tion from the sampling unit in order to eliminated100
the potential error of sample misidenfication.cf. 20101
2) We wanted to increase precision and reliability102
of the IRMS measurements for CO2 in air sam-103
ples and optimize sample preparation steps. 3) We104
wanted to reduce the time required per IRMS anal-105
ysis of a CO2 in air sample to increase throughput106
in the laboratory and reduce storage times of the107
samples.108
Methodology109
Field setup110
Three devices are used in our field setup (Fig. 1),111
consisting of an home-built air inlet selection unit,112
followed by an InfraRed Gas Analyzer (IRGA)113
for CO2 mixing ratios (model LI-840, LI-COR,114
Lincoln, Nebraska, USA) and a sample mani-115
fold at the end. This sample manifold is a116
modified and improved version of the device117
termed Automated Sampler of Air (ASA) by Theis118
et al. 19 . It contains 33 glass flasks sample con-119
tainers connected to three multiport Valco-valves120
(EMTMA2ST12MWE, VICI, Schenkon, Switzer-121
land) allowing independent filling of each individ-122
ual sample container with sample air. We con-123
tinue to use the abbreviation “ASA” to refer to the124
portable air sampling unit described here, because125
its key components (the Valvo-valves) and its func-126
tion as sample manifold have not changed with127
respect to the Theis et al. 19 version, despite the128
modifications described here.129
During field deployment, a single inlet is se-130
lected from a series of continuously purged air in-131
lets (Synflex™Type 1300, formerly known as Dek-132
abon™, Gembloux SA/NV, Belgium; ID 4 mm,133
≈ 1 L min−1). After a particle filter (Gelman,134
LI-COR), a T-split diverts the airflow (a) to the135
IRGA and a subsequent small pump (DC12/8FK,136
Fürgut GmbH, Germany) inside the inlet selec-137
tion unit, and (b) to an ASA sample inlet. The138
flow through the IRGA is kept at a continuous139
rate of 0.9 L min−1 (Fig. 1), within the manufac-140
turer supplied specifications for the IRGA. Once141
inside the ASA (Fig. 2), the sample air is pushed142
by a pump, diverted on activation of a solenoid143
valve (EVT307-5D0-02F-Q, SMC, Weisslingen,144
Switzerland) through a drying column containing145
magnesium perchlorate (Fluka, Switzerland) and146
is filtered (SS-4FW-2, Swagelok, USA) before be-147
ing pushed further through 300 mL glass flasks148
(Ernst Keller & Co AG, Basel, Switzerland) or149
10 mL stainless steel loops (SL10KSTP, VICI,150
Schenkon, Switzerland) connected to the multiport151
Valco-valves with≈ 0.9 L min−1. At one of the 12152
positions of each Valco-valve a short stainless steel153
capillary is used as low volume bypass to allow154
aligning multiple Valco-valves in series, typically155
three or four per ASA. To create an over-pressure156
2
of at least 50 kPa in the sample containers, we157
changed the original Theis et al. 19 design and the158
position of the Teflon membrane pump (N811K-159
DC, KNF, Germany) in combination with a poppet160
check valve (SS-6C-MM-1, Swagelok, USA) and161
an adjustable flow meter (V-100, Vögtlin, Switzer-162
land) at the exit. By having pressurized sam-163
ple containers, the chance of contamination dur-164
ing transport and laboratory analysis is minimized.165
In the laboratory (Zurich, 400 m above sea level,166
a.s.l.), the pressure excess (pressure above ambi-167
ent) directly after the pump inside the ASA (Fig. 2)168
was typically found to be ≈ 90 kPa and ≈ 50 kPa169
before the adjustable flow meter. This is suffi-170
cient for the collection of samples at alpine loca-171
tions (e.g. >2000 m a.s.l.), though higher pres-172
sures could be reached at the expense of flow rates.173
Although multiple ASAs can be used in series174
for sampling by using the bypass position of the175
solenoid valves, we have chosen a parallel setup176
utilizing a flow split with poppet check valves (SS-177
6CA-MM-3, Swagelok, USA) to prevent any back-178
flow from ASAs with inactive pumps or from the179
open connections after removal of one ASA. The180
position of the IRGA (Fig. 1) parallel to the ASA181
has shown no discernible different results. As an182
advantage over a sequential setup our parallel ver-183
sion allows to continue the concentration measure-184
ments of the sample gas even if the ASAs are dis-185
connected or inactive. The IRGA can alternatively186
be positioned directly before the ASA to directly187
analyze the gas that subsequently flows through the188
sample containers of the ASA.189
The inlet selection unit and ASAs are config-190
ured and operated by a field computer via RS-191
232 serial communication lines connected to dig-192
ital controllers (C-Control I STATION 2.0, Con-193
rad Electronics GmbH, Germany), programmed to194
operate the rotation valves, solenoid valve, pump,195
and digital flow meter, and to provide status infor-196
mation for later use in post-processing of CO2 con-197
centration and stable isotope data. The keypad and198
LCD display of the digital controllers are used to199
confirm correct operation of the devices before and200
during field deployment. The serial communica-201
tion, data storage and post processing are handled202
by scripts written in Perl language.203
After having completed an in situ field sam-204
pling sequence, the ASAs with up to 33 or 44 gas205
sample containers per ASA are transported back to206
the laboratory for subsequent (same day) isotope207
ratio analysis.208
Laboratory setup209
The precise determination of δ 13C and δ 18O val-210
ues in CO2 of large numbers of air samples im-211
plies a precise and reproducible sampling tech-212
nique as well as an automated and easy-to-use213
coupling of the sample containers (glass flasks or214
steel loops) to the Isotope Ratio Mass Spectrom-215
eter (DeltaplusXP, Finnigan MAT, Bremen, Ger-216
many).217
A series of multiposition valves are used for218
the flow path of the sample preparation (Fig.219
3). A 6-position dead-end path Valco-valve220
(ASD6MWE,VICI, Schenkon, Switzerland) and a221
4-port 2-position Valco-valve (AC4UWE, VICI,222
Schenkon, Switzerland) allow the alignment of up223
to four independent reference air gas bottles (lab-224
oratory air gas cylinder with different CO2 mix-225
ing ratios and δ -values) or helium, using the same226
sample preparation path as the gas sampled with227
the ASA allowing referencing according to the228
Identical Treatment (IT) principle.21 A feed cap-229
illary delivers pure He to the ASA (Fig. 3, valves230
1 and 2), allowing a pressure build-up in the glass231
flasks that flushes the sample gas at a rate of about232
5 mL min−1 through a water trap (Nafion dryer) to233
the cryogenic focus trap where condensable gases234
(mainly CO2 and N2O) are cryogenically trapped.235
After diverting the non-condensable gases to a vent236
(Fig. 3, valve 4), the cryogenically trapped sample237
is thawed and subsequently flushed by He into the238
Gas Chromatograph column (Poraplot Q 25 m ×239
320 nm i.d., Varian, Walnut Creek, USA, held at240
24 ◦C) to allow separation of CO2 from N2O and is241
subsequently led to the Isotope Ratio Mass Spec-242
trometer for analysis (Fig. 3, valve 5). The trap-243
ping efficiency was checked beforehand with an244
IRGA (LI-840, LI-COR, Lincoln, Nebraska, USA)245
behind the frozen cryogenic trap. In contrast to246
Theis et al. 19 , who used a Precon (Finnigan MAT)247
hooked up to the Isotope Ratio Mass Spectrome-248
ter, we modified the Gasbench II system (Finnigan249
MAT) to directly interface with individual ASA250
units. This modification of the Gasbench (Fig. 3,251
bottom panel) comprises the replacement of the252
“Gas Chromatograph”-type split22 by a ConFloIII-253
like split23 and the replacement of the stainless254
3
steel sample loop with a home-built cryogenic fo-255
cus trap (1/16" stainless steel capillary filled with256
Ni-wire) at the 8-port valve inside the Gasbench,257
which is configured to operate as 6-port valve (Fig.258
3, valve 4). A second 4-port 2-position Valco-259
valve (AC4UWE, VICI, Schenkon, Switzerland)260
inside the sample preparation path (Fig. 3, valve261
3) operates as a vent to release the pressure ex-262
cess inside the sample containers and allows for263
high flow purging of the sample preparation path264
(10 mL min−1) and the Isotope Ratio Mass Spec-265
trometer flow path (17 mL min−1) with pure He.266
In our system without Precon this would otherwise267
not have been possible. Without the posibility to268
flush with high flow (50 mL min−1) as shown by269
Theis et al. 19 , our measurement time would have270
been 835 s. With the help of the pressure vent and271
related high He flows we are able to reduce the272
analysis time per sample to 610 s, a period com-273
parable to Theis et al. 19 .274
The cryogenic trap and all valves in the Gas-275
bench, the external referencing unit, the rotary276
valve systems of the ASA and an automated liquid277
nitrogen refill procedure are computer controlled278
by modified Isodat script language (ISL) scripts,279
available in the vendor supplied ISODAT NT soft-280
ware package (Ver. 2.0 SP2.63, Finnigan MAT).281
To avoid overloading of the cryogenic trap282
with sample gas of high CO2 concentration (>283
1000 µmol mol−1) and to circumvent a possible284
non-linearity of the Gasbench and Isotope Ra-285
tio Mass Spectrometer combination with signal286
strength a, the signal strength of each sample is ad-287
justed to be close to that of the Isotope Ratio Mass288
Spectrometer reference by changing the cryogenic289
trapping period depending on the sample concen-290
tration. We first tested the relation between the291
sample CO2 concentration and the cryogenic trap-292
ping period empirically for each of the different293
sample container volumes and tube lengths, deter-294
mined the best fit (Fig. 4) and tested the results295
with known dilutions of a CO2 in air mixture of296
a known stable isotope composition. Furthermore,297
to improve stable conditions for the cryogenic trap,298
the liquid nitrogen (LN2) level in the Dewar was299
kept within the 95–100% range, either by manual300
refill or automated refill. To facilitate partly unat-301
tended operation (12 hours) and thus a certain level302
of autonomy of this setup, we installed a LN2 re-303
fill system that consists of a balance measuring the304
weight loss of the evaporating LN2 from the Dewar305
in combination with timing signals received from306
the Isotope Ratio Mass Spectrometer (Fig. 5) and307
that is supplied by a 30 L LN2 tank (up to 48 hours308
operation). Empirically, de-icing of the Dewar is309
required every 12 hours.310
As a last step, the ISODAT NT software config-311
uration had to be adjusted to our modified setup, in312
particular the measurement timing schedule (Fig.313
6). First, the sample preparation steps of flush-314
ing the capillaries with He and sample gas through315
the input lines before the gasbench, which is con-316
ventionally done between two measurements us-317
ing a “pre-script”, is scheduled during and in par-318
allel with the IRMS analysis for the next measure-319
ment. A second important modification that helped320
to save time was achieved by switching to high He321
flow rates while purging the input line capillaries322
before and in the Gasbench as described above.323
Third, changes to the ISODAT NT software con-324
figuration for the Gasbench allowed for variably325
timed operations parallel to the measurement of326
the reference standards (multitasking) within the327
“chromatography” part of the measurement time328
schedule. Thus, the cryogenic trapping period329
could be varied within the chromatogram sequence330
without disturbing other tasks (e.g. the measure-331
ment of the reference standards) and was no longer332
required to be executed before the chromatogram333
(e.g. in a pre-script). The combination of the de-334
scribed modifications allowed to shorten the IRMS335
measurement time per sample and increased the336
number of samples analyzed per day.337
Isotope ratio analysis338
The carbon and oxygen isotopic composition ofthe CO2 is expressed as the relative difference ofits isotope abundance ratio relative to that of an in-ternational standard. This difference, usually ex-pressed in per mill, is defined as
δ 13C[�]V–PDB =[(13C/12C)Sample
(13C/12C)V–PDB−1
]·103 (1)
δ 18O[�]V–PDB–CO2 =[
(18O/16O)Sample
(18O/16O)V–PDB–CO2
−1]·103
(2)aThe effect should be < 0.06�V−1 for δ 13C for the reference standards, according to the vendor supplied instruction manual.
4
Post-run off-line calculation and drift correc-339
tion for assigning the final δ 13C and δ 18O val-340
ues on the V-PDB (Vienna PeeDee Belemnite) and341
V-PDB-CO2 scaleb were done following the IT342
principle as described by Werner and Brand 21 .343
The δ 13C and δ 18O values of the laboratory air344
standards (Zurich CO2-in-air standards) were de-345
termined at the Max-Planck-Institut für Biogeo-346
chemie (MPI-BGC, Jena, Germany) according to347
Werner et al. 25 . The accurate assignment of the348
corresponding δ -values on the V-PDB and the V-349
PDB-CO2 scale was performed in Jena by mea-350
suring the Zurich CO2-in-air standards versus the351
“Jena-Reference AirSet” (J-RAS) as standard ref-352
erence material (SRM).e.g. 24 Any isotope ratio353
data presented in this article are reported in [�]354
deviation from V–PDB and V–PDB–CO2 for 13C355
and 18O, respectively. Typically several measure-356
ments of a laboratory reference standard are placed357
at the beginning and at the end of each measure-358
ment series, for post-calculation of corrections.359
Quality control (QC) standards are used to eval-360
uate this correction procedure.361
Results and discussion362
After ensuring linearity, we tested the effects of the363
variable cryogenic trapping period of our ASA–364
Gasbench–Isotope Ratio Mass Spectrometer setup365
on δ 13C measurements for a range of CO2 concen-366
trations and for different sample containers used367
(glass or metal). Furthermore, we tested the per-368
formance (precision, accuracy) of the δ 13C mea-369
surements for typical use of the described ASA–370
Gasbench–Isotope Ratio Mass Spectrometer setup.371
Linearity tests with gases of different CO2 in372
air or He mixing ratios have shown a strong re-373
lationship between the IRMS peak amplitude and374
the offset between the δ 13C (or δ 18O) of CO2 in375
a sample and its δ 13C (or δ 18O) reference value376
(Fig. 7). For δ 13C, this offset increases strongly377
with lower relative peak amplitude. We suppose378
the origin of this effect is the signal to noise ratio379
of the analysis. Thus, to ensure measurement inter-380
comparability, it is required to correct for this ef-381
fect, e.g. by optimizing the peak amplitudes of the382
samples to a limited range close to the amplitudes383
of the Isotope Ratio Mass Spectrometer reference384
gas. This optimization in effect means that the385
cryogenic trap should freeze the same amount of386
CO2 for each sample, independent of sample con-387
tainer and capillary volumes. Due to differences388
in pressure build-up as function of container vol-389
ume, the relationships between CO2 concentration390
of the sample and the trapping period required for391
a peak amplitude close to the reference have to be392
determined empirically (cf. Fig. 4, Table 1) and393
were thus tested by analyzing a broad range of di-394
lutions of a CO2 in air mixture with CO2 free air395
(Fig. 8, top panel). The resulting relative ampli-396
tudes for this range of diluted samples (Fig. 8, bot-397
tom panel) are between 85 and 125%, well within398
the typical variability of peak amplitudes (cf. Fig.399
7), reflecting the quality of the chosen fit function400
and the inaccuracy caused by the low time res-401
olution of the variable trapping period (Fig. 4).402
This last aspect is mostly defined by the 1 s time403
resolution of the Isotope Ratio Mass Spectrome-404
ter chromatogram procedure, for which the inaccu-405
racy increases at higher CO2 concentrations. For406
example, CO2 delivered by steel capillary for the407
concentration range [355,390] µmol mol−1 and408
[1365,1502] µmol mol−1 are represented by a 34 s409
and 20 s trapping period, respectively. If very410
high concentrations (> 5000 µmol mol−1) are ex-411
pected, the relative peak amplitude of the samples412
could be allowed to be > 100%. However, a new413
empirical relation would need to be determined414
for lower flow rates of sample through the cryo-415
genic trap or the Isotope Ratio Mass Spectrometer416
software for the variable trapping period (see Ap-417
pendix) would need to be changed to use a time418
resolution shorter than 1 s. In any case, applica-419
tion of the empirical relations of trapping period420
and sample concentration requires not only that the421
concentration needs to be known prior to IRMS422
analysis, but also that the CO2 concentration data423
from the IRGA (Fig. 1) needs to be collected and424
processed prior to the laboratory analysis.425
Since we modified the Isotope Ratio Mass426
Spectrometer setup substantially, we tested the per-427
formance (i.e. precision and accuracy) by mea-428
bThe virtual non-existing standard V-PDB is defined by adopting a δ 13C value of +1.95 � and a δ 18O value of –2.2 � forNBS 19 exactly. Via assigning these δ -values the hypothetical mineral V-PDB or rather the CO2 produced from it would be thestandard for δ 13C and δ 18O values. The term V-PDB-CO2 refers to the oxygen isotopic composition of the CO2 evolved fromthe mineral by reaction with water-free H3PO4 at 298 K. For details, see e.g. Ghosh et al. 24
5
suring δ 13C (and δ 18O) of a laboratory reference429
standard and QC standards that passed through the430
sample flow path (Fig. 9) or were sampled by431
the ASA beforehand (Fig. 10). The overall pre-432
cision of δ 13C measurements was determined to433
be <0.08� (σ ) for samples with standards stored434
in glass flasks inside an ASA (N=33), <0.11�435
(σ ) for samples with standards stored in stainless436
steel loops inside an ASA (N=44) and <0.06�437
(σ ) for directly supplied standards (N=5), over the438
course of several measurement campaigns between439
February 2006 and March 2008. The decrease in440
precision with increasing sample numbers (N=5,33441
or 44) suggests that reference standards must be in-442
cluded in the sample sequence more frequently to443
correct for possible drift effects. The slight differ-444
ence between the QC standard I and II (Fig. 9) can445
be explained by methods to determine the respec-446
tive reference value. For the QC standard I the ref-447
erence value was determined by the ETH IsoLab448
based on an average difference (N=5) to the labo-449
ratory reference standard. For the second QC stan-450
dard on the other hand, the reference value was de-451
termined by an internationally acknowledged lab-452
oratory against several international standards with453
high precision. In general, based on periodic mea-454
surements of standards (mostly QC Standard I) us-455
ing the ASAs, the overall accuracy was determined456
to be 0.11±0.04� (σ ), reflecting measurements457
with the ASA–Isotope Ratio Mass Spectrometer458
setup during one year, i.e. March 2007 to March459
2008. For δ 13C, we did not find an effect of the460
surface properties (e.g. volume, surface:volume,461
surface material) of the stainless steel loop ver-462
sus the glass flask sample containers on the pre-463
cision and accuracy of stored samples. However,464
for δ 18O, the used stainless steel loop contain-465
ers appeared inadequate and would require exten-466
sive pre-treatment, such as the removal of resid-467
ual water from the surfaces that can otherwise ex-468
change oxygen atoms in an equilibrium reaction469
with CO2 and thus potentially change the stable470
isotope composition of the sample. A treatment471
with long periods of dry air flushing in combina-472
tion with heating, as suggested by Gemery et al. 16 ,473
makes steel loops far less practical for δ 18O mea-474
surements than their counterpart, i.e. glass flask475
as sample containers. We evaluated the reliability476
of the filling procedure of the ASAs and the influ-477
ence of transport of samples from the field to the478
laboratory by filling the same ASA in the field and479
sebsequently in the laboratory with the same stan-480
dard gas. The resulting δ 13C and δ 18O measure-481
ments showed no significant difference (∆δ 13C=-482
0.04�, ∆δ 18O=0.05�, N=5) between the respec-483
tive filling locations (Sophia Etzold, Institute of484
Plant Sciences, ETH Zurich, Switzerland, unpub-485
lished data).486
For studies using the Keeling plot approach,487
e.g. to determine the signature of the respiration488
source via a statistical regression approach, the489
optimization of peak amplitudes provides a clear490
and essential improvement for intercomparability491
of δ 13C measurements.8 Small (systematic) errors492
in δ 13C values would lead to much larger uncer-493
tainty in the determination of the respiration sig-494
nature (the intercept of the Keeling plot regres-495
sion line).4,11 Our results show that the system de-496
scribed here not only can provide precision of at497
least 0.1� with an accuracy of 0.11±0.04� (σ )498
for δ 13C, but also allows unattended operation in499
the field and retain a measurement time per sam-500
ple of 610 s. This clearly fullfills the quality crite-501
ria necessary to perform gradient measurements of502
stable isotope ratios of CO2 in air for the study of503
atmosphere–biosphere interactions.504
Acknowledgments505
Peter Plüss (ETH) and Patrick Flütsch (ETH) are506
kindly acknowledged for their extensive techni-507
cal support. We would like to thank Matthias508
Saurer (PSI), Willi A. Brand (MPI-BGC), Michael509
Rothe (MPI-BGC) and Sophia Etzold (ETH) for510
their advice and helpful discussions. Our work has511
also benefited from discussions with Peter Weigel512
and Andreas Hilkert (Thermo Fischer). This work513
has been supported by the Swiss National Science514
Foundation (SNF), grant 200021-105949.515
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7
Appendix599
The ISL script code used for multitasking the con-600
centration dependend activation of the cryogenic601
trap is described in the following example.602
1 s c r i p t TrapTimer603
2604
3 i n c l u d e " l i b \ s t d i s l . i s l " ;605
4 i n c l u d e " l i b \ I n s t r u m e n t . i s l " ;606
5 i n c l u d e " l i b \ GasBench_ l ib . i s l " ;607
6608
7 f u n c t i o n TrapTimerFun ( number A)609
8 {610
9 number B = (−10.393* l o g (A) ) −96;611
10 r e t u r n B ;612
11 }613
12 main ( )614
13 {615
14 number nA = _GetSequenceNumber ( "616
C o n c e n t r a t i o n " , 6 0 ) ;617
15 number nB = c a l l TrapTimerFun ( nA) ;618
16 number nC = 150 − nB ;619
17 number nD = _RefGe t P ro f i l eNumber ( "ASA620
" , " S t a r t " , 0 ) ;621
18 number nE = abs ( _GetTickCount ( ) − nD)622
;623
19624
20 i f ( nE > ( nC * 1000) )625
21 {626
22 _S e t ( " Gas Bench / Valco " ,LOAD) ;627
23 _ U s e r I n f o ( " CryoTrap a c t i v e ! " , 0 , 0 ) ;628
24 }629
25 }630
The sample concentration is read from the se-631
quence table and converted to a trapping period via632
an empirically fitted function (code line 14–15) as633
shown in Figure 4, from which a target start time634
in seconds is calculated (16). The elapsed time is635
calculated (17–18) and compared to the target start636
time in order to decide if the trap should be set to637
an activated state or not (20–24). For correct cal-638
culation of the elapsed time, the start time of the639
chromatogram process was stored from the inter-640
nal millisecond counter with the command code641
1 numbers nF = ( _GetTickCount ( ) ) ;642
2 _Re gSe tP ro f i l eNumb er ( "ASA" , " S t a r t " ,643
nF ) ;644
3 c a l l S t a r t c h r o m a t o g r a m ( ) ;645
within the Acquisition ISL script used for this646
method. Please note that the included lines (1–2)647
are added just before the chromatogram is started648
(3). For effective use of the script in the ISODAT649
NT acquisition software, it was incorporated into650
the Gasbench configuration as an ActionScript de-651
vice. The execution of the script code (or Action-652
script device) within the acquisition method was653
scheduled each second during a specific time win-654
dow as shown in Figure 6. Contrary to the ISL655
delay command for delayed operations, the pro-656
posed solution does not interfere with the timing657
of other tasks during the acquisition and proved658
to be a reliable and effective way of performing659
time variable tasks parallel to tasks with fixed tim-660
ing during the IRMS chromatogram part of a mea-661
surement. Figure 4 shows different regression fits.662
Based on this, we have used different fitting func-663
tions for different concentration ranges, or applied664
a look-up-table approach using if statements. We665
were not able to program the power function fits666
(y = a · xb) with decimal values of “b” in the ISL667
script, and have relied on Taylor or logarithmic668
functions instead.669
FIGURE CAPTIONS670
Figure 1: Overview of improved setup, from sam-671
pling air in the field (top) to measurement by Iso-672
tope Ratio Mass Spectrometer (bottom). Commu-673
nication connections and sample gas flow paths674
are indicated by broken lines and thick lines, re-675
spectively. After field operation, the ASA is trans-676
ported to the lab and interfaced with the Isotope677
Ratio Mass Spectrometer.678
Figure 2: Flow diagram of the ASA showing the679
difference between field (top) and laboratory (bot-680
tom) operation. The ASA is shown in field sam-681
pling mode: the flow is diverted by the solenoid682
valve, the air sample is dried, filtered and pushed683
through the Valvo-valve (shown here in bypass684
loop position). During sampling, an adjustable685
flow regulator (set to ≈ 0.9 L min−1) and a pop-686
pet check valve (one-way, opening at >7 kPa) help687
create a pressure excess in the glass flask or stain-688
less steel loop sample containers. Per ASA, three689
or four Valvo-valves with sample containers are690
connected in series, adding up to a maximum total691
of 33 or 44 samples per unit. Two manual three-692
way valves are used to switch between field and693
laboratory setup.694
Figure 3: Flow diagram of the laboratory setup.695
Shown here is the situation for flow of He (valve 1)696
with sample air from the ASA (2) to the Gasbench697
8
(3) and subsequent cryogenic focus trap (4), while698
at the same time the Gas Chromatograph and the699
inlet of the Isotope Ratio Mass Spectrometer are700
flushed with He (4 and 5). See text for details.701
Figure 4: Empirical relations between sample CO2702
concentration and trapping periods required for703
peak amplitudes that are equal to the Isotope Ra-704
tio Mass Spectrometer reference gas, for samples705
delivered to the Isotope Ratio Mass Spectrome-706
ter from three different types of containers (glass707
flasks, steel loops, steel capillary). For glass flask708
sample containers, two fit functions are shown. For709
fit parameters and further details, see Table 1 and710
Appendix.711
Figure 5: Decision diagram for the automated liq-712
uid nitrogen (LN2) refilling system used for the713
cryogenic trap. Activation of the solenoid valve for714
LN2 flow into the Dewar depends on fulfillment of715
the conditions for the weight of the cryogenic trap716
Dewar (A), the temperature of the balance (B) and717
a signal from the Isotope Ratio Mass Spectrometer718
during a specific period of the IRMS measurement719
protocol (C) and a waiting period after filling.720
Figure 6: The timeline of the Isotope Ratio Mass721
Spectrometer measurement protocol implemented722
in the ISODAT NT software. The flow path of the723
input line and Gasbench is flushed with sample air724
(S) and the cryogenic trap is lowered into liquid725
nitrogen before sample air is let into the trap to be726
frozen (FS) for a period that is variable and derived727
from the sample CO2 mixing ratio. After the trap728
is raised to thaw, the cryogenically focused content729
is carried by He through the Gas Chromatograph to730
the Isotope Ratio Mass Spectrometer for analysis.731
Meanwhile, the sample inlet tubes are flushed with732
pure He (He) or the next sample (Snext) with higher733
flow rate (+).734
Figure 7: Linearity performance of δ 13C analysis735
using the modified Gasbench, expressed as the de-736
viation from the δ 13C reference value against the737
relative peak amplitude (A) in the chromatogram.738
See text for details.739
Figure 8: Application of concentration-dependent740
variable cryogenic trapping periods in the Gas-741
bench for measurements of δ 13C. In the top panel,742
the deviation from the δ 13C reference value is743
shown for different dilutions of a CO2 in air mix-744
ture with a constant δ 13C value. In the bottom745
panel, the deviation from the δ 13C reference value746
is shown for the corresponding relative peak am-747
plitude in the chromatogram. The SD for the δ 13C748
measurements is 0.04� (N=13).749
Figure 9: Deviation between measured δ 13C750
(δSample) and reference δ 13C (δRef) for a laboratory751
reference standard and two quality control (QC)752
standards that were directly suplied through steel753
capillaries, at different dates. The deviations of QC754
standard I (top) and QC standard II (bottom) are on755
average –0.02±0.04� and 0.06±0.05�, respec-756
tively. The maximum observed SD for each five-757
sample series of reference standard and QC stan-758
dards (I and II) is 0.05.759
Figure 10: The deviation between measured δ 13C760
(δSample) and reference δ 13C (δRef) for a laboratory761
reference standard and a quality control (QC) stan-762
dard that was sampled beforehand in glass flasks763
(3 min of ≈ 1 L min−1 flushing into an ASA).764
The maximum observed SD for each five-sample765
series of reference standard is 0.05 and 0.07 for766
δ 13C and δ 18O, respectively. For QC standard I,767
the average deviations from their reference value768
are 0.08±0.04� and –0.19±0.05� for δ 13C and769
δ 18O, respectively. The QC standard I is the same770
as shown in the top panel of Figure 9.771
Table 1: Fit parameters for the relationships be-772
tween sample CO2 concentrations and trapping pe-773
riods for different types of sample containers. See774
text and Figure 4 for details.775
9
FIGURES AND TABLES776
777
Figure 1: Overview of improved setup, from sam-pling air in the field (top) to measurement by Iso-tope Ratio Mass Spectrometer (bottom). Commu-nication connections and sample gas flow pathsare indicated by broken lines and thick lines, re-spectively. After field operation, the ASA is trans-ported to the lab and interfaced with the IsotopeRatio Mass Spectrometer.
778
Figure 2: Flow diagram of the ASA showing thedifference between field (top) and laboratory (bot-tom) operation. The ASA is shown in field sam-pling mode: the flow is diverted by the solenoidvalve, the air sample is dried, filtered and pushedthrough the Valvo-valve (shown here in bypassloop position). During sampling, an adjustableflow regulator (set to ≈ 0.9 L min−1) and a pop-pet check valve (one-way, opening at >7 kPa) help
create a pressure excess in the glass flask or stain-less steel loop sample containers. Per ASA, threeor four Valvo-valves with sample containers areconnected in series, adding up to a maximum totalof 33 or 44 samples per unit. Two manual three-way valves are used to switch between field andlaboratory setup.
779
Figure 3: Flow diagram of the laboratory setup.Shown here is the situation for flow of He (valve 1)with sample air from the ASA (2) to the Gasbench(3) and subsequent cryogenic focus trap (4), whileat the same time the Gas Chromatograph and theinlet of the Isotope Ratio Mass Spectrometer areflushed with He (4 and 5). See text for details.
●●
●●
● ● ● ● ●
●
●
●
●
●● ●
●● ●
0
20
40
60
80
100
0
20
40
60
80
100
300 500 700 900 1100 1300 1500
CO2 concentration [µµmol mol−−1]
Ent
rapm
ent p
erio
d [s
]
●
●
Glass flasks (ASA); y = a1·x1+a2·x
2+a3·x3+b
Glass flasks (ASA), alt. fit; y = a·xb
Steel loops (ASA); y = a·ln(x)+bSteel capillary (Ref. inlet); y = a·ln(x)+b
780
Figure 4: Empirical relations between sample CO2concentration and trapping periods required forpeak amplitudes that are equal to the Isotope Ra-tio Mass Spectrometer reference gas, for samplesdelivered to the Isotope Ratio Mass Spectrome-ter from three different types of containers (glassflasks, steel loops, steel capillary). For glass flasksample containers, two fit functions are shown. Forfit parameters and further details, see Table 1 andAppendix.
10
781
Figure 5: Decision diagram for the automated liq-uid nitrogen (LN2) refilling system used for thecryogenic trap. Activation of the solenoid valve forLN2 flow into the Dewar depends on fulfillment ofthe conditions for the weight of the cryogenic trapDewar (A), the temperature of the balance (B) anda signal from the Isotope Ratio Mass Spectrometerduring a specific period of the IRMS measurementprotocol (C) and a waiting period after filling.
782
Figure 6: The timeline of the Isotope Ratio MassSpectrometer measurement protocol implementedin the ISODAT NT software. The flow path of theinput line and Gasbench is flushed with sample air(S) and the cryogenic trap is lowered into liquidnitrogen before sample air is let into the trap to befrozen (FS) for a period that is variable and derivedfrom the sample CO2 mixing ratio. After the trapis raised to thaw, the cryogenically focused contentis carried by He through the Gas Chromatograph tothe Isotope Ratio Mass Spectrometer for analysis.Meanwhile, the sample inlet tubes are flushed withpure He (He) or the next sample (Snext) with higherflow rate (+).
11
●●●
●●
● ●●●●●
●
●
●●
●
●
●
●
●●
●
●●●●
●
●●
●
●
●
●●
●●●●
−0.4
−0.2
0.0
0.2
0.4
−0.4
−0.2
0.0
0.2
0.4
0 50 100 200 400
(ASample / ARef) × 100 [%]
18O
● CO2 in air (400 µµmol mol−−1)CO2 in He (4000 µµmol mol−−1)
●●●
●●
●●●
●●●●
●●●●
●
●●
●●●●●●
●●●
●●
●
●●●
●●
●●
−0.2
0.0
0.2
0.4
−0.2
0.0
0.2
0.4
0 50 100 200 400
δδ Sam
ple
− δδ
Ref
[‰
]
(ASample / ARef) × 100 [%]
13C ● CO2 in air (400 µµmol mol−−1)CO2 in He (4000 µµmol mol−−1)
783
Figure 7: Linearity performance of δ 13C analysisusing the modified Gasbench, expressed as the de-viation from the δ 13C reference value against therelative peak amplitude (A) in the chromatogram.See text for details.
−0.2
0.0
0.2
−0.2
0.0
0.2
0 300 600 900 1200 1500 1800
CO2 concentration [µµmol mol−−1]
13C
2007−07−18
−0.2
0.0
0.2
−0.2
0.0
0.2
0 50 100 150 200
13C
2007−07−18
δδ S
ampl
e −
δδR
ef
[‰]
(ASample / ARef) *100 [%]784
Figure 8: Application of concentration-dependentvariable cryogenic trapping periods in the Gas-bench for measurements of δ 13C. In the top panel,the deviation from the δ 13C reference value isshown for different dilutions of a CO2 in air mix-ture with a constant δ 13C value. In the bottompanel, the deviation from the δ 13C reference valueis shown for the corresponding relative peak am-plitude in the chromatogram. The SD for the δ 13Cmeasurements is 0.04� (N=13).
●●●●●
●●●●
●●●
●●●
●●●●●
−0.2
0.0
0.2
−0.2
0.0
0.2
02:00 06:00 10:00 14:00 18:00
13C
2008−02−26 ● QC standard I Reference standard
●●●
●●●●●●●
●●
●●● ●
●●●●
●●●●●
●●●●● ●●●●
● ●●●●●
●●●
●●
−0.2
0.0
0.2
−0.2
0.0
0.2
16:00 20:00 00:00 04:00 08:00
13C
2008−03−13
δδS
ampl
e −
δδR
ef
[‰]
Time of day [h]
● QC standard II Reference standard
785
Figure 9: Deviation between measured δ 13C(δSample) and reference δ 13C (δRef) for a laboratoryreference standard and two quality control (QC)standards that were directly suplied through steelcapillaries, at different dates. The deviations of QCstandard I (top) and QC standard II (bottom) are onaverage –0.02±0.04� and 0.06±0.05�, respec-tively. The maximum observed SD for each five-sample series of reference standard and QC stan-dards (I and II) is 0.05.
●●●●●●
●●●●●●●●●●●●●●
●●
●●●●●●●●●●
●
−0.4
−0.2
0.0
0.2
0.4
−0.4
−0.2
0.0
0.2
0.4
16:00 18:00 20:00 22:00 00:00
13C
2008−03−12 ● QC standard I Reference standard
●●●
●●●●
●●●
●●●●●●
●●●●●●●●
●●●●●●●
●●
−0.4
−0.2
0.0
0.2
0.4
−0.4
−0.2
0.0
0.2
0.4
16:00 18:00 20:00 22:00 00:00
18O
2008−03−12
● QC standard I Reference standard
δδS
ampl
e −
δδR
ef
[‰]
Time of day [h]786
Figure 10: The deviation between measured δ 13C(δSample) and reference δ 13C (δRef) for a laboratoryreference standard and a quality control (QC) stan-dard that was sampled beforehand in glass flasks(3 min of ≈ 1 L min−1 flushing into an ASA).The maximum observed SD for each five-sampleseries of reference standard is 0.05 and 0.07 forδ 13C and δ 18O, respectively. For QC standard I,the average deviations from their reference valueare 0.08±0.04� and –0.19±0.05� for δ 13C andδ 18O, respectively. The QC standard I is the sameas shown in the top panel of Figure 9.
787
12
Table 1: Fit parameters for the relationships between sample CO2 concentrations and trapping periods fordifferent types of sample containers. See text and Figure 4 for details.
Fit function Fit parametersGlass flasks (in ASA) y = a1 · x1 +a2 · x2 +a3 · x3 +b a1 =−3.71 ·10−1, a2 = 3.536 ·10−4,
a3 =−1.215 ·10−7, b = 174Steel loops (in ASA) y = a · ln(x)+b a =−15.37, b = 144
Steel capillary (Ref. & QC standards) y = a · ln(x)+b a =−10.39, b = 96
13