Introduction
Long-term monitoring of trace gases that are increasingly abundant in the
atmosphere is fundamental for the analysis of the imbalance of their sources
and sinks and for the prediction of future environmental change on Earth.
Nitrous oxide (N2O) is one such trace gas, with global warming
potential that is 220 times as great as that of carbon dioxide (CO2),
and is the most important stratospheric ozone-depleting gas of this
century (Myhre et al., 2013; Ravishankara et al., 2009). Its globally
averaged concentration, given as a mole fraction, was about 324 nmol mol-1 (10-9 moles per mole of dry air) in 2011
(Hartmann et al., 2013) and
increases by 0.73 nmol mol-1 a-1
(Ciais et al., 2013). Sources of N2O
include natural and agricultural soils, aqueous environments such as oceans,
rivers, and lakes, industrial processes such as fossil fuel combustion,
biomass burning, and animal and human wastes
(Ciais et al., 2013); its major sink is
photochemical decomposition in the stratosphere.
Although concentration analyses yield quantitative information related to
trace gases straightforwardly, it is often difficult to differentiate the
sources contributing to the increase of such gases in the atmosphere,
especially for N2O. Natural abundance ratios of stable isotopes of the
elements that compose trace gas molecules have qualitative information
related to the origin and production–decomposition processes of the gases
because isotope ratios are generally different among different substrates.
Moreover, they can change during physical, chemical, and biological
processes. Regarding N2O, measurements of the nitrogen isotope ratio
(15N /14N) for the atmosphere and various sources since the 1980s
have revealed that the imbalance of isotopically light N2O from surface
sources and isotopically heavy N2O refluxed from the stratosphere after
its partial decomposition causes a progressive decrease in the
15N /14N isotope ratio of tropospheric N2O (Ishijima et
al., 2007; Röckmann et al., 2003a; Sowers et al., 2002). Furthermore, a
technique developed for measuring isotopomers of N2O
(14N15N16O and 15N14N16O) expanded
conventional isotopic analysis to isotopocule analysis by which ratios of
NNO molecules substituted with stable isotopes of nitrogen or oxygen at any
site relative to 14N14N16O are obtained and by which
production and decomposition pathways can be differentiated in greater
detail (Toyoda et al., 2015 and references therein).
Compared to concentration analysis, stable isotope and isotopocule analyses
require (1) larger sample amounts, (2) more time and labor to extract and
purify the target compound from the sample, and (3) larger and more
expensive apparatus. Although recently developed tunable diode laser
absorption spectroscopy (TDLAS) relaxes some of the requirements above, and
although it has some potential for on-site monitoring of stable isotope / isotopocule ratios of trace gases (Harris et al., 2014; Mohn et al.,
2012; Tuzson et al., 2008), mass spectrometry combined with flask sampling
still holds advantages for high-precision isotopic monitoring at polar
regions or remote areas and flight observation using a balloon or an
airplane because of smaller sample volume requirements.
In most currently used mass spectrometric analytical methods for N2O
isotopocules, air samples are first passed through chemical adsorbents to
remove CO2 and water vapor. Then, N2O is concentrated on
chemically inert adsorbents or inner walls of narrow tubes at liquid
nitrogen temperatures. It is further purified on a capillary column of a gas
chromatograph (GC) and is introduced directly into an isotope ratio monitoring mass spectrometer (IRMS). The analysis of a single sample takes
30–60 min. The precision reported in earlier studies is typically
0.1–0.5 ‰ for 1 nmol of N2O
(e.g., Toyoda et al., 2001) (See Sect. 2.4 for notation
of isotopocule ratios), which is worse than the ultimate precision expected
from the shot-noise limit of the IRMS (Potter et al.,
2013) and which is insufficient to resolve the secular trend of atmospheric
N2O isotopocule ratios. This low precision is partly caused by
incomplete separation of interfering components such as CO2 and
fluorinated hydrocarbons, or by imprecise manual handling during sample
preparation.
To improve the precision of the isotopocule ratio analysis of atmospheric
N2O and to reduce the labor for complicated sample preparation
procedures for mass spectrometric analysis, several automated methods have
been reported. Röckmann et al. (2003b) improved the
precision of fragment ion (NO+) analysis by modifying the gas
chromatographic purification of N2O from interfering species such as
halocarbons and less volatile compounds (Röckmann et al.,
2003b). Röckmann and Levin (2005) and Potter
et al. (2013) reported further improvement in the precision by partially or
fully automating sample preparation steps and by slightly increasing the
sample size.
In addition to the mass spectrometric method, an automated sample preparation
system has been reported, which can be coupled to a quantum cascade laser
absorption spectrometer (QCLAS) for the monitoring of atmospheric isotopocules of
N2O (Harris et al., 2014; Mohn et al., 2010). However, previously
reported automated methods entail several shortcomings. For example, they
are designed to measure pressurized air samples such as ambient air drawn by
pumps or air collected into glass bottles or metal cylinders using pumps.
For that reason, they are not applicable to samples at ambient or subambient
pressure.
Here we present an automated preconcentration system that offers enhanced
flexibility in terms of sample gas pressure and N2O concentration. The
novel system encloses a vacuum line and a computer program that controls
valves to inject samples of a designated amount.
Preparation system
The preparation system developed in this study consists of a sample
injection unit, cryogenic concentration unit, purification unit, and
cryofocusing unit (Fig. 1). It is placed in a steel rack (60 cm width, 80 cm
depth, 150 cm height) with wheels attached, and is connected to a gas
chromatograph–isotope ratio monitoring mass spectrometer (GC-IRMS). Details
of each unit are presented below.
Sample injection unit
This unit consists of a multi-position six-port switching valve (E4SD6MWE;
Valco Instruments Co. Inc., Houston, TX, SV1 in Fig. 1) equipped with an
electric actuator, air-actuated diaphragm shut-off valves (FPR-ND-71-6.35-2;
Fujikin, Osaka, Japan, V1-V5 in Fig. 1), a pressure gauge (VSHT21; Valcom
Co. Ltd., Toyonaka, Japan, PG1 in Fig. 1), a capacitance manometer (Barocel
Model 600; BOC Edwards, Wilmington, MA, PG2 in Fig. 1), a Pirani vacuum
gauge (GP-2A; ULVAC, Inc., Chigasaki, Japan, VG in Fig. 1), a vacuum pump
system (turbo drag pump TMH 071 P and diaphragm pump MVP 015-2 with a
controller; Pfeiffer Vacuum GmbH, Asslar, Germany, VP in Fig. 1), three
custom-made glass bottles, and stainless steel (ss) tubing.
Schematic portraying the sample preparation system developed in
this study: CT, chemical trap; FM, flow monitor; GC, gas chromatograph;
IRMS, isotope ratio mass spectrometer; LN, liquid nitrogen; MFC, mass flow
controller; NV, needle valve; OS, open split interface; P, pressure
regulator; PC, purification column; PG, pressure gauge; SV, electrically
actuated switching valve; T, trap; V, air-actuated diaphragm valve; VG,
vacuum gauge; VP, vacuum pump. A–E denote parts of the vacuum line or glass
bottles that are used to expand the sample, the volume (cm3) of which
is also shown.
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A sample flask (made of either glass or ss) or a gas cylinder is manually
connected to one of the switching valve ports with an ss connector (Cajon
Ultra-Torr or Swagelok; Swagelok Company, Solon, OH). The tubing between the
diaphragm valve (V1, V2, or V3) and the flask/cylinder valve is evacuated by
manually operating the valves and vacuum pump via control software (see
below). Then, all the diaphragm valves are closed, the flask valve is opened
by hand, and a computer program for sample preparation (see Sect. 2.5) is started.
First, sample gas pressure in the flask is measured using the pressure gauge
by expanding the sample gas into the vacuum line until V4. Based on the
pressure and the volume of the flask and the sample size to be injected, the
“sample expanding option” and final pressure of the sample injected into
the vacuum line is calculated. Seven options exist for sample expansion into
the calibrated volume in the vacuum line from 100 cm3 (option no. 1) to
510 cm3 (option no. 7). This expansion is realized by a combination of
the three glass bottles (C, D, and E in Fig. 1) with different volumes.
Next, an aliquot of the sample in the flask is expanded by sequential
open–close operation of diaphragm valves. The pressure is monitored using
the manometer. When the pressure agrees with the precalculated value within
±5 %, either V1 or V3 (when sample is analyzed, Fig. 1) or V2 (when
standard gas is analyzed) and V4 are closed, the pressure is recorded, and
the injected sample amount is calculated.
Concentration unit
This unit consists of a chemical trap (CT in Fig. 1), an electrically
actuated two-position six-port switching valve (E4C6UWE; Valco Instruments
Co. Inc., Houston, TX, SV2 in Fig. 1), a U-shaped concentration trap (T1 in
Fig. 1), and a mass flow controller (SEC-E40; Horiba Stec Co. Ltd., Kyoto,
Japan, MFC in Fig. 1). The chemical trap is a glass tube (9 mm outer
diameter (o.d.), 20 cm long) packed with Mg(ClO4)2 (8–24 mesh; Wako Pure
Chemical Industries Ltd., Osaka, Japan), NaOH on support (Ascarite, 20–30
mesh; Thomas Scientific), and Mg(ClO4)2 (20–48 mesh) in series
with approximately equal length. The T1 is an ss tube (1/4 inch o.d., 30 cm long) packed with glass beads (Flusin GH 60–80 mesh; GL
Sciences Inc., Tokyo, Japan).
First, the concentration trap is purged with ultra-pure He (> 99.9999 %, Japan Air Gases Ltd., Tokyo, Japan) at 100 cm3 min-1
for > 10 s by switching SV2 to the “inject” position. The He is
purified in advance through a column packed with molecular sieves 5A, active
charcoal, and molecular sieves 13X in series (PC in Fig. 1). Next, SV2 is
switched to the “load” position and the trap is cooled with liquid
nitrogen in an ss dewar which is driven up and down by a custom-made
air-actuated stage and which is filled with liquid nitrogen from an
automatic liquid nitrogen supply system (Koshin Ltd., Tokyo, Japan). Then
V5, V15, and valves relevant to the sample injection option (V9–V14) are
opened. The sample gas in the calibrated volume is transferred to the
concentration trap through the chemical trap by He carrier gas at 30 cm3 min-1. The He is purified in a similar manner to that
described above. When more than two glass bottles are filled with the sample
gas, the transfer is conducted sequentially. The transfer time is set so
that the total volume of He which flows through the bottle is twice the
bottle volume.
Purification and cryofocusing unit
This unit consists of two electrically actuated two-position six-port
switching valves (SV3 and SV4 in Fig. 1, E4C6UWE; Valco Instruments Co.
Inc., Houston, TX), a gas chromatograph (GC-8AIT; Shimadzu Corp., Kyoto,
Japan. GC1 in Fig. 1) equipped with a thermal conductivity detector (TCD),
and a U-shaped cryofocusing trap (T2 in Fig. 1). The GC column is an ss tube
(4 mm o.d., 3 m length) packed with Porapak Q (80–100 mesh; Waters Corp.,
MA). It is kept at 60 ∘C. The cryofocusing trap is an ss tube
(1/16 inch o.d., 70 cm long) with no packing material.
Initially, SV3 and SV4 are set to the load position. After the sample
concentration step is completed, SV2 is switched to the inject position.
The concentration trap is heated to 70 ∘C by lowering the liquid
nitrogen dewar and turning on a sheathed electric heater attached to the
trap. The concentrated trace gases are transferred to the GC column with
purified He at 20 cm3 min-1. When 2 min have passed after the GC
injection, the cryofocusing trap is cooled with liquid nitrogen by moving up
another ss dewar. Three minutes later, SV4 is switched to the inject
position. Purified N2O from the GC is focused on the trap for 2 min.
Time sequence of the sample preparation procedure. Horizontal
arrows on the top indicate the periods for evacuation of the inlet line (a),
sample injection (b), cryogenic concentration of N2O on T1 (c),
purification of N2O by GC1 (d), cryofocusing of N2O on T2 (e), and
injection of N2O into GC2 (f). See also Fig. 1.
![]()
Flow chart of the algorithm for sample injection. n denotes the
sample size in cubic centimeters at 25 ∘C and 1 atm.
nmeas or nmeas(j) is the actual sample size calculated from
measured pressure. vb is the partial volume (cm3) of the vacuum
line indicated by B in Fig. 1. v(i) is the calibrated volume that
corresponds to the sample expanding option i. vS is sum of the volume
of sample flask and partial volume A. PG1(j) and PG2(j) are outputs
of pressure gauges 1 and 2 (in kPa) (Fig. 1). Pcalc(i) and
PG2calc(i) are calculated pressures that correspond to the sample
expanding option i.
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Injection to GC-IRMS
After the cryofocusing step is completed, SV4 is switched to the load
position, the liquid nitrogen dewar is moved down, and the cryofocus trap is
heated to 70 ∘C similarly, as in the case of the concentration
trap. The N2O is injected into another GC (GC6890; Agilent Technologies
Inc., Santa Clara, CA, GC2 in Fig. 1) with He (2 cm3 min-1). It is
further purified with the GS Carbon PLOT column (0.32 mm inner diameter (i.d.), 3 µm
film thickness, 30 m; Agilent Technologies Inc.) maintained at 35 ∘C. The purified N2O is finally injected into an IRMS (MAT252; Thermo
Fisher Scientific K.K., Yokohama, Japan) via an interface that includes a
gas dryer with a permeation tube and two open split interfaces for the
sample and reference gas (GC-combustion interface; Thermo Fisher Scientific
K.K., slightly modified).
Mass spectrometric analysis of N2O isotopocules is conducted as
described elsewhere (Toyoda and Yoshida, 1999; Toyoda et
al., 2015). Briefly, molecular (N2O+) and fragment (NO+)
ions of N2O are analyzed in independent runs. Solving the following
equations and applying correction for the rearrangement or scrambling
reactions during fragmentation, the isotopocule ratios are obtained as delta
values.
45R=15Rα+15Rβ+17R46R=18R+(15Rα+15Rβ)17R+15Rα15Rβ31R=15Rα+17R17R=A(18R)γδ15Ni=15Rsamplei/15Rstandardi-1(i=α,β,or bulk)
δ18O=18Rsample/18Rstandard-1SP=δ15Nα-δ15Nβ
In Eqs. (1)–(6), 45R and 46R respectively denote the measured ion-beam
intensity ratios of m/z 45/44 and 46/44 in molecular ion analysis; 31R shows
a 31/30 ratio by fragment ion analysis; 15Rα,
15Rβ, 17R, and 18R respectively denote the abundance
of ions 14N15N16O+, 15N14N16O+,
14N14N17O+, and 14N14N18O+ relative
to 14N14N16O+. In Eq. (4), A=0.00937035 and
γ=0.516 (Kaiser et al., 2003). In Eq. (5),
δ15Nbulk denotes the average isotope ratios for
15N /14N. The subscripts sample and standard respectively denote
the isotope ratios for the sample and the standard. SP denotes site preference. International standards
for N and O isotope ratios are atmospheric N2 and Vienna Standard Mean Ocean
Water (VSMOW), respectively.
Operation by PC software
A personal computer (NI PXI-1042Q with a controller NI PXI-8196 and I/O
boards NI PXI-6221, NI PXI-4351, NI PXI-8421, and NI PXI-6514; National
Instruments Corp., Austin, TX) and programming software (LabVIEW Ver. 8.2;
National Instruments Corp., Austin, TX) were used to activate the solenoid
valves that regulate compressed air for air-actuated shut-off valves and the
air-actuated up–down stages, the multi/two-position switching valves, the
vacuum pump system, the automatic liquid nitrogen supply system, and the
temperature controller for the heaters. The PC also received analogue data
from the pressure and vacuum gauges and from the manometer, received the TCD
signal, and synchronized the GC-IRMS data acquisition with the end of the
sample preparation procedure. The timing of each regulation function is
presented in Fig. 2.
The program developed in this study includes a special algorithm to adapt
the sampling procedure to the prevailing sample pressure and N2O
concentration. As described briefly in Sect. 2.1, it has a user interface to
obtain information related to the sample: the flask volume and the sample
size to be injected. When the actual sample gas pressure is measured and
obtained, it automatically determines the optimal procedure for sample
injection. Figure 3 shows a flow chart for the algorithm.
Results and discussion
Sample injection to the system
The time required for sample injection depends on the sample expanding
option (see Sect. 2.1). It takes about 5 min when a 300 cm3
aliquot of air, which contains ca. 4 nmol of N2O in the case of ambient
air (ca. 320 nmol mol-1), is injected from a 1 L flask pressurized to
about 2.5 atm (option no. 7). When the flask volume or inner pressure is
lower, more time is needed because the number of repetitive sample diffusion
steps increases and one valve was operated 5 s after actuating another valve
to equilibrate the pressure in the inlet line and to avoid potential
fractionation of isotopocules.
When a smaller sample volume with high N2O concentration is measured,
sample injection is completed in a minute or less. However, the performance
of quantitative sample injection becomes poor for samples with more than 10 µmol mol-1 N2O because the system cannot fully adjust the
introduction of a small amount of sample (< 10 cm3). Moreover,
the relative error of the pressure measurement increases for low pressure.
Such highly concentrated samples are better introduced after dilution with
N2 or He. Modification of one glass bottle (e.g., bottle C) to enable
manual injection with a microsyringe is also possible.
Concentration, purification, and cryofocusing of N<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math></inline-formula>O
During cryogenic concentration of N2O, the flow rate and flowing time
of the He carrier gas should be optimized carefully to ensure quantitative
recovery of N2O and thus also minimize contamination of subsequent
analyses (blank values). Our preliminary tests showed that each glass bottle
(C, D, or E) is purged completely when the total volume of He is more than
twice the bottle volume. This result indicates that laminar flow is
predominant in the bottle. Turbulent flow, which is expected to cause
exponential dilution and to result in the consumption of a larger amount of
He to sweep out the initial sample gas, is negligible.
The main purpose of the purification step is separation of N2O from
CO2 and compounds that are less volatile than N2O. CO2 is
1000 times more abundant than N2O in ambient air samples. Its
isotopocules have the same mass as those of N2O. Therefore, it often
interferes with mass spectrometric analysis of N2O molecular ions. We
tested two column packing materials for this purpose: Porapak Q and silica
gel (dimension of the column was identical to that of Porapak Q, 60–80 mesh; GL Sciences Inc., Tokyo, Japan). Although silica gel has the unique
property of eluting N2O before CO2, their separation took longer
than in Porapak Q. The separation was not complete, even at 50 ∘C
with the flow rate of 15–50 cm3 min-1. We also strove to separate
CO2 and N2O without using chemical adsorbents, which revealed a
condition under which CO2 and N2O are separated almost completely
in preliminary experiments using a thermal conductivity detector and a
mixture of CO2 and N2O in N2 bath gas (mixing ratios of
CO2 and N2O were ca. 250 µmol mol-1). However, a small
CO2 peak was observed on the m/z 44 chromatogram after separation by
the second GC. Mass ratios 45R and 46R showed dependence on the area
of CO2 peak, which indicates that separation on the first column was
insufficient for precise isotopocule ratio analysis of N2O. Therefore,
we inserted the chemical CO2 trap before cryogenic concentration.
Volatile compounds such as halocarbons and hydrocarbons have longer
retention time than that of N2O on columns typically used for N2O
analysis. Some of them are known to hamper the chromatography of successive
runs caused by their very slow elution (Röckmann et al., 2003b). Similar to
previous studies, such compounds were prevented from being transferred to
the next step and were backflushed to vent by switching the flow path in the
present system.
The cryofocusing step was necessary to inject the N2O purified in the
high-flow system to the low-flow capillary GC-IRMS system. To achieve a
quantitative N2O recovery, the timing of the cryofocusing step was
optimized to trap the eluent from the first column only while N2O was
released.
Optimization of GC-IRMS analysis and measurement precision
We tested two fused-silica capillary columns for the separation of N2O
from other constituents in the second GC, a porous polymer PLOT column (HP
PLOT Q, 0.32 mm i.d., 20 µm film thickness, 30 m; Agilent Technologies
Inc.) and a PLOT column with a monolithic carbon layer (GS Carbon PLOT). The
latter column was found to have benefits for the separation of CO2,
N2O, and other interfering compounds such as fluorinated hydrocarbons
(Fig. 4). A shortcoming was that the retention time of N2O at the
optimized condition became longer than that obtained with the porous polymer
column, which was used in previous studies (Potter et al., 2013;
Röckmann et al., 2003b).
Typical chromatogram of background air sample obtained in a
fragment-ion (NO+) monitoring run. The x axis shows elapsed time after the start of
heating of the cryofocusing trap. After the peaks of reference N2O
injected from GC-IRMS interface (no. 1–no. 4), sample N2O peak appears
(no. 5). The peak elution about 100 s later is only detected on m/z 31 trace
and is CF+ derived from a fluorinated carbon species.
![]()
Precision of isotopocule ratio measurements as a function of
sample size.
![]()
Comparison of the analytical precision obtained in this study and
those values reported in the literature.
Reference
Sample size
Precision (1 standard deviation) (‰)
Analytical
Notes
(cm3 of
δ15Nbulk
δ15Nα
δ15Nβ
SP
δ18O
time
ambient air)
(min)
Toyoda et al. (2001)
100
0.1–0.5
0.5–1
0.5–1
1–2
0.1–0.5
25
Manual system withMAT252 (n=3)
Toyoda et al. (2013)
300
0.1
0.3
0.4
0.6
0.3
35
Manual system withMAT252 (n=3)
Röckmann et al. (2003b)
125–167
0.1
0.3a
0.4b
0.6b
0.2
NAc
Automated system withDelta Plus XL (n= 5–20)
Röckmann and Levin (2005)
333
0.06
NA
NA
NA
0.09
20
Automated system withDelta Plus XP
Mohn et al. (2010)
10 000
NA
0.24
0.17
0.29b
NA
ca. 30
Automated system with quantum cascade laser (n=136)
Wolf et al. (2015)
8000
0.12
0.20
0.12
0.22
0.10
ca. 30
Fully automated systemwith QCLAS (n=331)
Potter et al. (2013)
420
0.05
0.11
0.14
0.21
0.10
NA
Fully automated systemwith MAT253 (n= 3–5)
This work
320
0.09
0.19
0.30
0.45
0.23
40
Fully automated systemwith MAT252 (n=3)
a Obtained with 420 cm3 air; b estimated from the reported
precision for δ15Nα, δ15Nβ,
or δ15Nbulk; c not available or not
described.
Example of measurement results conducted on a single day.
Measurement no.
Sample
δ15Nbulk
δ15Nα
δ15Nβ
δ18O
SP
1, 2
S
-2.69
-4.17
-1.20
21.22
-2.97
3, 4
X1
6.58
17.18
-4.02
43.05
21.20
5, 6
X1
6.80
16.63
-3.04
43.86
19.68
7, 8
S
-2.62
-4.16
-1.08
21.34
-3.08
9, 10
X2
7.71
18.51
-3.09
44.54
21.61
11, 12
X2
7.96
18.50
-2.59
44.45
21.09
13, 14
S
-2.58
-4.43
-0.73
21.28
-3.70
15, 16
X3
6.02
15.22
-3.17
43.15
18.39
17, 18
X3
6.18
16.20
-3.85
43.68
20.05
Average
S
-2.63
-4.25
-1.00
21.28
-3.25
SD
(n=3)
0.05
0.15
0.25
0.06
0.39
X1
6.69
16.91
-3.53
43.46
20.44
(n=2)a
0.11
0.28
0.49
0.41
0.76
X2
7.83
18.51
-2.84
44.49
21.35
(n=2)a
0.12
0.01
0.25
0.05
0.26
X3
6.10
15.71
-3.51
43.41
19.22
(n=2)a
0.08
0.49
0.34
0.27
0.83
a Difference/2 is shown.
The degree of precision of the measurements was evaluated with the
standard deviation of repeated analyses (n=3) of synthetic air (349 nmol mol-1 N2O) pressurized in an aluminum cylinder that had been
calibrated against the international isotopic standard and which was used as
a working standard (Toyoda et al., 2013). As presented in Fig. 5, precision of δ15Nbulk, δ18O, and SP values
measured on a single day are typically better than 0.1, 0.2, and 0.5 ‰, respectively, when
more than 4 nmol (which corresponds to about 300 cm3 of the synthetic
air) of N2O is injected. The peak area of major ions m/z 44 and 30
showed good linearity with respect to the sample size (data not shown). The
N2O concentration was obtained by comparison of the peak area
normalized to the specific sample size between the sample and the laboratory
standard. The resulting precision of the concentration measurement is better
than 0.5 % (coefficient of variation, n=3). In addition, measured
isotopocule ratios are independent of the sample size of 4–8 nmol. Results
show that routine analyses of atmospheric air samples can be conducted with
samples of 320 cm3 so that measurements of each sample are sandwiched
by those of the working standard (Table 2).
The performance of the developed system is presented along with that of
previous works in Table 1. The precision and required sample size of this
work is comparable to that of similar automated GC-IRMS systems. It takes 40 min
for a single run, which means that a total of 80 min is necessary to
obtain a single set of δ15Nbulk, δ18O, and SP
on some mass spectrometers that are incapable of simultaneous monitoring of
five ions (m/z 44, 45, 46, 30, and 31). This might be a shortcoming of the
present system, but it presents advantages in terms of flexibility of the
sample pressure and sample size.