Fully automated, high-precision instrumentation for the isotopic
analysis of tropospheric N
2
O using continuous flow isotope ratio
mass spectrometry
Katherine E. Potter
*
, Shuhei Ono and Ronald G. Prinn
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue,
Cambridge, MA 02139, USA
RATIONALE: Measurements of the isotopic composition of nitrous oxide in the troposphere have the potential to bring
new information about the uncertain N
2
O budget, which mole fraction data alone have not been able to resolve.
Characterizing the expected subtle variations in tropospheric N
2
O isotopic composition demands high-precision and
high-frequency measurements. To enable useful observations of N
2
O isotopic composition in tropospheric air to reduce
N
2
O source and sink uncertainty, it was necessary to develop a high-precision measurement system with fully automated
capabilities for autonomous deployment at remote research stations.
METHODS: A fully automated pre-concentration system for high-precision measurements of N
2
O isotopic composition
(d
15
N
b
, d
15
N
a
, d
18
O) in tropospheric air has been developed which combines a custom liquid-cryogen-free cryo-trapping
system and gas chromatograph interfaced to a continuous flow isotope ratio mass spectrometry (IRMS) system. A
quadrupole mass spectrometer was coupled in parallel to the IRMS system during development to evaluate peak
interference. Multi-port inlet and fully-automated capabilities allow streamlined analyses between in situ air inlet, air
standards, flask air sample, or other gas source in exactly replicated analysis sequences.
RESULTS: The system has the highest precision to date for
15
N site-specific composition results (d
15
N
a
0.11%, d
15
N
b
0.14% (1s)), attributed mostly to uniformity of analytical cycles and particular attention to fluorocarbon interference
noted for
15
N site-specific measurements by IRMS. Air measurements demonstrated the fully automated capacity and
performance.
CONCLUSIONS: The system makes substantial headway in measurement precision, possibly defining the limits of IRMS
measurement capabilities in low concentration N
2
O air samples, with fully automated capabilities to enable high-
frequency in situ measurements. Copyright © 2013 John Wiley & Sons, Ltd.
Nitrous oxide (N
2
O) is a significant greenhouse gas and a
main contributor to stratospheric ozone destruction. Since
the beginning of the industrial era atmospheric levels have
risen from around 270 ppb to presently above 320 ppb,
generally attributed to anthropogenic fixed nitrogen and
agricultural expansion.
[1–3]
As N
2
O is a long-lived, well-
mixed gas, variation in its mole fraction in the troposphere
is subtle on all timescales. Since the improvement
in measurement precision for in situ high-frequency
observations of N
2
O mole fraction that began around 1994,
a discernible mole fraction seasonal cycle has emerged from
long-term data, currently with an amplitude ~0.3 to 0.9 ppb
atop the ~320 ppb mean,
[4–7]
as well as subtle short-term
variability useful for regional emissions assessments.
[8,9]
These mole fraction data have been analyzed and modeled,
but have seemingly reached the limit of information which
can be extracted from whole N
2
O observations about the
N
2
O budget with large uncertainties remaining.
[7,10,11]
Knowledge of the isotopic composition of N
2
O in the
troposphere lends valuable additional information about
N
2
O tied to the specific isotopic signatures and fractionation
associated with atmospheric N
2
O sources, sinks, and other
influences to allow reduction in N
2
O budget uncertainty.
Despite this promised utility, measurements of tropospheric
N
2
O isotopic composition are scarce. With the development
of sufficiently precise in situ high-frequency isotopic
instrumentation, measurements of the slight variations in
the tropospheric isotopic composition add information about
the controlling processes of atmospheric N
2
O. Stable isotopic
ratios of N
2
O linked to source and sink isotopic signatures
can provide the much-needed additional constraints on
emissions and counter-balancing stratospheric sink.
For the stable isotopic analysis of N
2
O, the rare singly
substituted d
18
O and d
15
N values (
14
N
15
N
16
O,
15
N
14
N
16
O,
14
N
14
N
18
O, relative to the abundant
14
N
14
N
16
O) are
considered here. Nitrous oxide is an asymmetric molecule
(N
b
N
a
O), and the
15
N site-specific isotopomer ratios present
a unique opportunity to investigate an additional set of
parameters beyond the d
15
N
bulk
value, measurable only with
* Correspondence to: K. E. Potter, Department of Earth,
Atmospheric, and Planetary Sciences, Massachusetts Institute
of Technology, 77 Massachusetts Avenue, Cambridge, MA
02139, USA.
E-mail: kep@mit.edu
Copyright © 2013 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2013, 27, 1723–1738
Research Article
Received: 21 March 2013 Revised: 6 May 2013 Accepted: 7 May 2013 Published online in Wiley Online Library
Rapid Commun. Mass Spectrom. 2013, 27, 1723–1738
(wileyonlinelibrary.com) DOI: 10.1002/rcm.6623
1723