Fully automated system for stable isotopic analyses of dissolved nitrous oxide at natural abundance levels

<jats:p>Isotopic and isotopomeric analyses of dissolved nitrous oxide (N<jats:sup>2</jats:sup>O) can provide important information about the mechanisms of production and consumption of this climatically important trace gas. Increased spatial and temporal data coverage for N<jats:sup>2</jats:sup>O isotopic measurements are needed to improve our understanding of the production of N<jats:sup>2</jats:sup>O in a variety of aquatic environments, such as coastal and open ocean areas. A new method is presented for automated measurement of δ<jats:sup>15</jats:sup>N<jats:sup>bulk</jats:sup>, δ<jats:sup>18</jats:sup>O, and site preference of dissolved nitrous oxide in natural waters using a GC‐Pal autosampler and a Finnigan Delta<jats:sup>Plus</jats:sup> isotope ratio mass spectrometer and is capable of analyzing 30 samples in 22 h without user attention after the start of the run. The system was first tested with samples produced by injecting our N<jats:sup>2</jats:sup>O reference gas and/or N<jats:sup>2</jats:sup>O test mixtures, ranging in isotopic composition and concentration, into N<jats:sub>2</jats:sub>‐sparged seawater. The method was then applied to samples containing a large range of N<jats:sup>2</jats:sup>O concentrations, δ<jats:sup>15</jats:sup>N<jats:sup>bulk</jats:sup>, δ<jats:sup>18</jats:sup>O, and site preference values from two high‐resolution depth profiles from the Arabian Sea. The results showed precise determination of isotopic values of N<jats:sup>2</jats:sup>O from 160 mL aqueous samples over the concentration range normally found in the oceans (5–80 nmol L<jats:sup>−1</jats:sup>). Sample standard deviation for triplicate 160 mL samples at 6 nmol L<jats:sup>−1</jats:sup> N<jats:sup>2</jats:sup>O is better than 0.3‰ for δ<jats:sup>15</jats:sup>N<jats:sup>bulk</jats:sup>, 0.5‰for δ<jats:sup>18</jats:sup>O, and 1.0‰ for <jats:sup>15</jats:sup>N site preference. The automated technique described here requires relatively minor modifications of existing N<jats:sup>2</jats:sup>O purge and trap systems and can be implemented broadly for a variety of applications.</jats:p>