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

Matthew R. McIlvin and Karen L. Casciotti

Limnol. Oceanogr. Methods 8:54-66 (2010) | DOI: 10.4319/lom.2010.8.54

ABSTRACT: Isotopic and isotopomeric analyses of dissolved nitrous oxide (N2O) can provide important information about the mechanisms of production and consumption of this climatically important trace gas. Increased spatial and temporal data coverage for N2O isotopic measurements are needed to improve our understanding of the production of N2O in a variety of aquatic environments, such as coastal and open ocean areas. A new method is presented for automated measurement of δ15Nbulk, δ18O, and site preference of dissolved nitrous oxide in natural waters using a GC-Pal autosampler and a Finnigan DeltaPlus 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 N2O reference gas and/or N2O test mixtures, ranging in isotopic composition and concentration, into N2-sparged seawater. The method was then applied to samples containing a large range of N2O concentrations, δ15Nbulk, δ18O, and site preference values from two high-resolution depth profiles from the Arabian Sea. The results showed precise determination of isotopic values of N2O from 160 mL aqueous samples over the concentration range normally found in the oceans (5-80 nmol L-1). Sample standard deviation for triplicate 160 mL samples at 6 nmol L-1 N2O is better than 0.3‰ for δ15Nbulk, 0.5‰ for δ18O, and 1.0‰ for 15N site preference. The automated technique described here requires relatively minor modifications of existing N2O purge and trap systems and can be implemented broadly for a variety of applications.