A continuous flow isotope ratio mass spectrometry method for high precision determination of dissolved gas ratios and isotopic composition

Chawalit N. Charoenpong, Laura A. Bristow, Mark A. Altabet

Limnol. Oceanogr. Methods 12:323-337 (2014) | DOI: 10.4319/lom.2014.12.323

ABSTRACT: Dissolved gas ratios and isotopic compositions provide essential information about the biological and physical mechanisms influencing N2, O2, and Ar in aquatic systems. Current methods available are either limited by overall cost, labor-intensive sample collection and analysis, or insufficient precision. Here, we present a new highly accurate and robust method for sample collection and subsequent simultaneous measurement of the dissolved gas ratios (N2/Ar and O2/Ar) and isotopic compositions (δ15N2 and δ18O2) in seawater. The relatively simple sampling procedure using low cost materials enables collection of hundreds to more than a thousand discrete samples on a single research cruise. Samples can be preserved and stored at room temperature and maintain their integrity for many months. Laboratory analysis employs an on-line extraction system coupled to a multi-collector isotope ratio mass spectrometer (IRMS). A continuous flow of He carrier gas completely degasses the sample, and passes through the preparation and purification system before entering the IRMS for analysis. The use of this continuous He carrier permits short analysis times (less than 8 min per sample) as compared with current high-precision methods. In addition to reference gases, calibration is achieved using air-equilibrated water standards of known temperature and salinity. Assessment of reference gas injections, air equilibrated standards, as well as samples collected in the field shows the accuracy and precision of this new method to be equal to or better than current standard techniques.