Seawater DIC analysis: The effects of blanks and long‐term storage on measurements of concentration and stable isotope composition

Dissolved inorganic carbon (DIC) concentration and stable isotope composition measurements (delta C-13) are central to the study of biological and chemical processes in the ocean. We examined a small volume (1 mL) method using continuous-flow isotope ratio mass spectrometry (CF-IRMS) for making both measurements in a single sample run. Prior exploration of this approach has yielded variable results. Effects of concentration, isotopic composition, storage time, and the gas used to flush the sample tubes were tested using standard solutions. Multiple isotopic composition standards permitted calculation of an apparent blank that develops due to slow rates of gas exchange through the septum. A blank correction is applied to the delta C-13 measurements, giving precision and accuracy better than 0.1 parts per thousand after 6 months of storage and similar to 0.2 parts per thousand after 18 months of storage. CF-IRMS concentration measurements on field samples collected near Tatoosh Island, off the Pacific coast of Washington, U.S.A., were compared to independent measurements (Dickson, CDQC Laboratory, UCSD) on the same water, the latter approach involving collection of 500 mL of seawater fixed with mercuric chloride. Concentration measurements by CF-IRMS on seawater samples (precision similar to 1-2%) are correctable to the Dickson results, though scatter in this relation exceeds analytical precision. The DIC and delta C-13 measurements on coastal seawater, in conjunction with salinity measurements, indicated strong effects of biological activity on carbon dynamics relative to possible effects of mixing water masses. In sum, this small volume methodology has application to field situations and laboratory experiments where larger volumes are logistically challenging and storage space is limited.