Radiocarbon in Dissolved Organic Carbon by Uv Oxidation: Procedures and Blank Characterization at Nosams

Abstract

This study describes a procedural blank assessment of the ultraviolet photochemical oxidation (UV oxidation) method that is used to measure carbon isotopes of dissolved organic carbon (DOC) at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS). A retrospective compilation of Fm and δ13C results for secondary standards (OX-II, glycine) between 2009 and 2018 indicated that a revised blank correction was required to bring results in line with accepted values. The application of a best-fit mass-balance correction yielded a procedural blank of 22.0 ± 6.0 µg C with Fm of 0.30 ± 0.20 and δ13C of –32.0 ± 3.0‰ for this period, which was notably higher and more variable than previously reported. Changes to the procedure, specifically elimination of higher organic carbon reagents and improved sample and reactor handling, reduced the blank to 11.0 ± 2.75 µg C, with Fm of 0.14 ± 0.10 and δ13C of –31.0 ± 5.5‰. A thorough determination of the entire sample processing blank is required to ensure accurate isotopic compositions of seawater DOC using the UV oxidation method. Additional efforts are needed to further reduce the procedural blank so that smaller DOC samples can be analyzed, and to increase sample throughput.

Radiocarbon in Dissolved Organic Carbon by Uv Oxidation: An Update of Procedures and Blank Characterization at Nosams

“…The method is basically the same as that in Xu et al (2021) with several procedural changes. The most notable change relates to handling and exposure of the water-cooling probe: After the UV pretreatment of Milli-Q water (for standards, blanks, and small volume samples), the water-cooling probe is now placed in a cylindrical glass container with 65/40 socket filled with fresh Milli-Q water (Figure 1, right).…”

“…Another procedural modification, compared to the Xu et al (2021) paper: the halogen trap, previously using a potassium iodine solution, was replaced with a copper wool flow-through Figure 1 The DOC water-cooling probe used at NOSAMS. At left, the water-cooling probe is exposed to air.…”

“…Radiocarbon ( 14 C) measurements in DOC provide key insights into the age and geochemical cycling mechanisms of this important global carbon reservoir. A recent paper documented the procedures utilized for DOC radiocarbon measurements at the National Ocean Science AMS facility (NOSAMS; Xu et al 2021). A major component of that paper was to characterize the procedural blanks and to describe modifications to the procedures intended to minimize blank contributions.…”

“…A major component of that paper was to characterize the procedural blanks and to describe modifications to the procedures intended to minimize blank contributions. In that effort, the DOC UV oxidation blank was reduced from 22 μg C to 11 μg C. This note describes some recent modifications to the procedure, described by Xu et al (2021), that further reduces the blank to 2.6 ± 0.6 μg C and doubles the throughput.…”

Isotopic Evidence for Sources of Dissolved Carbon and the Role of Organic Matter Respiration in the Fraser River Basin, Canada

“…Results have been revised based on the recent NOSAMS 14 C-DOC procedural blank correction assessment (Xu et al 2021). Analytical precision for NOSAMS DOC and DIC 14 C Fraction modern (Fm) values was better than 0.005, and for ETH DOC 14 C was better than 0.04.…”