Analytical improvements and assessment of long-term performance of the oxidation-denitrifier method

RATIONALE The analysis of the nitrogen (N) isotopic composition of organic matter bound to fossil biomineral structures (BB-δ N) using the oxidation-denitrifier (O-D) method provides a novel tool to study past changes in N cycling processes. METHODS We report a set of methodological improvements to the O-D method, including: (i) a method for sealing the reaction vials in which oxidation of organic N to NO takes place; (ii) a recipe for bypassing the pH adjustment step before the bacterial conversion of NO to N O; and (iii) a storage method of recrystallized dipotassium peroxodisulphate (K S O ) under Ar atmosphere.  RESULTS The new method eliminates the occasional contamination and vial breakage, and increases sample throughput. The protocol for bypassing of pH adjustment does not affect BB-δ N, and it significantly reduces processing time. Storage of K S O reagent under Ar atmosphere produces stable oxidation blanks over more than two years. We report analytical blanks, accuracy and precision for this methodology from eight users over the course of ~ 3.5 years of analyses at the Max Planck Institute for Chemistry. Our method produces analytical blanks characterized by low N content (0.30 ± 0.13 nmol N, 1σ, n=204) and stable δ N (– 2.45 ± 3.50 ‰, n=204). The analysis of reference amino acid standards USGS40 and USGS65 indicates an overall accuracy of  – 0.23 ± 0.35 ‰ (1σ, n=891). The analysis of in-house fossil standards gives similar analytical precision (1σ) across a range of BB-δ N values and biominerals: zooxanthellate coral standard PO-1 (6.08 ± 0.21 ‰, n=267), azooxanthellate coral standard LO-1 (10.20 ± 0.28 ‰, n=258), foraminifera standard MF-1 (5.92 ± 0.28 ‰, n=243) and tooth enamel AG-Lox (4.06 ± 0.49 ‰, n=78). CONCLUSIONS The proposed analytical improvements significantly increase sample throughput without compromising analytical precision or accuracy down to 1 nmol of N.