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

The analysis of the nitrogen (N) isotopic composition of organic matter bound to fossil biomineral structures (BB-delta N-15) 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 (a) a method for sealing the reaction vials in which the oxidation of organic N to NO3- takes place, (b) a recipe for bypassing the pH adjustment step before the bacterial conversion of NO3- to N2O, and (c) a method for storing recrystallized dipotassium peroxodisulfate (K2S2O8) under Ar atmosphere. Results: The new sealing method eliminates the occasional contamination and vial breakage that occurred previously while increasing sample throughput. The protocol for bypassing pH adjustment does not affect BB-delta N-15, and it significantly reduces the processing time. Storage of K2S2O8 reagent under Ar atmosphere produces stable oxidation blanks over more than 3.5 years. We report analytical blanks, accuracy, and precision for this methodology from eight users over the course of similar to 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, 1 sigma, n = 195) and stable delta N-15 (-2.20 +/- 3.13 parts per thousand, n = 195). The analysis of reference amino acid standards USGS 40 and USGS 65 indicates an overall accuracy of -0.23 +/- 0.35 parts per thousand (1 sigma, n = 891). The analysis of in-house fossil standards gives similar analytical precision (1 sigma) across a range of BB-delta N-15 values and biominerals: zooxanthellate coral standard PO-1 (6.08 +/- 0.21 parts per thousand, n = 267), azooxanthellate coral standard LO-1 (10.20 +/- 0.28 parts per thousand, n = 258), foraminifera standard MF-1 (5.92 +/- 0.28 parts per thousand, n = 243), and tooth enamel AG-Lox (4.06 +/- 0.49 parts per thousand, n = 78). Conclusions: The methodological improvements significantly increase sample throughput without compromising analytical precision or accuracy down to 1 nmol of N.