Performance evaluation of nitrogen isotope ratio determination in marine and lacustrine sediments: An inter-laboratory comparison

Abstract

Nitrogen isotopes of organic matter are increasingly studied in marine biogeochemistry and geology, plant and animal ecology, and paleoceanography. Here, we present results of an inter-laboratory test on determination of nitrogen isotope ratios in marine and lacustrine sediments. Six different samples covering a wide range of total nitrogen content and d15N values were analyzed by eight different laboratories using their routine procedures. The laboratories were asked to measure three batches with three replicates for each sample to assess accuracy and variability within and among laboratories; this permits assessment of repeatability and reproducibility, which are essential in meta analysis of the increasing database on d15N values in marine sediments. The grand average d15N values for individual samples ranged from 1.65–10.90‰. One laboratory exhibited an average bias of 0.27‰ compared to the mean of all other laboratories. Apart from one sample, which showed an exceptionally high overall standard deviation (OSD) of 0.51‰, the analytical precision (1 s) averaged 0.24‰, ranging from 0.18–0.31‰ for individual samples. Out of the eight participating laboratories, two showed a significantly elevated within-laboratory standard deviation (WLSD) of 0.41‰ and 0.32‰ compared to an average WLSD of 0.15‰for the other laboratories. The WLSD was inversely correlated with the ratio of peak height to peak width, which was taken as a simple measure of peak shape. Moreover, our data also revealed an inverse correlation between total nitrogen content and measurement precision. These correlations may provide guidance for improving the measurement precision of individual laboratories. Based on the results of this round robin test, we have estimated the expanded measurement uncertainty on the 2r level to 0.45‰ for sediment samples with a nitrogen content >0.07 wt%. Sediment samples with lower nitrogen contents cannot be measured with sufficient precision without additional precautions and care should be taken when interpreting d15N signatures and records for sediments with nitrogen concentrations <0.07 wt%.