Practical considerations for improved reliability and precision during compound specific analysis of δ15N in amino acids using a single combined oxidation-reduction reactor

RATIONALE There has been increased interest in the analysis for δ 15 N in amino acids to gain simultaneous insight into both trophic relationships and source producers within ecosystems. New developments in gas chromatography combustion isotope ratio mass spectrometry equipment has led to variable outcomes in performance due to limited information about best practices for new systems. METHODS Precision for δ 15 N in amino acids using the single combined oxidation-reduction reactor is improved across a sequence of analyses if the reactor is oxidized for a substantial period (2 h), immediately followed with a conditioning run of alkanes prior to analysis for N, and the liquid N 2 CO 2 trap is left immersed throughout. A five point calibration curve using amino acids with a range of δ 15 N values from −2.4‰ to +61.5‰ was used in combination with a 13 amino acid mixture to correct for offsets during derivatization. RESULTS Combining the improved setup with normalization techniques using both internal and external standards allows for a reliable throughput of ~25 samples per week. It allowed for a reproducible level of error of <±0.5‰ within standards repeated 10 times across each sequence and a sample error of (±0.18‰), which is lower than analytical error typically associated with δ 15 N-amino acid analysis (±1‰). CONCLUSIONS A few practical considerations regarding oxidation and conditioning of the combustion reactor allow for increased sequence capacity with the single combined oxidation-reduction reactor. These considerations combined with normalization techniques result in a higher throughput and reduced analytical error during analysis of δ 15 N in amino acids.