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

Abstract

AbstractRATIONALEThere has been increased interest in the analysis for δ15N 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.METHODSPrecision for δ15N 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 N2 CO2 trap is left immersed throughout. A five point calibration curve using amino acids with a range of δ15N values from −2.4‰ to +61.5‰ was used in combination with a 13 amino acid mixture to correct for offsets during derivatization.RESULTSCombining 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 δ15N-amino acid analysis (±1‰).CONCLUSIONSA 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 δ15N in amino acids.