Practical considerations for improved reliability and precision during determination of δ15N values in amino acids using a single combined oxidation–reduction reactor

Abstract

Rationale: There has been increased interest in the measurement of δ15N values in amino acids (AAs) to gain simultaneous insight into both trophic relationships and the composition of biogeochemical sources used by producers at the base of the food web. A new combustion reactor design in gas chromatography/combustion isotope ratio mass spectrometry (GC/C-irMS) equipment has brought to light variable outcomes in performance, highlighting the need for better information about best practices for new systems. Methods: Precision for δ15N values 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) and subsequently maintained throughout the sequence with 12–17 s seed oxidation before each run during GC/C-irMS. 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 13–15 amino acid mixture to consistently normalize measurements to internationally calibrated reference materials. Results: Combining this oxidation method with normalization techniques using both internal and external standards provided a reliable throughput of ~25 samples per week. It allowed for a reproducible level of precision of <±0.5‰, n = 10 within a derivatized standard mixture across each sequence and an average sample precision of ±0.27‰ n = 3, which is lower than the analytical precision typically associated with δ15N values for 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 the measurement of δ15N values in amino acids.