Practical considerations for improved reliability and precision during determination of δ 15 N values in amino acids using a single combined oxidation-reduction reactor

There has been increased interest in the measurement of δ N 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. Precision for δ N 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 δ N 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. 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