High-precision measurement of phenylalanine δ15N values for environmental samples: a new approach coupling high-pressure liquid chromatography purification and elemental analyzer isotope ratio mass spectrometry.

Rationale: Compound-specific isotope analysis of individual amino acids (CSI-AA) is a powerful new tool for tracing nitrogen (N) source and transformation in biogeochemical cycles. Specifically, the δ(15)N value of phenylalanine (δ(15)N(Phe)) represents an increasingly used proxy for source δ(15)N signatures, with particular promise for paleoceanographic applications. However, current derivatization/gas chromatography methods require expensive and relatively uncommon instrumentation, and have relatively low precision, making many potential applications impractical.

Methods: A new offline approach has been developed for high-precision δ(15)N measurements of amino acids (δ(15)N(AA)), optimized for δ(15)N(Phe) values. Amino acids (AAs) are first purified via high-pressure liquid chromatography (HPLC), using a mixed-phase column and automated fraction collection. The δ(15)N values are determined via offline elemental analyzer-isotope ratio mass spectrometry (EA-IRMS).

Results: The combined HPLC/EA-IRMS method separated most protein AAs with sufficient resolution to obtain accurate δ(15)N values, despite significant intra-peak isotopic fractionation. For δ(15)N(Phe) values, the precision was ±0.16‰ for standards, 4× better than gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS; ±0.64‰). We also compared a δ(15)N(Phe) paleo-record from a deep-sea bamboo coral from Monterey Bay, CA, USA, using our method versus GC/C/IRMS. The two methods produced equivalent δ(15)N(Phe) values within error; however, the δ(15)N(Phe) values from HPLC/EA-IRMS had approximately twice the precision of GC/C/IRMS (average stdev of 0.27‰ ± 0.14‰ vs 0.60‰ ± 0.20‰, respectively).

Conclusions: These results demonstrate that offline HPLC represents a viable alternative to traditional GC/C/IMRS for δ(15)N(AA) measurement. HPLC/EA-IRMS is more precise and widely available, and therefore useful in applications requiring increased precision for data interpretation (e.g. δ(15)N paleoproxies).