UC Santa Cruz

Compound specific isotope analysis of individual amino acids (CSI-AA) is a powerful tool for tracing nitrogen (N) source and transformation in biogeochemical cycles. Specifically, the d15N value of phenylalanine (d15NPhe) represents an increasingly used proxy for baseline d15N values, with particular promise for paleoceanographic applications. Further, multiple studies have shown that d15N values of Phe and glutamic acid (Glu) can be coupled to provide precise estimates of trophic position (TP), while simultaneously decoupling baseline d15N values from the effects of trophic transfer. However, current derivatization/gas chromatography (GC) methods require expensive and relatively uncommon instrumentation, and have relatively low precision, posing significant issues for many potential applications. Here we present a new approach to CSI-AA based on a high-pressure liquid chromatography (HPLC) purification of underivatized amino acids, followed by offline elemental analysis-isotope ratio mass spectrometry (EA-IRMS). The utility of this approach is demonstrated in two different applications using natural materials: 1) the construction of a high precision paleoceanographic record from the d15NPhe in proteinaceous coral tissue and 2) the TP estimation of five marine organisms using the coupled d15N values of Glu and Phe. Both demonstrations were compared to values measured by an established GC based method. In all cases both methods produced equivalent values, however, values generated by the new HPLC/EA-IRMS approach had higher average precision in all cases. These results demonstrate that this approach represents a viable alternative to traditional GC based methods for d15NAA measurement and may therefore allow significant expansion of CSI-AA applications, requiring only commonly available instrumentation to produce high precision d15NAA values.