Precise Measurement of Phenylalanine δ15N Values via Elemental Analysis-Isotope Ratio Mass Spectrometry Following Purification with High-Pressure Liquid Chromatography: A New Tool for Fine-Scale Paleo-Nitrogen Cycle Reconstructions
Abstract
Compound specific isotope analysis of individual amino acids (CSI-AA) in organic paleoarchives is emerging as a powerful tool for reconstructing the paleo-nitrogen (N) cycle. Because the δ15N of phenylalanine (Phe) remains almost unchanged with diagenesis or trophic transfer it has been demonstrated to be the most promising AA proxy for the δ15N of primary production. However, the precise measurement of AA δ15N values is currently limited by the standard gas chromatography-isotope ratio mass spectrometer (GC-IRMS) approach. The key problem with this approach is that GC-IRMS δ15N precision (±1‰) is almost an order of magnitude lower than typical bulk δ15N measurements (±0.1‰), posing a significant problem for assessing fine scale changes within paleo-climate records. Additionally, required instrumentation is both expensive, and not widely available. Here we present an offline approach in which Phe is purified via high-pressure liquid chromatography (HPLC), using an analytical scale, mixed-phase column and automated fraction collection. δ13C and δ15N values of the fractions are then determined via standard elemental analysis-isotope ratio mass spectrometry (EA-IRMS). We evaluate the precision of this HPLC-EA-IRMS method vs. traditional GC-IRMS for Phe δ15N values of isotopic AA standards and various proteinaceous marine samples (marine coral, sediment, and cyanobacteria). Typical HPLC-EA-IRMS δ15N precision is ±0.1‰ for isotopic AA standards and ±0.5‰ for marine coral, sediment, and cyanobacteria samples compared to ±0.5‰ and ±1.0‰ for AA standards and samples analyzed by GC-IRMS. In order to demonstrate the viability of our approach, we present a comparison of a Phe δ15N record produced from a deep-sea bamboo coral specimen from Monterey Bay, CA, using our offline HPLC-EA-IRMS method vs. standard GC-IRMS. Each method produced equivalent Phe δ15N values within error, however, the HPLC-EA-IRMS method produced Phe δ15N values with over twice the precision of the traditional GC-IRMS method (HPLC-EA-IRMS standard deviation = 0.27‰ ± 0.14‰, GC-IRMS standard deviation = 0.60‰ ± 0.20‰). Our results suggest that this HPLC-EA-IRMS method represents a powerful, yet widely available tool for reconstructing fine scale N-cycling processes within paleoceanographic records.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B43C0404B
- Keywords:
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- 0469 BIOGEOSCIENCES / Nitrogen cycling;
- 0473 BIOGEOSCIENCES / Paleoclimatology and paleoceanography;
- 4894 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Instruments;
- sensors;
- and techniques;
- 4994 PALEOCEANOGRAPHY / Instruments and techniques