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Practical considerations in the determination of compound-specific amino acid δ15N values in animal and plant tissues by gas chromatography-combustion-isotope ratio mass spectrometry, following derivatisation to their N-acetylisopropyl esters

Authors


R. P. Evershed, Organic Geochemistry Unit, School of Chemistry, Cantock's Close, University of Bristol, Bristol BS8 1TS, UK.

E-mail: R.P.Evershed@bristol.ac.uk

Abstract

RATIONALE

Stable nitrogen isotope (δ15N) values of bone collagen are routinely used to inform interpretations of diet and trophic positions within contemporary and ancient ecosystems, yet the underlying physiological and biochemical factors which contribute to the bulk collagen δ15N value remain little understood. Determination of individual amino acid (AA) δ15N values in animal and plant proteins can help to elucidate the cycling of nitrogen and inform predictions of palaeodiet and ecology.

METHODS

In this study we present a methodology for the measurement of amino acid δ15N values using gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). Amino acid standards of known δ15N values were derivatised to their N-acetylisopropyl (NAIP) esters and purified through Dowex ion-exchange resin to determine any isotopic fractionation associated with derivatisation and ion-exchange chromatography. The effect of starch on AA δ15N values was also determined by hydrolysing bone collagen with and without the presence of starch.

RESULTS

The amino acids derivatised to their NAIP esters give values within ±0.8‰ of their δ15N values measured separately by elemental analyser (EA)-IRMS, with a precision of better than 0.8‰. The δ15N values of AAs after Dowex ion-exchange chromatography were within ±0.9‰ of their values prior to ion-exchange chromatography. The AA δ15N values of bone collagen hydrolysed with and without starch were within ±0.8‰.

CONCLUSIONS

Hydrolysis of lipid-extracted plant material followed by purification of AAs using Dowex ion-exchange resin and derivatisation to their NAIP esters is a suitable protocol for the accurate determination of individual plant and animal AA δ15N values by GC-C-IRMS. Copyright © 2012 John Wiley & Sons, Ltd.

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