Determination of Arginine δ15N Values in Plant and Animal Proteins by Gas Chromatography–Combustion–Isotope Ratio Mass Spectrometry

Nitrogen (N) stable isotope techniques are widely used in ecology, archaeology, and forensic science to explore trophic relationships and provenances of organisms and materials, most widely using bulk δ 15 N values of whole organisms, tissues, or other materials. However, compound-specific isotope values can provide more diagnostic isotope “fingerprints” and specific information about metabolic processes. Existing techniques for nitrogen isotope analysis allow the determination of δ 15 N values of 14 amino acids (AAs), accounting for ca. 75% of plant protein and collagen N. The majority of remaining N is from arginine, comprising 16 and 14% of collagen and plant protein N, respectively. We therefore aimed to develop a method to detect arginine and determine its δ 15 N value (δ 15 N Arg ) by gas chromatography–combustion–isotope ratio mass spectrometry (GC−C−IRMS), to further contribute to the understanding of the metabolic routing of this important AA. We demonstrate that arginine, as its N -acetyl isopropyl ester, is amenable to GC analysis using a 15 m midpolarity DB-35 column, eluting with baseline resolution from other AAs. The recorded δ 15 N value by GC−C−IRMS was within the error of that of the underivatized compound determined by elemental analyzer–isotope ratio mass spectrometry (EA−IRMS). The newly developed GC−C−IRMS method was applied to modern plant protein and cattle collagen, enabling their δ 15 N Arg values to be related to AA biosynthesis. Determination of archaeological cattle collagen δ 15 N Arg values confirmed the suitability of this method to provide further insights into past diets and ecosystems. Bulk collagen δ 15 N value reconstruction including δ 15 N Arg values better reflect the measured bulk values, as the isotopic ratio of 91% of collagen N can now be determined at the compound-specific level.