Compound-specific isotope analysis of amino acids for aquatic systems – Problems, challenges, solutions: A review

Compound-specific isotope analysis of individual amino acids (AAs) is a promising method for characterizing resource flow and transformation in aquatic systems, encompassing microbial domains to macroscopic consumers. This review delves into two key areas of research concerning AAs in aquatic ecosystems. Firstly, carbon isotope analysis is employed to explore carbon sources assimilated for AAs synthesis, and secondly, nitrogen isotope analysis is used to clarify ecosystem baselines and trophic dynamics of organisms. Surprisingly, literature provides limited discussions on the limitations or critical challenges faced during AAs isotope analysis over the past decade. To address this gap, this review focuses on challenges encountered in method development, sample preparation, and technical aspects, including instrumental design, and δ¹³C and δ¹⁵N analysis of AAs. Recommendations are provided to increase the accuracy and reliability. The review also highlights the significance of comprehensive understanding and detailed methodology to address challenges during AA isotope analysis in aquatic systems.