Nitrogen isotope ratios of amino acids in formalin-fixed biological specimens retain stable and reliable isotopic signatures that can be used to reconstruct historical changes in ecosystem nitrogen baselines and trophic structure over decades.

When an organism dies and its tissues are preserved in formalin, the chemical reacts with certain parts of proteins but does not remove or add any nitrogen atoms. This means the nitrogen isotope pattern in each amino acid stays exactly as it was when the organism was alive. Source amino acids like phenylalanine keep their original isotope value because they are not broken down after being incorporated into proteins. Trophic amino acids like glutamic acid retain their elevated isotope value because the metabolic processes that enriched them before death leave behind stable chemical bonds. These preserved patterns in archived specimens allow scientists to read the organism’s diet and position in the food chain from decades ago, just as if the tissue had been frozen fresh.