Evaluation of the quantity and the carbon isotopic composition of amino acids by using diverse sample residues after lipid extraction.

Compound-specific stable isotope analysis (CSIA) becomes a critical tool in the fields of ecology and biogeochemistry to trace basal carbon sources such as amino acids (AAs) and fatty acids (FAs) in natural environments. However, in many studies, either AAs or FAs have been analyzed due to restricted sample amounts. The aim of this study is to report a single-sample preparation method by identifying AAs using lipid-free residues derived from a typical FA analytic procedure. The capability of the lipid-free residues was evaluated by determining AA quantities and carbon isotopic compositions (δC) across diverse sample matrices, including soil, suspended particulate organic matter (SPM), fish tissue, and cultured cyanobacteria (N = 3). Our result showed that the use of lipid-free residues after a typical lipid extraction procedure in most samples did not significantly reduce the AA quantities, relative to the conventional AA method (done by using whole samples), except in case of samples of soil. Moreover, δC compositions of the most AAs (from essential to nonessential AAs) in all samples were not significantly different, with an average isotope difference of < 2.1‰ between the two methods. Overall results indicate that the residual parts after extracting lipid fraction are useful for reliable AA quantification and individual isotopic information. Ultimately, one sample preparation for determining FA and AA data enables us to characterize one (or multiple) basal carbon source(s) within rare samples despite their limited abundance. Graphical abstract.