A RAPID FTIR-BASED METHOD FOR QUALITY ASSESSMENT OF COMERCIAL L-THEANINE SUPPLEMENTS

L-theanine (C₇H₁₄N₂O₃) is a naturally occurring amino acid primarily found in green tea (Camellia sinensis), with functional properties associated with stress reduction, improved sleep quality, cognitive support and mild anxiolytic effects. Although it can be consumed through tea infusions, its concentration may vary significantly due to factors such as plant part, harvest season, geographical origin and processing. To ensure more precise and consistent intake, many individuals turn to dietary supplementation. However, adulteration in commercial supplements remains a recurrent issue that may compromise their composition, efficacy and safety. In this context, the present study proposes an analytical methodology for evaluating the quality of L-theanine supplements available in Brazil. The strategy combines Fourier-transform infrared spectroscopy (FTIR) and high-resolution mass spectrometry (HRMS) to identify the active ingredient, detect impurities or potential adulterants and quantify the compound. FTIR-ATR was used as a rapid and non-destructive method for L-theanine quantification. Samples from commercial brands were analyzed and a calibration curve was constructed using pure L-theanine at different concentrations. The most significant spectral features appeared around 3300 cm⁻¹, related to N–H stretching vibrations, which are typical of amino acid structures and amine groups. Theanine content was estimated using a single-point standard addition approach (S₀/S₁₀), focusing on regions of maximum spectral depth. Data processing was performed using Python libraries such as scikit-learn and numpy, applying linear regression supported by high R² values. Complementarily, HRMS analysis confirmed the molecular identity of L-theanine in the reference sample by detecting a predominant ion at m/z 175.1081, corresponding to the protonated form [M+H]⁺. The developed methodology demonstrates potential as a low-cost, scalable and accessible screening tool for quality control of dietary supplements, contributing to consumer safety and improved regulation through the integration of spectroscopy and chemometrics.