AAs were selected for this study as they cover a broad and diverse range of classes with both parent compounds and metabolites. Table shows the AAs and metabolites investigated during this project along with AA abbreviations used in this paper. More detailed information regarding AAs and metabolites may be found in the supplementary information (Table ). Analytical standards and deuterated (stable isotope-labelled) standards were obtained from Sigma-Aldrich (Gillingham, UK), TRC (Toronto, Canada), LGC (Middlesex, UK), or MCE (Cambridge, UK). The methanol used was HPLC-grade (Sigma-Aldrich), the water was of 18.2 MΩ quality (Elga, Marlow, UK); and the purity of formic acid, used as the mobile-phase additive, was >95% (Sigma-Aldrich). Glassware was deactivated using 5% dimethylchlorosilane in toluene (Sigma-Aldrich) to avoid losses via adsorption.