For the study, 58 AAs and 26 metabolites were investigated using a method that has previously been developed within the group covering a broad range of AA classes (table ), further chemical information on targets may be found in the supplementary information (table ). Analytical standards and internal standards (deuterated labelled standards) were purchased from the following companies: Sigma-Aldrich (Gillingham, UK), TRC (Toronto, Canada), LGC (Middlesex, UK), or MCE (Cambridge, UK). All methanol (MeOH) used was HPLC-grade and formic acid (>95 % purity), were both obtained from Sigma-Aldrich. All glassware used in this project was deactivated using 5 % dimethylchlorosilane in toluene to avoid any loss of target analytes via adsorption. Regarding solid phase extraction (SPE), Oasis HLB (60 mg, 3 mL) cartridges were used for the extraction of target analytes, purchased from Waters (Manchester, UK). Polypropylene LC vials, along with Whatman GF/F 0.7 μm filters, were also obtained from Waters.