4 aminobutyric acid

L-norepinephrine hydrochloride, (±)−dopamine hydrochloride, serotonin hydrochloride, epinephrine hydrochloride, tryptamine, (±)−octopamine hydrochloride, (±)−synephrine, tyramine hydrochloride, β-phenethylamine, D,L-metanephrine hydrochloride, adenine riboside, 4-aminobutyric acid, L-tyrosine, L-tryptophan, L-tryptophan-d5 (TRP-D5) and 4-aminobutyric acid-2,2,3,3,4,4-d6 (GABA-D6) were purchased from Sigma-Aldrich Corporation (Milan, Italy). The deuterated internal standards, (±)−norepinephrine-2,5,6,α,β,β-d₆ HCl (NE-D6), 2-(3,4-dihydroxyphenyl)ethyl-1,1,2,2-d₄-amine (DA-D4), serotonin-α,α,β,β-d₄ creatinine sulfate complex (5HT-D4), (±)−epinephrine-2,5,6,α,β,β-d₆ (E-D6), tryptamine-α,α,β,β-d₄ HCl (TRY-D4), (±)−p-octopamine-α,β,β-d₃ HCl (OCT-D3), 2-(4-hydroxyphenyl)ethyl-1,1,2,2-d₄-amine HCl (TYRA-D4), and 2-phenyl-d₅-ethylamine (β-PEA-D5) were obtained from CDN Isotopes (Pointe-Claire, Quebec, Canada). The L-tyrosine-ring-d4 (TYRO-D4) was from Cambridge Isotope Laboratories, Inc. (Tewksbury, MA, USA). The purity of all analytes and labeled internal standards was ≥98%.
Water was purified with a Milli-Q Elix purification system (Millipore, Bedford, MA, USA). High-purity MS-grade solvents (formic acid, methanol, and acetonitrile) were obtained from Fluka (Milan, Italy) and used without further purification.