methanol (MeOH) (Fisher Scientific), water from Milli-Q Plus, LC/MS-grade
acetonitrile (Fisher Chemical), and formic acid (Merck). The standards
used for quantitation were Cell Free Amino Acid Mixture-15N (Sigma-Aldrich), glucose-d2, γ-aminobutyric
acid-d2 (GABA-d2), acetylcholine-d9, oleic acid-d9 (FA 18:1-d9),
LPC 19:0, PC 11:0/11:0. Solvents for PA nano-DESI analysis were MeOH/H2O (9:1) with 0.1% formic acid or ACN/MeOH (9:1) with 0.1%
formic acid (