L leucine

The purity assessed CRMs of
the target AAs (l-Proline, l-Valine, l-Isoleucine, l-leucine, l-phenylalanine, and l-tyrosine)
were obtained from the National Metrology Institute of Japan (NMIJ,
Japan). The following isotopic labeled Aas for the ISTDs were obtained
from Cambridge Isotopes Laboratory (Andover, MA): l-Proline
(U-¹³C₅, 99%; ¹⁵N, 99%; Pro*), l-Valine (U-¹³C₅, 97–99%;¹⁵N,97–99%; Val*), l-Isoleucine (U-¹³C₆, 99%; ¹⁵N, 99%; Ile*), l-Leucine (U-¹³C₆, 97–98%; ¹⁵N, 97–98%;
Leu*), l-Phenylalanine (U-¹³C₉, 99%; ¹⁵N, 99%; Phe*), and l-Tyrosine (¹³C₉, 99%; Tyr*). All other chemicals used in this study are summarized
in the Supporting Information.

PC(12:0/13:0), PE(12:0/13:0), SM(d18:1/12:0) and Cer(d18:1/12:0) were purchased from Avanti Polar Lipids (Alabaster, AL, USA). Stable isotope labeled references were purchased from Cambridge Isotope Laboratories (Andover, MA, USA), including: Glycine (2-¹³C, ¹⁵N), L-Alanine (2,3,3,3-D₄), L-Valine (D₈), L-Leucine (5,5,5-D₃), L-Methionine (methyl-D₃), L-Phenylalanine (ring-¹³C₆), L-Tyrosine (ring-¹³C₆), L-Aspartic acid (2,3,3-D₃), DL-Glutamic acid (2,4,4-D₃), L-Ornithine: HCl (5,5-D₂), L-Citrulline (5,5-D₂) and L-Arginine (5-¹³C, 4,4,5,5-D₄). Deuterium-labeled carnitine and acylcarnitines (NSK-B set) were purchased from Cambridge Isotope Laboratories (Tewksbury, MA, USA), including ²H₃-Acetylcarnitine (C₂), ²H₃-Propionylcarnitine (C₃), ²H₃-Butyrylcarnitine (C₄), ²H₉-Isovalerylcarnitine (C₅), ²H₃-Octanoylcarnitine (C₈), ²H₉-Myristoylcarnitine (C₁₄), and ²H₃-Palmitoylcarnitine (C₁₆). HPLC-grade ammonium acetate was purchased from Sigma-Aldrich (St. Louis, MO, USA), and HPLC-grade formic acid and acetonitrile were purchased from Thermo Fisher Scientific (Waltham, MA, USA). All solutions were prepared by LC-MS ultra-pure water.