Abstract: The present invention relates to newborn screening kits, methods, stable isotopically-labeled internal standards or internal standard solution for high throughput screening and analysis of metabolic disorders using liquid chromatography mass spectrometry (LC-MS) are provided. The metabolic disorders can be amino acid, organic acid or fatty acid oxidation disorders, and particularly urea cycle disorders or deficiencies, hyperammonemia, Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH), and/or argininosuccinic aciduria. The newborn screening kits, methods, stable isotopically-labeled internal standards or internal standard solution are particularly useful for newborn screening (NBS) of metabolic disorders.