A method for detecting amino acids in body fluids is described. The method includes derivatizing the body fluid amino acids (e.g. plasma amino acids), separating the derivatized amino acids by liquid chromatography followed by and subjecting the identification by mass spectrometry. The identity of each amino acid from the body fluid is determined by comparing to a set of structurally similar amino acid standards, which preferably are added to the body fluid sample as internal standards. Use of the method of diagnose an individual with a metabolic disorder is also provided.