The sample treatment used was: ten capsules or five tablets were first homogenized (by grinding for tablets) and then, the required quantity was weighed to obtain a final concentration of 60 μg/mL of l-amino acid (an intermediate concentration within the calibration curve) after four consecutive water extractions. For Capsules#3 the final concentration was 120 μg/mL for l-Phe and 45 μg/mL for l-Tyr due to their content in the sample. For each extraction, 20 mL of water were added to the sample and the mixture was shaken (5 min), sonicated (5 min), and centrifuged (18,353 × g) for 5 min at 25 °C. All the supernatants were collected in an appropriate volumetric flask that was completed with water.