Amino acids playing important roles in metabolic processes are often included in dietary supplements whose use has largely expanded over the last 20 years not only in patients with particular deficiencies, but also in athletes and even common people that want to enrich their regular daily diet. In the present study, a bare silica Kinetex core-shell 2.6 μm HILIC column was used for separation of some important hydrophilic amino acids and amino acids-like molecules i.e., aspartic acid, creatine, carnitine, arginine and the tripeptide glutathione (GSH), by optimizing the chromatographic conditions for their determination in complex alimentary supplements. The contribution of partition, adsorption and ion exchange on the retention mechanism was studied by varying parameters such as water content and the counter-ion concentration in the mobile phase. Optimum conditions employed a Phenomenex Kinetex core-shell 2.6 μm HILIC (100 × 4.6 mm i.d.) column and a mobile phase of acetonitrile/potassium phosphate buffer (12.5 mM; pH = 2.8) 85:15, v/v, at the flow rate of 1.4 mL/min, using UV detection at 200 nm. A reference HPLC method for the selective determination of GSH by using 1,4-naphthoquinone as derivatization reagent was also introduced for comparative purposes.
The developed HILIC method was validated and applied to the analysis of the considered compounds in dietary supplements. Interestingly, in some of the real samples, oxidized glutathione which is an inactive impurity of GSH, was found at the level of about 20%. The proposed study confirms the importance of simple analytical methods for a rigorous quality control of dietary supplements containing unstable active ingredients.