Determination of lipoic acid by flow-injection and high-performance liquid chromatography with chemiluminescence detection

A new flow-injection (FI) and high performance liquid chromatography (HPLC) with chemiluminescence detection method has been proposed for the determination of α-lipoic acid (LA). The assay is based on the measurement of chemiluminescence (CL) produced during the reaction of α-lipoic acid with potassium permanganate in a sodium hexametaphosphate medium (pH 3). This reaction is accompanied by a weak CL, which is greatly increased in the presence of a formaldehyde solution. The proposed FI method allows the determination of LA over the range: 0.5–20 μg mL−1 with LOD 4 × 10−3 μg mL−1. An introduction of HPLC into the flow manifold improves selectivity of the method and allows the determination of LA in a complex sample. The chromatographic linear range is 2.5–30 μg mL−1 with LOD 1.774 μg mL−1. Chromatographic separation was achieved by isocratic elution (acetonitrile/potassium dihydrogen phosphate, pH 3, adjusted with phosphoric acid): 30/70 using a Cosmosil 5C18-MS-II (4.6 mm × 150 mm I.D.) column at a flow rate of 1.0 mL min−1. The presented methods were utilized to determine the α-lipoic acid content in “Alfa-lipoic acid” capsules and in food products.