Quantitative determination of alpha lipoic acid in dietary supplements using HPLC with different detection modes

Alpha lipoic acid (α-LA) is a natural antioxidant used both in the prevention and treatment of various oxidative stress related diseases [1]. The present paper deals with the development of a rapid and selective method for the quantitative determination of the α-LA content in dietary supplements. The method is based on HPLC with coulometric electrode array detection (CEAD) and ESI-MS detection modes. First, α-LA was extracted with methanol by sonication. The chromatographic separation for CEAD was then achieved isocratically [acetonitrile/methanol/50 mM potassium dihydrogen phosphate 305:65:630, v/v/v, pH 3] using an ACE 3-C-18 column (150×3mm, 3µm particle size) at a flow rate of 0.45ml/min. For ESI-MS (negative mode), an ACE 3-C-18 column with 2.1mm I.D. was used. The mobile phase consisted of 0.1% acetic acid in water/acetonitrile 55:45, v/v and was used at a flow rate of 0.2ml/min. Both chromatographic methods were validated and the results were in good correlation [Table 1]. High sensitivity and selectivity make these methods useful in pharmacological studies.

Table 1: Content of α-lipoic acid in dietary supplements

Supplement	Claimed (mg/g)	Found (mg/g)
		HPLC-CEAD	HPLC-MS
a	34.9	33.6	33.9
b	0.7	0.4	0.5
c	1.3	0.9	0.9
d	20.6	12.1	13.2
e	13.1	9.9	10.5
f	1.0	0.4	0.4

Acknowledgements: 1) Higher Education Commission, Pakistan 2) Austrian Exchange Service, Austria.

References: [1] Navari-Izzo, F. et al. (2002) Plant Physiol Biochem 40: 463–470.