Determination of alpha lipoic acid in dietary supplement preparations and in drug formulations

Abstract

Alfa lipoinska (tioktinska) kiselina se koristi u lečenju dijabetesne polineuropatije. Zbog svojih antioksidativnih svojstava, prisutna je u brojnim dijetetskim suplementima sama, kao i u kombinaciji sa aminokiselinama, L-karnitinom i drugim jedinjenjima. Dostupni podaci za kvantitativno određivanje alfa lipoinske kiseline su malobrojni. Usled toga, cilja našeg rada bio je da se razvije i validira TLC metoda za određivanje alfa lipoinske kiseline posle derivatizacije sa paladijum (II) hloridom. Za razdvajanje alfa lipoinske kiseline i njenog redukovanog oblika korišćene su RPTLC ploče veličine 20×10 cm, uz mobilnu fazu propanol-2 : methanol : aceton : voda : sirćetna kiselina u odnosu 6:4:2:8:0,2 v/v/v/v/v. Nakon razvijanja, hromatografske ploče su potapane u rastvor paladijum (II) hlorida, a žute zone formiranog kompleksa su merene na 375 nm. Retenciona vremena alfa lipoinske kiseline i njene redukovane forme su 45 i 32 nm. Zavisnost površine signala i količine nanete supstance ispitana je korišćenjem linearne regresione jednačine za opseg koncentracija 1 – 3 μg i polinomalne regresione jednačine drugog stepena za koncentracioni opseg 0,5 – 5 μg. Za datu metodu dobijene vrednosti koeficijenta korelacije (r=0,999), limit kvantifikacije (0,3 μg), rikaveri (98,5 – 105,2%) i preciznost (0,9 – 2,9%) su zadovoljavajući. Validirana hromatografska metoda je primenjena za određivanje alfa lipoinske kiseline u doziranim oblicima i dijetetskim suplementima. Nađeni sadržaj lipoinske kiseline (98,5 – 102,0%) u doziranim oblicima je u propisanim granicama, dok je u dijetetskim suplementima varirao u rasponu od 50 – 185%.

Alpha lipoic (thioctic) acid is a drug used for the treatment of diabetic polyneuropathy. Due to its antioxidant properties alpha lipoic acid nowadays widely used in dietary supplement preparation alone and in combination with amino acids, L-carnitine and other compounds. There are not so many data available on the quantitative determination of alpha lipoic acid in dietary supplements. Therefore, the aim of these investigations was to develop and validated TLC method for determination of alpha lipoic acid after derivatization by Palladium (II) chloride reagent. The separation of alpha lipoic acid was performed on RPTLC plates (20 × 10 cm) using propanol-2 : methanol : acetone : water : acetic acid (6:4:2:8:0.2 v/v/v/v/v) as mobile phase. The plates were immersed in solutions of Palladium (II) chloride reagents and yellow spots were scanned at 375 nm. The retention times of alpha lipoic acid and its reduced form were 45 and 32 mm, respectively. Relationship of the peak areas and the amount of the substance applied was evaluated using the linear (1 -3 μg/spot) and second degree polynomial regression function (0.5 – 5 μg/spot). For the proposed procedure coefficient of correlation (r=0.999), limit of quantification (0.3 μg/spot), recovery (98.5 – 105.2 %) and precision (0.9 – 2.9%) were found to be satisfactory. The developed method was applied for determination of alpha lipoic acid in drugs dosage formulations and in dietary supplement preparation. The content of lipoic acid were found to be 98.5 – 102.0% in drug dosage formulations and 50 – 185.0% in some of dietary supplement preparations.