Electrochemical characterization ond determination of vitamin E

U ovom radu su ispitivana elektrokemijska svojstva α-tokoferola, uporabom cikličke i diferencijalne pulsne voltametrije. Cilj rada bio je ispitati topljivost α-tokoferola u različitim otapalima, utjecaj pH, brzine polarizacije i koncentracije na njegova oksido-redukcijska svojstva, istražiti moguću adsorpciju α-tokoferola na površinu radne elektrode te odrediti α-tokoferol u realnim uzorcima pomoću HPLC metode. Prilikom ispitivanja topljivosti α-tokoferola najbolja se pokazala smjesa etanola i vode u omjeru 70:30. U cikličkom voltamogramu α-tokoferola uočen je anodni strujni vrh pri potencijalu, Ep,a = 0,175 V koji se odnosi na ireverzibilnu oksidaciju α-tokoferola. Utvrđeno je da je oksidacija α-tokoferola difuzijski kontroliran proces jer je dobivena linearna ovisnost struje oksidacijskog strujnog vrha (Ip,a) o drugom korijenu brzine polarizacije elektrode ( 1/2) te je nagib pravca log Ip,a vs log iznosio oko 0,5. U diferencijalnom pulsnom voltamogramu α-tokoferola je također detektiran jedan oksidacijski strujni vrh pri potencijalu, Ep,a = 0,070 V, koji je linearno rastao s povećanjem masene koncentracije α-tokoferola u koncentracijskom području od 0,43 mg dm-3 do 70,41 mg dm-3. Tekućinskom kromatografijom visoke djelotvornosti ispitivan je sadržaj α-tokoferola u realnim uzorcima te stabilnost otopina i najbolji omjer suhe tvari i smjese za ekstrakciju. Za ekstrakciju α-tokoferola iz praškastog dodatka prehrani kao najbolji omjer utvrđen je 1:1,25, dok je za peršin i kelj najbolji omjer bio 1:50. Da bi se utvrdila ponovljivost mjerenja napravljena su tri uzorka ekstrakta peršina u omjeru 1:50, uzorci su ostavljeni 1h na tamnom i zatim pripremljeni na isti način kao i prethodni. Svaki uzorak injektiran je dva puta Srednja vrijednost rezultata mjerenja za prvo injektiranje je 0,833 mg/100g, dok za drugo injektiranje srednja vrijednost iznosi 0,823 mg/100g. Standardna devijacija rezultata dobivenih za prvo injektiranje iznosi 0,020, a za drugo 0,019. Relativno standardno odstupanje za rezultate dobivene za prvo injektiranje je 2,425%, a za drugo 2,299%. Interval pouzdanosti metode je za prvo injektiranje uzorka ±0,023, a za drugo 0,021.In this paper, the electrochemical properties of α-tocopherol were investigated by using cyclic and differential pulse voltammetry. The aim of the work was to examine the solubility of α-tocopherol in different solvents, the influence of pH, scan rate and concentration on its oxido-reduction properties, to investigate the possible adsorption of α-tocopherol on the surface of the working electrode, and to determine α-tocopherol in real samples using the HPLC method. When testing the solubility of α-tocopherol, the mixture of ethanol and water in a ratio of 70:30 proved to be the best. In the cyclic voltammogram of α-tocopherol, an anodic peak current was observed at the potential, Ep,a = 0,175 V, which corresponds to the irreversible oxidation of α-tocopherol. It was determined that the oxidation of α-tocopherol is a diffusion-controlled process, because a linear dependence of the oxidation peak current (Ip,a) vs the square root of scan rate ( 1/2) was obtained, and the slope of the line log Ip,a vs log was around 0,5. One oxidation peak current was also detected in the differential pulse voltammogram of α-tocopherol at the potential, Ep,a = 0,070 V, which increased linearly with the increase of mass concentration of α-tocopherol in the concentration range from 0,43 mg dm-3 to 70,41 mg dm-3. The content of α-tocopherol in real samples, the solutions stability, and the best ratio of dry matter to the extraction mixture were examined by high-performance liquid chromatography. For the extraction of α-tocopherol from the powdered food supplement, the best ratio was 1:1,25, while for parsley and kale the best ratio was 1:50. In order to determine the repeatability of the measurements, three samples of parsley extract were made in a ratio of 1:50, the samples were left for 1 hour in the dark and then prepared in the same way as the previous one. Each sample was injected twice. The mean value of the measurement results for the first injection is 0,833 mg/100g, while for the second injection the mean value is 0,823 mg/100g. The standard deviation of the results obtained for the first injection is 0,020, and for the second 0,019. The relative standard deviation for the results obtained for the first injection is 2,425% and for the second injection 2,299%. The confidence interval of the method is ±0.023 for the first injection of the sample, and 0,021 for the second