Abstract
A new voltammetric method for the simultaneous determination of vitamin E and vitamin K present in different types of commercially available food supplements has been developed. This electroanalytical method is based on the ex situ adsorptive accumulation of these biologically active compounds onto the surface of a solid glassy carbon electrode (GCE) with subsequent electrochemical detection by square-wave adsorptive stripping voltammetry in 0.01-mol L−1 HNO3 containing 0.1-mol L−1 KCl at pH 2.08. Due to reversible electrochemical reactions of phylloquinone, a subsequent voltammetric detection of both vitamins in anodic mode can be performed. Since individual forms of vitamins E and K usually exhibit nearly identical electrochemical behavior, it is therefore impossible to distinguish individual forms (quinones and tocopherols) and determine their molar concentrations in this way. Thus, the values of vitamin content were expressed as mass equivalent of phylloquinone and α-tocopherol as they are the most biologically active forms. Despite the high sensitivity, relatively short linear ranges were obtained due to the interaction (competition) of both vitamins during adsorption onto the freshly polished surface of the GCE from a 50% aqueous–acetonitrile mixture. The obtained results showed that the voltammetric approach is a very simple and low-cost analytical method that can be used in analyses of food supplements.
Highlights
Vitamin E and K belong to a group of fat-soluble vitamins [1,2,3,4], which are classified as non-polar organic compounds [5]
All the lipophilic vitamins present in a sample can be accumulated onto the nonpolar surface of a freshly polished glassy carbon electrode (GCE) from an optimum aqueous–organic mixture, and after applying a negative deposition potential over a period of time, they can be anodically oxidized sequentially in one step according to their different standard redox potentials [4]
The presented square-wave adsorptive stripping voltammetry (SWAdSV) method is assessed in terms of analytical parameters, namely the calibration range, limit of detection, precision, accuracy and feasibility of using the standard addition method in a real analysis
Summary
Vitamin E and K belong to a group of fat-soluble vitamins [1,2,3,4], which are classified as non-polar organic compounds [5]. All the lipophilic vitamins present in a sample can be accumulated onto the nonpolar surface of a freshly polished glassy carbon electrode (GCE) from an optimum aqueous–organic mixture, and after applying a negative deposition potential over a period of time (electrochemical reduction of phylloquinone to phyllohydroquinone; H2VK1), they can be anodically oxidized sequentially in one step according to their different standard redox potentials [4]. The developed analytical method is based on an adsorptive accumulation of these biologically active compounds onto the surface of a GCE with subsequent electrochemical detection using SWV in 0.01-mol L−1 HNO3 containing 0.1-mol L−1 KCl (pH 2.08). The presented square-wave adsorptive stripping voltammetry (SWAdSV) method is assessed in terms of analytical parameters, namely the calibration range, limit of detection, precision (repeatability), accuracy (recovery) and feasibility of using the standard addition method in a real analysis
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