## 1 Introduction  
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]. Vitamin E (VE), known as the most active lipophilic antioxidant, exists as four tocopherol and tootrienol isomers [3; 4; 6; 7] whose sources include vegetable oils, nuts and seeds of plants [8; 9]. Vitamin K present in green plants (phylloquinone; VK1) and produced by bacteria (menaquinone; VK2) [10; 11; 12; 13; 14], is widely used in the diet for its anti-hemorrhagic properties [11; 12]. VE is essential in the protection of fatty acid chains in the lipoprotein bilayer in the cytoplasmic membrane [3; 4], because it preferentially reacts with peroxy radicals to form harmless oxidation products which are reduced back by the reduction properties of ascorbic acid (vitamin C) [15]. Due to VE's significant physiological effects, avitaminosis may contribute to an increased risk of developing several serious diseases of civilization, such as fertility disorder [16], cardiovascular diseases (heart attack and stroke) [1], Alzheimer's disease [17] and renal impairment [18]. VK is essential for the carboxylation reaction of glutamic acid, which is known to be a precursor of blood-clotting factors [19]. Its deficiency results in a deactivation of prothrombin, causing hemorrhage [20], prosthetic valve failure [21] and bone formation disorders [11]. From all of the above information, it is evident that analysis of the presence and levels of these vitamins is justified, especially for food quality control. For Czech legislation based on that from the European Union, the reference analytical method used for the determination of VE and VK in foodstuffs is high-performance liquid chromatography (HPLC) in either normal-phase or reversed-phase systems [22; 23; 24; 25] with spectrophotometric detection, known as CSN EN 12822 (560055) and CSN EN 14148 (560053), respectively. Moreover, it is worth mentioning that a recently investigated electrochemical detection method for both vitamins could be potentially used [1; 2], however, this electrochemical detection has not been implemented for the simultaneous determination of VE and VK. Several scientific papers suggest that there is a real chance to simultaneously determine VE and VK in various foodstuffs using adsorptive stripping voltammetry (AdSV) [2]. Principally, 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; H2 VK1), they can be anodically oxidized sequentially in one step according to their different standard redox potentials [4]. Compared to the standard HPLC method, its relatively high sensitivity and fast and easy sample preparation for analysis can be seen as a great advantage. However, AdSV is unable to recognize individual vitamin forms due to overlap of their peak current signals. Hence, a sum of individual forms is usually expressed as the concentration equivalent of the most biologically active form [1]. In this study, a simple and rapid AdSV procedure is presented with square-wave voltammetry (SWV) as an electroanalytical technique for the simultaneous determination of VE and VK in commercial food supplements. 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\({}^{-1}\) HNO\({}_{3}\) containing 0.1-mol L\({}^{-1}\) KCl (pH 2.08).