## 1 Introduction  
The interest in selenium compounds grew as we learned more about its role in the human body. Today, it is widely recognized that this element is an essential part of antioxidant enzymes, and has important role in scavenging free radicals, regulating their content during biochemical reactions and protecting cells from oxidative stress [1; 2; 3]. Long-term oxidative stress can cause the development of many chronic diseases, such as cancer, diabetes, cardiovascular disease, and immune dysfunction. Moreover, the reduction of reactive oxygen and nitrogen species is important for the prevention of aging-related diseases. From the other side, prolonged intake of selenium in high doses can lead to health problems, such as hair loss, muscle tremors, stomach upset, kidney failure, and enhanced risk of type 2 diabetes [4]. The recommended dietary allowance of selenium is in the range of 50-70 ug per day for adults, and it is based on the concentration of selenium in plasma or serum as well as the activity of glutathione peroxididases (selenium-dependent antioxidant enzymes) in plasma [5; 6]. The soils in Europe are relatively poor in Se, thus, selenium consumed with the diet does not always provide sufficient intakes of this element. For this reason, Se-enriched foods and different supplements have been proposed to solve these problems [7; 8]. One of the selenium sources in the diet may be the products of plant origin. Some plants could accumulate inorganic selenium species from soil and transform them into active selenoamino acids, which are more bioavailable and less toxic than Se inorganic forms [9; 10]. Selenomethionine (SeMet) and Se-methylselenocysteine (MeSeCys) are the main selenoamino acids naturally occurring in dietary sources. It was reported that these compounds display strong antioxidant activities and cytotoxicity against varied cancer cells [11]. As various selenium chemical forms have different biological utilizations, toxicities, and nutrition values, detailed knowledge of the contents of individual selenium compounds is important to evaluate their impact on human health. Moreover, some regulations require specification of the active ingredients of foodstuffs or supplements, thus, speciation analysis is necessary [12; 13; 14; 15]. Liquid chromatography and electrophoretic separation, both coupled to inductively coupled plasma mass spectrometry detection, are the most frequently used techniques for selenium speciation analysis [14; 15]. However, analysis of selenium species in food samples and supplements creates difficulties due to the possible instability of Se compounds in extraction media and during storage [16; 17; 18; 19; 20; 21; 22]. The biggest problems presented in the literature data are connected with the oxidation of selenomethionine during the preparation of various kinds of samples [17; 18; 21; 22; 23]. [PERSON] et al. reported that selenomethionine oxide (SeMetO) had started to appear as soon as selenised yeast, a very popular Se supplement, was dried and tableted [24]. The presence of SeMetO was found even in soil samples [25]. Probably a sample matrix has a great impact on the stability of selenium species. It was found that acidification of a sample significantly increases the stability of inorganic selenium compounds, but such treatment caused a drastic degradation of SeMet in the extract of dietary supplements based on Se-enriched yeast [18]. One of the methods applied for preventing SeMet oxidation is the addition of thiols, e.g., dititoreitol (DDT) or \(\beta\)-mercaptoethanol (\(\beta\)ME) [21; 25]. However, as we reported earlier, single addition of these thiols was not enough to prevent the Se oxidation process in tea extracts, but it was proved that the state of equilibrium is established between the oxidized and reduced form of SeMet [22]. In this work, we present expanded studies on the stability of selenium compounds as a function of DTT and \(\beta\)ME concentration on the example of beetroot juice samples.