## 1 Introduction  
Oxidative stress can be associated with many pathologies [1, 2]. Together with other compounds (vitamin C, vitamin E, thiotic acid, glutathione, etc. ), zinc provides protection against oxidative damage [1, 2, 3]. Zinc is the second most abundant metal in the human body [4]. Practically, it is an oligo-element essential for human health because it is a component of proteins involved in cell structures [5] and on the other hand it is a cofactor for more than 300 enzymes involved in many processes, including the cellular respiration, immunity, DNA and protein synthesis, metabolism, cell division, etc. [3, 4, 5, 6, 7]. Thus, its deficiency can lead to serious damage, being responsible for different pathologies, for example, the acute deficiency decreases the innate and adaptive immunity and chronic deficiency increases the production of pro-inflammatory cytokines and leads to severe inflammatory diseases such as rheumatoid arthritis [5]. Obese patients showed alteration of zinc metabolism and that can lead to endocrine disorders. Zinc-\(\alpha\)2-glycoprotein (Zn-2\(\alpha\)-G) is an adipokine with lipid-mobilizing and anti-inflammatory activity. Thus, low levels of serum zinc and Zn-2\(\alpha\)-G were identified in obese patients [8]. On the other hand, blood clotting disturbances are related to zinc intake: hypozincemia leads to poor aggregation of platelets and prolongation of bleeding time and, opposite, hyperzincemia leads to increased coagulability [9]. The zinc supplementation is efficient in the management of patients with type 2 diabetes mellitus (DM2), which exbibit hypozincemia (possibly due to reduced gastrointestinal absorption). It was observed that zinc supplementation in DM2 patients resulted in reduced glycated hemoglobin percentage [10]. Likewise, zinc supplementation was proved to reduce severity and duration of diarrhea in children because of intestinal mucosa regeneration and facilitation of electrolyte transport [11]. According to another study, zinc is linked to oxidative stress and may be important in prevention and treatment of sarcopenia [12]. Additionally, zinc proved to be effective in the treatment of acne vulgaris, in several clinical trials [13]. Active transport of zinc inside prostate cells is performed by a zinc transport protein (ZIP1). ZIP1 downregulation may be involved in the decrease in zinc inside malignant cells. Zinc treatment of human prostate tumors was demonstrated [14]. Nevertheless, zinc may exhibit several adverse events and may interact with quinolones and tetracycline (decreasing antibiotic absorption) [15]. On the other hand, using zinc in the treatment of neonatal hyperbilirubinemia is not encouraged [16]. Therefore, dietary supplements (DS) can be useful in treatment of several pathologies, but special attention must be paid to nutrivigilance issues [15]. Several zinc complexes with active pharmaceutical ingredients were synthetized and tested in order to improve biological properties of the parent molecules [17; 18; 19; 20; 21]. Thus, for improving their efficiency and reducing the risks, the quality control of DS is essential for their biopharmaceutical profile. Many papers were published describing quantitative determination of zinc in pharmaceutical and biological matrices: neutron activation analysis [22], AAS [23; 24], voltammetry [25; 26], differential-pulse polarography [27], spectrofluorimetric analysis [28], electrodialysis coupled with AAS [29], ion-selective electrode [30], capillary zone electrophoresis [31], solid phase photometry [32; 33], derivative spectrophotometry [34; 35; 36; 37], and 0 order spectrophotometry [38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54]. Nowadays pharmacopoeias recommend quantification on zinc in dietary supplements by means of atomic absorption spectrometry (AAS) [55]. The AAS method requires expensive equipment and relatively expensive reagents. Zinc forms binary [56] and ternary [57] complexes with Xylenol Orange (XyO) in acidic buffered media.