## 1 Introduction  
Urine is one of the oldest and most basic human specimens used to evaluate the presence, severity and cause of diseases within the kidney and urinary tract [1]. Several diseases can be traced back to an imbalance of trace elements [2, 3, 4, 5]. Since the early 1990s, trace elements have been attracting attention in a wide range of scientific areas related to human health such as clinical analysis, nutritional diagnosis and environmental analysis [6, 7, 8, 9, 10]. These elements may be either essential or non-essential. Essential elements (manganese (Mn), cobalt (Co), copper (Cu), zinc (Zn), molybdenum (Mo)) are those that are required for normal physiological processes in human beings [11]. Many elements are part of protein complexes that are required for enzymatic activities and may also play structural roles in tissue or cell membranes. Non-essential trace elements (aluminum (Al), vanadium (V), arsenic (As), cadmium (Cd), tin (Sn), lead (Pb)) are considered toxic and are not essential for physiological processes [9, 11]. Moreover, essential trace elements can cause toxicity if they reach excessive levels in the human body. Chelation therapy is commonly used by physicians to counteract excessive and harmful levels of non-essential trace elements. Recently, several researchers have claimed that chelation therapy can also help in treating many other conditions, including heart disease [12, 13], autism [14], Alzheimer's disease [15], and diabetes [12]. Chelation therapy is used to remove metals (e.g., Al, V, chromium (Cr), Mn, iron (Fe), Co, Cu, Zn, As, selenium (Se), Mo, Cd, Sn, barium (Ba), TI, Pb) from the body by the formation of a chelate complex. Chelatorsare chelating ligands that are used during chelation therapy. Chelators--such as EDTA (ethylenediaminetetraacetate), BAL (British anti-Lewisite), DMSA (2,3-dimercaptosuccinic acid), DMPS (2,3-dimercapto-1-propanesulfonic acid), and DPA (2- amino-3-methyl-3 sulfanylbutanoic acid)--mobilize metals from tissues and maintain the chelate complex during circulation to the kidneys and subsequent excretion in the urine [16]. Chelation therapy is a very effective way to remove several metals from the body. The elements selected to be measured in this work were therefore chosen based on their importance in chelation therapy and on their relevance to biological processes. Al is a non-essential trace element and has no physiological function in the human body. Al occurs naturally in soil, water, and air. Small amounts of Al are released into the environment from coal-fired power plants and incinerators. An exceedingly small amount of the Al in food or water can enter the human body through the digestive tract. However, most of this absorbed element is excreted by the human body. Nevertheless, an overload of Al over a long period of time can trigger disorders in the brain (such as Alzheimer's), and damage the liver, kidneys and bones [17]. Moreover, an overload of this element may lead to inflammatory effects or oxidative stress [18]. V is one of the minerals that is regularly ingested through our daily diet. As an ultra-trace element, V occurs only in very small amounts in the body. The highest levels of V are found in the liver, kidneys and bones. An overload of this element may cause chronic bronchi, lungs and intestine disease. Furthermore, visual disturbances, nausea and cardiac arrhythmia have been described [19; 20]. Another ultra-trace but essential element is Mn. It is found in the liver, kidneys, pancreas, and in bone marrow. Mn is absorbed through the small intestine and is transported through the blood plasma. Mn is mainly excreted via bile and urine. Due to today's good nutrition, Mn deficiency symptoms are hardly possible nowadays. Skeletal changes, coagulation disorders, and disturbed sperm formation were also observed [21]. In humans, a lack of Mn is suspected to cause oxidative stress and bone structure problems [22].