“Trace elements” are such building blocks of our planet and all living organisms, which, although occurring in rather modest concentration levels, are indispensable for a plethora of metabolic processes. Especially since the last decades, we observe enormously increasing efforts devoted by the scientific community to investigate, characterize and quantify trace elements. So-called “essential trace elements” are important constituents of human food, animal fodder, plant fertilizers, or cultivation media to form biotechnologically relevant microbes. Trace elements travel from the soil through the food chain, starting from phytoplankton until reaching our dinner tables; apart from food, drinking water is another important source for trace element uptake by all organisms. This makes trace elements interesting for diverse scientists such as analytical chemists, biochemists, geologists, physiologists, zoologists, and botanists .
In dependence on the scientific realm, different definitions for the terminus “trace elements” are found. An analytical chemist considers an element in a given sample with an average concentration of less than 100 ppm on an atomic counting basis or less than 100 μg/g on a mass basis, a “trace element”. In contrast, biochemists define “trace elements” as those elements, which, although present only in tiny amounts, are needed to maintain the physiological balance of an organism, often acting as cofactors in enzymatic reactions; this biochemical definition encompasses various heavy metals (iron, copper, nickel, vanadium, cobalt, manganese, molybdenum, chromium, and zinc), some nonmetals (boron and iodine), and certain metalloids (selenium, silicon, and arsenic). This definition implies a daily requirement for “essential trace elements” by humans in amounts between 50 μg and 18 mg/day . To become susceptible toward metabolizing by animals and humans, some trace elements need to undergo transformation by microbes into complex bioavailable forms, as observed in the case of cobalt, which is utilized mainly as cyanocobalamin (vitamin B12) [3, 4]. Moreover, geologists define trace elements by concentrations not surmounting 1 pro mille of a rock or mineral. In addition, the term “trace element” is frequently used when analyzing the elemental composition of igneous rocks, hence those rocks formed by magma. In mineralogy, trace elements can substitute network-forming ions in mineral crystal structures; here, trace elements are not vital to a mineral’s defined composition and do not appear in the mineral’s chemical formula. However, as well known in the case of quartz, those metals occurring in trace quantities result in characteristic coloration of the minerals; for example, the substitution of silicon by iron in traces gives the quartz amethysts its famous purple coloration .
Keeping with trace elements playing a role in human metabolism, we find at least two of them as so-called “vital poisons” in the periodic system of elements, namely chromium and arsenic, well-known toxins, which, nevertheless, are essential for the functioning of our metabolism. Apart from the 14 undisputed essential trace elements mentioned in the above paragraphs, others, such as fluorine, strontium, or lithium, are suggested to also display biological functions in humans, which, although not clearly elucidated yet mechanistically, are evidenced by element deprivation effects in diverse metabolic studies [1, 6]. In addition, limited circumstantial evidence for certain benefits or biological function in mammals is reported in the case of the metals aluminum, rubidium, cadmium, germanium, tin, and lead, the so-called “ultra-trace elements” . Only for prokaryotes, a physiological role of tungsten  and lanthanum  is generally accepted. What is not included in this list are all those elements which are present in significant amounts in our body, such as the four biological basic elements hydrogen, oxygen, carbon, and nitrogen and the macroelements sulfur, chlorine, phosphorus, magnesium, sodium, potassium, and calcium.
2. Determination of trace elements
However, trace elements are not only blessing but also curse by generating environmental and health-related concerns when exceeding certain concentration levels. In the context of environmentally sensitive trace elements, it is often not easy to draw a clear line between the desired concentration range of a trace element and the range in which it already exerts toxicity . Aquatic environments, soil, organisms, food, fodder, energy carriers like coal, and airborne particles are targets, which are potentially contaminated with trace elements; this, in turn, provokes the need for modern, reliable, and fast tools for determination of trace element in diverse matrices. Although many problems caused by trace elements are already well managed in the meanwhile, constant vigilance in environmental exposure, application, and nutritional supply of trace elements still displays the