Term (also microelement) for chemical elements that only occur in low concentrations (traces). If the concentration is extremely low, it is also referred to as an ultra-trace element. As a rule, in geochemistry, material components and accompanying elements of rocks and minerals in concentrations below 0.1% or 1000 ppm (millionths) are called trace elements. In analytical chemistry, the threshold value is 100 ppm or 100 µg/g (= 100 mg/kg) or 0.01 %. In biology, essential trace elements are chemical elements that are essential for a living organism (usually in relation to humans) and occur in proportions of less than 50 mg/kg in the organism.
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