This is what we found on Wikipedia regarding the Trace Amount:

A trace element is a chemical element of a minute quantity, a trace amount, especially used in referring to a micronutrient, but is also used to refer to minor elements in the composition of a rock, or other chemical substance.

In nutrition, trace elements are classified into two groups: essential trace elements, and non-essential trace elements. Essential trace elements are needed for many physiological and biochemical processes in both plants and animals. Not only do trace elements play a role in biological processes but they also serve as catalysts to engage in redox – oxidation and reduction mechanisms. Trace elements of some heavy metals have a biological role as essential micronutrients.