Many mineral elements occur in plant and animal tissues in such minute amounts that early workers were unable to measure their precise concentrations with analytical methods then available. They were therefore described as occurring in traces, hence the term " trace element." This is still in use despite the development of modern analytical laboratory techniques such as atomic absorption spectrometry and neutron activation analysis which have an ability to measure all trace elements in the smallest of biological samples with great precision and accuracy. In fact, it could be argued that since the development of these highly sophisticated techniques, the term 'trace' has become scientifically obsolete.

A trace element is considered as essential for both man and animals if it meets the following criteria; a) It is present in all healthy tissues, b) Its concentration from one species to the next is fairly constant.

c) Depending on the species studied, the amount of each element has to be maintained within its required limit if the functional and structural integrity of the tissues's to be safeguarded, and the growth, health, and fertility to remain unimpaired.

d) Its withdrawal induces reproducably the same physiological and/or structural abnormalities. e) Its addition to the diet either prevents, or reverses, the abnormalities.