Introduction

1.1.1 Essential Trace Elements

Food and beverages ingested by humans represent a potentially proficient pathway of exposure to toxic and nutritionally important minor and trace elements. Many mineral elements occur in living tissues, food and diets in such small amounts that they are frequently described as "traces" and the phrase "trace elements" arose to describe them. At the present time, less than one-third of the 90 naturally occurring elements are known to be essential for life.

The bulk of human body is composed of six major elements; oxygen, carbon, hydrogen, nitrogen, calcium and phosphorus and six minor elements; sulfur, potassium, sodium, chlorine, magnesium and silicon. The total percentage of minor and major elements in total body weight is 98.8 (Table 1.1). If six noble gases are excluded as unlikely to have a physiological function, 71 elements of the periodic system remain, and because of their low concentration in living matter, are termed the "trace elements".

The concentration of major and minor elements in living tissues can be expressed in grams per kilogram. On the other hand, the concentration of trace elements in living tissues varies between 0.01 and 100 mg kg-1 (Table 1.2). It may not be appropriate to classify them as essential or toxic elements. It is logically wrong to establish a category of "toxic" elements, because any element may be potentially toxic and this property is but a function of concentrations to which humans are exposed. Essentiality of the trace elements is established when a further reduction below the range of tolerable levels, better known as "range of safe and adequate intakes", results in a consistent and reproducible impairment of a physiological function.

These considerations suggest a logical classification of the 71 trace elements into those, with proven essentiality and the rest for which essentiality is "presently not known". This classification leaves room for the possibility that future research will include additional elements as essential. Each of the two categories can be subdivided according to their practical importance under given conditions; e.g., local, regional or national imbalance in the environment, industrial emissions or dietary habits. Some essential elements may not be of any nutritional concern at all, as in the case of magnesium in human nutrition, since it is in sufficiently high levels; others, such as selenium, may have the highest regional importance because of deficiency in one area and toxicity in another.

In recent years, there has been an increase in the realization of the importance of the role of trace elements in biological systems. The study of life processes shows that many vital functions are dependent on the presence of a specific trace element. Because of that, trace elements are one of the important nutrient factors for the growth and maintenance of human and animal life.

Food only, excluding intakes from water and air, normally supplies a major proportion of the total daily trace element intake by humans. Since the late 1950s, concerns over the introduction of trace elements and many other components into the environment as a result of human activities have greatly increased. Besides soil and water, food is also contaminated with trace metals by the introduction of mechanized farming, ever increasing use of chemicals, sprays, preservatives, food processing and canning. In order to get the minimum adverse impact, it is important to measure and continuously monitor their levels in various food items, total diet, water and inhaled air.

The concentrations of trace elements in food give important information about dietary habits of special group, health situation of individuals and origins of elements. Therefore, it is important to determine the daily dietary intake of trace elements, their concentrations and sources.

Recent developments of trace element research in the area of nutrition have led to a need to accurately and precisely determine the content of these micronutrients in food. In the past several decades, the analytical chemistry community has made great advances in improvement in sensitivity, selectivity and accuracy of analytical methodology.

In this book, we will present experimental techniques for the collection, preparation and determination of trace elements in food. All the modern techniques will be discussed in some detail so that it will be useful for both researcher and technical staff who are working in this area.

1.1.2 Classification of Trace Elements

The simplest definition of trace essential element is that it is required in small amount for the maintenance of life; its absence results in death or a severe malfunction of the organism.

All major and minor elements are important; besides that, some of the trace elements e.g; Cr, Fe, Co, Cu, Zn, Se, Mo and I are essential trace elements; and some of them; Mn, Si, Ni, B, V, and Sn are probably essential trace elements; and further some of them F, As, Cd, Pb, Al and Hg are considered potentially toxic, some possibly essential elements for animal and human life. Actually all essential elements may also be toxic in animals and humans if ingested at sufficiently high levels and for a long enough period (Fig.1.1). The above elements will be discussed in detail in Chapter 13.

Essential trace elements are required by man in amounts ranging from 50 µg day-1 to 20 mg day-1. The organism can neither grow nor complete its life cycle without the element in question. The element should have a direct influence on the organism and be involved in its metabolism. The effect of the essential element cannot be wholly replaced by any other element.

The bioavailibilities of the essential elements depend on their chemical form, the compositions of diet and health situation of the individuals. Thus, establishment of the optimum daily requirements and determination of actual daily intake of essential elements are important problems of trace element in nutrition.

The essential trace elements provide a classical example of required nutrients as described by Bertnard as early as 1911. An organism may go through several stages as the concentration of essential nutrient progresses from deficiency to excess. In absolute deficiency, death may result, with limited intake; the organism survives but may show marginal insufficiency. With increasing nutrient, a plateau representing the optimal function is reached. As the nutrient is given in excess, first marginal toxicity then mortal toxicity are...