Chemical Speciation in Natural Waters

Abstract

Chemical speciation may be defined as the determination of the individual concentrations of the various chemical forms of an element which together make up the total concentration of that element in a sample. The next decade will certainly see a blossoming of interest from chemists, biochemists, and biologists in techniques for chemical speciation. This is because it is becoming more and more apparent that both bioavailability and toxicity are critically dependent on the chemical form of the substance under test. Measurement of the total concentration of a nutrient or a trace element provides very little information about the bioavailability of the substance, since many vitamins and essential elements are now known to exist in a variety of chemical forms, the assimilability of which vary greatly.^1–3 For example, niacin and folic acid exist in some foods in bound forms which are unavailable to many species,⁴ and the same is true of a whole range of essential metals, including iron, zinc, cobalt, and chromium. For man, the most assimilable form of iron is heme iron from meat. Inorganic iron salts and simple organic complexes are far less available. The group of chromium-amino-acid complexes known as the glucose tolerance factor^1.5 provides most of the usable chromium, while cobalamin (vitamin B₁₂) is the only essential chemical form of cobalt.⁶ It has also been suggested that zinc-amino-acid complexes are more available than inorganic zinc.^2.7 Analysis of a diet for total iron, chromium, cobalt, or zinc, without some information about their chemical forms, is therefore almost worthless because a nutritional deficiency of these elements could exist despite their widespread distribution.