In the earlier discussion of the major elements in seawater, the arbitrary division between major and minor elements was set at a mass concentration of one milligram per kilogram, equal to one part per million. This is a convenient separation, because only salts present in concentrations somewhat greater than 1 mg kg-1 can have a detectable effect on the density and related physical properties of seawater. Most of the elements present in smaller concentrations are not conservative in seawater; by examining the distributions of these elements we may learn much about the biological and geochemical processes in the ocean. Furthermore, the analytical difficulties of measuring or even detecting many of the substances present in small concentrations in seawater often pose special problems that must be addressed during any consideration of these elements.

Analytical considerations

In approaching the analysis of any substance present in seawater at a concentration of, for example, around 1 µg kg−1, a value common for some important elements, it should first be realized that the substance is present in a matrix consisting of 35 000 000 µg of a complex mixture of salts and other substances. This fact alone can impose severe difficulties. Furthermore, when dealing with concentrations at this level or below, it is often not appreciated that equivalent or even much greater quantities of the substance to be analyzed may be introduced during collection at sea, as contaminants in the reagent-grade chemicals required for the analysis, in dust from the air, and on the walls or in the very composition of ordinary laboratory glassware and other equipment.