Markers in analytical chemistry

The growing use of markers in analytical literature in the 10 years, 1991-2000, is presented and discussed because of their relevance in modern analytical chemistry. The complementary and contradictory aspects of markers and others related words, such as tracer, indicator, index, labelling compound, etc., are clarified. To offer a general overview, several classifications of markers are outlined. The main distinction between markers is their internal or external fitness for purpose. Selected examples are assessed on this basis.