“…Many different chemical and physical preconcentration techniques have been proposed, including surface adsorption on activated carbon [18] electrodeposition [19], precipitation chromatography [20], liquid/liquid extraction [21], immobilized reagents [22], preconcentration on ion exchange treated polyurethane foam [23], plus a variety of other techniques well known to the analytical chemist. However, by far the most popular, and also the most successful, techniques have been those based on the use of ion exchange resin.…”