Determination of selenium in food supplements by differential-pulse cathodic stripping voltammetry in the presence of added copper

Selenium was determined by cathodic stripping voltammetry in a 1 mol l ^-1 HCl acid solution containing added Cu^II. In this medium, selenium was preconcentrated on the hanging mercury drop electrode and stripped cathodically in differential-pulse mode. After a deposition period of 1 min, 0.2 ng ml^-1 of selenium could be detected. The method was applied to the analysis of food supplements. Total selenium was determined after digestion of the sample with HNO₃ - HClO₄ and reduction to the electroactive Se^IV by heating with HCl. Inorganic selenium (i.e., selenite and selenate) was similarly determined after extraction with dilute sodium hydroxide solution and clean-up on activated carbon. Representative over-the-counter preparations have been analysed.