The maximum contaminant concentrations (Maximal Contaminant Level, MCL) of inorganic arsenic and selenium species in drinking water should not exceed 10 and 50 µg/L, respectively. Given that each of the two elements occurs in two oxidation levels – trivalent and pentavalent – a separation step is necessary prior to ICP/MS detection. This Application Note shows the simultaneous determination of the two arsenic (arsenite and arsenate) and selenium species (selenite and selenate). Separation takes place on the Metrosep Dual 3 - 100/4.0 column.