AbstractAn ICP-MS with an octopole reaction/collision cell was used for the multi-element determination of trace elements in sea-water. The use of a reaction or collision gas, respectively, reduces serious spectral interferences from matrix elements such as ArCl+ or ArNa+. On introducing He or H2 to the cell, detection limits of 0.3 ng L−1 for U to 20 ng L−1 for Fe and As were determined in 10-fold diluted sea-water. The trace elements V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Mo, Cd, Pb and U were determined in diluted sea-water samples (certified reference material: NASS-5, SLEW-3) with a trueness of ±20% to the certified values. For most of the elements an accuracy of better than 19% was achieved with no sample pre-treatment other than simple dilution.