In the frame of a certification campaign organised by BCR (Bureau Communautaire de Reference, Commission of the European Communities, Brussels), Al, Ca, Cl, Fe, Mg, Mn and Na were determined in two candidate reference materials, low mineral content fresh water CRM 398 and high mineral content fresh water CRM 399. The concentration of the elements ranged from 30 ng/g (for Mn) to 80-mu-g/g (for Ca). Metals present at relatively high concentrations were determined under operating conditions of non-optimal sensitivity, but with no loss in precision. The accuracy and precision of the results obtained by ICP-MS were checked by comparing with other analytical techniques and laboratories. The agreement was excellent. This study showed that ICP-MS not only yields excellent results for interference-free elements at trace levels, but is also competitive for less ideal elements e.g. Ca, Fe and Cl.