In the framework of a certification campaign, organised by the European Community Bureau of References (BCR), quadrupole inductively coupled plasma - mass spectrometry (ICP-MS) was used for the determination of the aqua regia soluble contents of Mn, Co, Ni, Cu, Zn, Cd, Hg and Pb in three different soil and sludge candidate reference materials (BCR CRM 141R, 144R and 146R). Especially for the transition metals, for which accurate quantitation is often hampered by the presence of spectral interferences, great care was taken in avoiding systematic errors due to overlap with signals from polyatomic ions and/or isobaric nuclides showing the same nominal mass as the nuclides of interest. Si-based interferences were avoided by additional HF-treatment of the samples obtained after aqua regia extraction. The presence of other potential interferences was checked by registration of signal behaviour plots (signal intensity as a function of the nebulizer gas flow rate) and/or measurement with a high resolution ICP - mass spectrometer. In order to obtain the highest accuracy possible, bath external standardization and the method of standard additions were used for calibration and the results obtained were compared. Hence it was shown that, despite the use of an internal standard, for some elements accurate results could only be obtained using the method of standard additions. Finally, for Hg and Pb, besides the calibration techniques already mentioned, also isotope dilution was used. For all materials analysed, the results obtained are in excellent agreement with the values finally certified.