This International Standard specifies a method for the extraction, with aqua regia, of trace elements from soils and similar materials, prepared according to ISO 11464, and containing less than about 20 % (m/m) organic carbon according to ISO 10694. Materials containing more than about 20 % (m/m) organic carbon will require treatment with additional nitric acid (see note 8). The resulting solution is suitable for the determination of trace elements using appropriate atomic spectrometric techniques. With high solute concentrations in extract solutions, spectral interferences and background enhancement should be expected. NOTE 1 Aqua regia will not totally dissolve most soils and similar materials, and the efficiency of extraction for particular elements differs from element to element. Such efficiency might also differ for the same element in different matrices. Users of this International Standard should carry out a programme of analysis using reference materials to assure themselves that the method given here is appropriate for their needs. Elements extractable in aqua regia cannot, therefore, be decribed as "totals"; conversely they cannot be regarded as the "bio-available" fraction, as the extraction procedure is too vigorous to represent any biological process.