Determination of the interchangeable heavy-metal fraction in soils by isotope dilution mass spectrometry

An isotope dilution technique using enriched stable isotopes is applied to determine the interchangeable heavy-metal fraction in soils. Metals in two soil samples are extracted at constant pH, with water, HH4NO3, and EDTA. A spike of enriched stable isotopes is added to the suspension of sample and eluant at the beginning of the extraction. The heavy-metal fraction which e xchanges with the added spike during the extraction is called the interchan geable fraction. The extractable heavy-metal fractions are obtained from th e heavy-metal concentrations in the eluates. Isotope ratios and concentrati ons are determined by HR-ICP-MS. The isotope dilution technique described e nables both the extractable and the interchangeable heavy-metal fractions t o be determined in the same experiment. The combination of both results giv es additional information on elemental availability under different conditi ons that cannot be obtained by analyzing the extractable heavy-metal fracti ons alone. It is demonstrated that in some cases different eluants just shi ft the distribution of the interchangeable fraction of an element between t he solid and liquid phases (e.g., Pb and Cd in a topsoil sample) while the amount of the interchangeable fraction itself remains constant. For other e lements, as Ni, Zn, and Cr, the use of different eluants (different pH, com plexing agents) sometimes enlarges the interchangeable fraction.