Mass spectrometric investigations of the kinetic stability of chromium andcopper complexes with humic substances by isotope-labelling experiments

Isotope-labelling exchange experiments were carried out to investigate the kinetic stability of Cr(III) complexes with humic substances (HS). To compa re the results with those of an ion, not expected to form kinetically stabl e HS complexes with respect to its electron configuration, Cu(II) was inves tigated under the same conditions. HS solutions of different origin were th erefore spiked with Cr-53(III) or Cu-65(II) after saturation of HS with chr omium and copper of natural isotopic composition. In fractions of metal/HS complexes with different molecular weight, obtained by ultrafiltration and HPLC/ICP-MS using size exclusion chromatography (SEC), respectively, the is otope ratios of chromium and copper were determined by ICP and thermal ioni sation mass spectrometry. Distinct differences in the isotopic composition of chromium were found in the permeate of the ultrafiltration compared with the corresponding unseparated solution, which indicates kinetically stable Cr(III)/HS complexes. On the other hand, the copper isotopic composition w as identical in the permeate and the unseparated solution, which shows that a total exchange df Cu2+ ions took place between free and HS complexed cop per ions. The SEC/ICP-MS experiments also resulted in a different isotopic distribution of chromium in the chromatographically separated complexes whe reas the copper complexes, separated by SEC, showed identical isotopic comp osition. The kinetic stability of Cr(III)/HS complexes could be explained b y the d(3) electron configuration of Cr3+ ions, a fact which is well known from classical Cr(III) complexes, and influences substantially the mobility of this heavy metal in the environment.