Micelle mediated methodology for the determination of free and bound iron in wines by flame atomic absorption spectrometry

A methodology utilizing the cloud point phenomenon for the determination of free and tannin-bound iron in wines is presented. The method employs precipitation of the tannins and other phenolic and insoluble compounds in the micelles of a non-ionic surfactant mixture (TX-100 and TX-45) upon increase of the solution temperature, which are subsequently separated from the initial solution by centrifugation. The surfactant-rich-phase containing the tannins and the insoluble iron fraction is directly aspirated into the nebulizer of a flame atomic absorption spectrometer after its uptake with a methanolic solution of HNO3. The supernatant is submitted to the same cloud point extraction procedure for the determination of free iron species in the presence of a chelating agent, ammonium pyrrolidine dithiocarbamate (APDC), in order to form water-insoluble complexes with free iron. The complexes are extracted in the micelles and directly analyzed after they are diluted in a methanolic solution as described above. The total content of iron was also determined by conventional methods for comparison, yielding satisfactory results. The calibration graph was rectilinear up to 0.35 mg 1(-1) Fe, with detection limits of 0.02 mg 1(-1) with a relative standard deviation of 2.4%. The method was successfully applied to red and white wines. (C) 2002 Elsevier Science B.V. All rights reserved.