Speciation of Cu, Fe and Mn in beer using ion exchange separation and size-exclusion chromatography in combination with electrothermal atomic absorption spectrometry

Abstract

Methods were developed for studying the speciation of Cu, Fe and Mn in beer. The charge of the metal species was determined using cation and anion exchange separation based on solid-phase extraction cartridges, and the molecular weight of the metal complexes was studied by size-exclusion chromatography. Electrothermal atomic absorption spectrometry was used off-line for the quantitative determination of the metals. It was found that the beer contained ligands that released Cu and Fe from the separation columns; therefore, special procedures for cleaning the columns (with EDTA) had to be developed. The speciation analysis indicated that Mn was probably present as Mn²⁺ in beer, whereas Fe was found as negatively charged complexes and Cu as both negative and positive species. Using globular proteins for calibration of the size-exclusion column, the complexes were found to be of the order of 4–12 kDa. The ligands responsible for the complexation of Fe and Cu may be compounds such as polymeric phenols or phytic acid. When the beer was spiked with Cu²⁺ and Fe³⁺, the added metal ions were recovered as negatively charged species, indicating that the beer contained free metal-complexing ligands. It was found that the total metal concentrations in beer could be determined by direct analysis of 1 + 9 diluted beer; the results agreed with those obtained after decomposition of the beer with HNO₃–H₂O₂.