Speciation analysis of elements in cereal flours by liquid chromatography–inductively coupled plasma mass spectrometry

Phosphorus, manganese, iron, cobalt, nickel, copper, zinc, and molybdenum species fractions were studied in maize and rye flour. Total and extractable contents of elements were determined by ICP–MS. Extracts of flours were prepared using 0.02 mol/l Tris–HCl buffer solution (pH 7.5) and 70% (v/v) ethanol as extractants, respectively. Both types of extracts were analysed by size exclusion chromatography (SEC) using a Superdex 75 HR 10/30 column on-line coupled to an ICP–MS. The 0.02 mol/l Tris–HCl buffer solution, pH 7.5 served as the mobile phase. Cobalt, nickel, zinc and molybdenum compounds were found in the low molecular mass region (<1 kDa). The main fraction of phosphorus compounds was found in the 3.5 kDa region. Remaining phosphorus compounds were detected in the high-molecular (>150 kDa) and the low-molecular mass (<1 kDa) regions. Minor amounts of copper and phosphorus (in both samples), zinc (in maize flour) and molybdenum (in rye flour) were found in the high molecular mass region (45 to >150 kDa). When the flours’ extracts are spiked with cupric ions, practically all copper is bound to low molecular mass compounds. The low molecular mass fractions of unspiked flours’ extracts obtained by SEC were further separated by reversed phase chromatography using a Kromasil C4 column and 10% methanol as the mobile phase.