Speciation of metal-carbohydrate complexes in fruit and vegetable samples by size-exclusion HPLC-ICP-MS

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Abstract
Kinetically inert and thermodynamically stable metal complexes with polysaccharides were detected in aqueous leachates and enzymatic digests of apple and carrot samples by size-exclusion chromatography with parallel refractometric and ICP-MS detection. The method developed allowed detection in the water-soluble fraction and the identification of a high molar mass polysaccharide fraction (>50 kDa) containing Pb, Ba, Sr, Ce and B, whereas other metals (Zn, Cu, Mg) eluted as complexes with low molar mass non-carbohydrate compounds. The majority of the metal-carbohydrate complexes were located in the solid water-insoluble fraction of the analysed samples. An extraction procedure with a mixture of pectinolytic enzymes was developed to release these species into the aqueous phase. The metal-binding carbohydrate component was identified as the dimer of rhamnogalacturonan-II, a pectic polysaccharide present in plant cell walls. The unidentified residual metal species contained less than 5% of the metals present.