Speciation of tin in lemon juice: An example of trace metal speciation in food

A scheme is presented for the separation and identification of different tin species in lemon juice. Soluble tin species are separated from other constituents by means of column-switching liquid chromatography and detected by electrothermal atomic absorption spectrometry (AAS). Other metals (copper, zinc, iron, calcium and magnesium) are detected in the same fraction by on-line flame-AAS. Gas chromatography/mass spectrometry of the organic ligands after derivatization confirmed that citric acid is the important ligand for metal binding in lemon juice. Tin is also partly bound to particulate matter, which is investigated by infrared spectroscopy and model experiments. It is shown that tin is most probably bound to carboxylic groups of pectin (analogously to calcium). Investigations of different lemon juices showed large variations of total tin, but few differences between tin species. Only one sample contained organotin compounds at a detectable level.