Determination of indium in natural waters by flow injection inductively coupled mass spectrometry

Two methods have been developed to measure indium (In) in natural waters by flow injection inductively coupled plasma mass spectrometry (ICPMS). One is the isotope dilution technique using an 113In enriched spike and the other utilizes natural yttrium present in the sample as an internal standard. In the former, optimization of the 113In spike to minimize error is often difficult for samples in which In concentrations are variable, whereas in the latter method, a separate determination of Y in the sample is necessary and hence more sample is required. Using about 1 litre of water sample 200 fold preconcentration of In was performed by solvent extraction and back extraction technique and then introduced into the ICPMS to measure the 113/115In or 115In/89Y ratios. The detection limits were 0.01-0.02 pmol kg-1 for both methods. Application of the methods to seawater samples yielded the concentrations of 0.06-0.15 pmol kg-1 for the Pacific and 0.6-1.5 pmol kg-1 for the Atlantic. The large inter-oceanic variation of In best resembles that of Al amongst the 3B group of elements in the periodic table. River and estuarine samples gave a more variable range of concentrations of 0.01-15 pmol kg-1. Most of the In supplied by rivers is removed by scavenging in the estuarine mixing zone, suggesting that the fluvial input of In to the ocean is small. (author)