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Ion-exchanger phase absorptiometry, in which the absorbance of a coloured species sorbed on the ion-exchanger phase is measured directly, was applied to the determination of trace amounts of iron, using flow analysis. The method was more sensitive than the corresponding solution-phase method by a factor of 100 or more. For the determination of iron in highly purified water, the sensitivity was further increased by on-line preconcentration. The system blank could be reduced by passing the carrier solutions through respective on-line purifying columns. After the sample had been introduced onto a small cation-exchange column, the iron concentrated on the column was eluted with an acetate carrier solution, and then mixed with the other carrier stream which contained a pulse of 4,7-diphenyl-1 ,I 0-phenanthrolinedisulfonate (DPPS) reagent solution. The coloured iron-DPPS complex, formed in the stream, was introduced into a flow-through cell, the light path of which had been partly filled with anion exchanger. The increase in attenuance due to the sorption of the coloured complex was measured continuously. The detection limit was found to be 0.01 pg dm-3, when an 80 cm3 water sample was used.

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