Ferric oxide will be dissolved in concentrated H2SO4 by bubbling carbon monoxide through the solution over heat for approximately four hours. The solution will then be derated by passing purified nitrogen through it for several minutes. The volume will then be adjusted using derated water. Two ml aliquot of the resultants 57 FeSO4 and 58 FeSO4 solutions will be transferred into opaque ampules. Each ampule will be purged with nitrogen and sealed. Ampules will contain either 200 µg/ml of total iron (as ferrous sulphate) and 150 µg/ml of 57Fe or 700 µg/ml of total iron (as ferrous sulphate) and 420 µg/ml of 58Fe. Aliquot from an individual ampule will be used to prepare 57Fe and 58Fe-spiked working solutions.