One of the major difficulties associated with the chemical separation usually required in activation analysis for trace elements has been the need to determine the chemical yield of the separation process. In a separation using the substoichiometric principle, equal amounts of a reagent capable of converting the irradiated element and its isotopic carrier to an easily-separable form are added to the prepared sample and to the standard, but the reagent is sufficient to react with only a part of the element and carrier which is present. By this means the specific activities of the extracts or precipitates are made proportional to the total activities, and the determination of the absolute chemical yield of the separation is rendered unnecessary. Furthermore, the use of a substoichiometric amount of reagent usually increases the selectivity of the separation process.