Indirect Determination of Soluble Trace Elements in Certified Reference Materials by Neutron Activation Analysis

Since 1983 we have analyzed aqua regia extracts from environmental reference materials of 5 different soils and 4 different sludges by neutron activation analysis. Aqua regia as such is not accepted for irradiation in a nuclear reactor, and therefore the solution has to be evaporated to dryness, mixed and dried to constant weight before an aliquot can be taken and subjected to instrumental neutron activation analysis (INAA). Results for leachable Cr and Zn were found to display greater variability than results for total contents of these elements, and the reasons for this will be discussed. For the 1994 certification campaign for one soil and two sludges we have chosen to determine the total content of trace elements in these reference materials by INAA before and after aqua regia leaching. The leachable contents are then found indirectly as a difference between the two results; in this way we eliminate the sources of uncertainty associated with the drying and handling of the aqua regia extracts, and we reduce the contributions from counting statistics.