Matrix problems in the certification analysis of botanical materials by neutron activation analysis

Plant materials often contain a mineral fraction as an inseparable part of the plant matrix. Methods for trace analysis in biological materials may not include the amount of determinand present in such mineral fractions, but for certification purposes it must be included. Instrumental methods of analysis, such as INAA, automatically include the total amount of an element, regardless of its chemical or physical form; other methods, including RNAA, determine only the amount of element in solution. For certification analysis either the entire sample has been completely dissolved, or the insoluble residue should not contain the determinand. In BCR certification analysis for As and Se by RNAA the irradiated sample was decomposed with sulphuric and nitric acids, and CRM 279 Sea Lettuce and CRM 402 White Clover left an insoluble residue. Unirradiated material was then digested without carrier addition, and the insoluble residue separated by filtration. No significant content of Se was found by INAA, but about 5% of the total amount of As was found in CRM 402. The ramifications of making a correction are discussed, and it is concluded that a correction for an insoluble fraction may lead to a positive bias.