Analysis of ultratrace lithium and boron by neutron activation and mass-spectrometric measurement of 3He and 4He.

A new technique for analysis of lithium and boron at ultratrace concentrations (less than 10(-8)g g-1) is described. The method consists of mass-spectrometric assay of 3He from decay of tritium produced by thermal-neutron reaction on 6Li, and 4He produced by thermal-neutron reaction on 10B. Two neutron-irradiation facilities were used: the McMaster reactor, which is 235U-enriched and light-water moderated; and a graphite-moderated thermal column attached to the 235U-enriched, heavy-water-moderated core at the National Bureau of Standards (NBS) reactor. In the McMaster irradiations, fast neutrons (greater than 0.2 MeV) induce the reactions 14N(n, 3H)12C, 12C(n, alpha)9Be, 16O(n, alpha)13C, and 14N(n, alpha)11B. These reactions become serious sources of error in samples such as human blood which have very low concentrations of lithium and boron, and high concentrations of nitrogen, carbon and oxygen. In the NBS thermal column, fast-neutron reactions are virtually absent, and only corrections for thermal-neutron capture by deuterium, and thermal-neutron (n, alpha) reactions on oxygen, sulfur, chlorine, potassium, and calcium need to be taken into account. Results are presented for various actual samples including human blood and its components, and some standard biological reference materials, to provide a realistic base for other workers to judge the reliability of the method.