An instrumental neutron activation method for multi-element geochemical mapping

Abstract

The application of rapid instrumental neutron activation analysis to multi-element geochemical mapping based on stream sediment and heavy mineral concentrate samples is described with reference to the 1:250,000 geochemical map series of Great Britain produced by the Institute of Geological Sciences, London. The activation method has been devised to extend the range of elements currently analysed by atomic absorption spectrophotometric, optical emission and X-ray fluorescence spectrometric methods. The following elements are determined (limits of detection are given in ppm in parentheses): Na (20), Al (500), K (1000), Sc (0.05), Ti (1000), Cr (5), V (20), Mn (5), Fe (500), Co (0.05), Zn (50), Se (5), As (2), Br (1), Rb (20), Mo (2), Ag (5), Cd (3), Sb (0.5), Ba (200), Cs (1), La (0.1), Ce (3), Sm (4), Eu (1), Hf (0.3), Ta (2), W (2), Ir (0.01), Au (0.005), Th (0.5), U (1). The precision and accuracy of the method compare favourably with standard methods of trace element analysis. Subsampling precision is good since 500-mg subsamples are employed and analytical precision for most elements is in the range of 5–15% at 95% confidence limits. No systematic error occurs in relation to other methods of analysis. Sample preparation is minimal as it is unnecessary to grind, dissolve, preheat or ignite, or to add buffers or take samples into solution. Forty samples can be analysed per man week, but the time taken to complete an analysis is dependent on the decay time necessary before counting which may be one month. The method is non-consuming and non-modifying, which is particularly useful if panning concentrates are to be analysed prior to mineralogical examination. The method is considered to have the greatest potential for orientation studies, error control and follow-up investigations.