Determination of ppm level contents of potassium in silicate materials by means of neutron activation analysis

Twenty to forty mgs silicate samples (olivine mega-crysts, meteorites, and standard rocks) were weighed, wrapped in aluminium foil, and irradiated with a flux of 8 X 1013 n cm-2 s-1 for 40 min in JRR-4 of JAERI, along with a standard sample. After cooling, the sample was fused with sodium peroxide and sodium hydroxide in a zirconium crucible containing the potassium carrier. The resulted cake was dissolved with water. After centrifugation, the supernatant was roughly neutralized. Potassium precipitated as potassium tetraphenylborate was collected on filter paper, washed, dried, weighed, and wrapped with cellophane tape. A photopeak of 1525 keV of neutron-induced was used for the determination of potassium. Chemical yields were checked both by gravimetrical and reactivation methods. The gravimetrical method gave systematically larger values than those obtained by the reactivation method. The factor varies from 1.04 to 1.42. This might suggest that some elements were coprecipitated with potassium. Analytical results of potassium for standard rocks, JB-1 and JP-1, were in good agreement with the literature values. The lowest value of potassium in silicate materials determined in this study was 4.3 ppm for an olivine megacryst sample. Based on the gamma-spectrum of this sample, the detection limit of potassium for this method was deduced to be 370 ppb for a sample size of 30 mg. Several sources of errors accompanied with the present method were examined and their maximum estimates were calculated.