Determination of tungsten in geologic materials by neutron activation analysis

A method is described for the determination of tungsten in geologic materials. After fusion with sodium peroxide, tungsten is isolated by extraction into chloroform with a-benzoinoxime, back extraction into aqueous potassium hydroxide, and precipitation with a-benzoinoxime. The activities of the 0.13 and 0.48 MeV gamma rays are measured. The activity of the 0.155 MeV gamma ray of ¹⁸⁸Re (¹⁸⁸W), added prior to the fusion, is measured to determine the yield; yields normally range from 40 to 90 percent. The coefficients of variation for less than part-per-million concentrations of tungsten are normally <20 percent; the limit of detection is 0.005 ppm for a 100-mg sample.