Spectrochemical determination of submicrogram amounts of tungsten in geologic materials

A rapid, sensitive, emission spectrographic method for the determination of tungsten in geologic materials has been developed. Sample fusion with potassium hydroxide followed by a hot-water leach renders the tungsten soluble. Acidification of the solution, reduction with titanium trichloride, complexing with thiocyanate, and ether extraction separate and concentrate the tungsten. An aliquot of the ether solution is pipetted into the cavity of a preformed graphite electrode containing 6 milligrams of sodium carbonate and is evaporated. The carbonate flux prevents formation of refractory tungsten carbide during arcing, resulting in nearly complete volatilization of tungsten during a short exposure period. Spectra from direct-current arc excitation of the tungsten are recorded on film and visually compared to standard spectra. A determination limit of 0.2-part-per-million tungsten is obtained from a 0.5-gram sample with a sample completion average of 30-50 samples per man-day.