Hf-176/Hf-177 Determination in Small Samples by a High-Temperature Sims Technique

This paper describes a new method for the analysis of the Hf isotopic composition in geological materials. This technique requires the separation and purification of Hf out of a complex matrix. The resulting Hf separate contains less than 0.2% Zr and less than 0.01% Ti. In the new mass spectrometric technique, called ''hot-SIMS'', the sample is held at a high temperature while being bombarded by a primary beam. The hot-SIMS technique has several orders of magnitude better ionization efficiency than conventional thermal ionization. Also, due to the high temperature at which the sample is held, the mass spectrum of the sputtered secondary ions contains only a limited amount of molecular ions; the formation of hydrides, especially, is inhibited by the high temperatures. This new technique now allows routine high-precision Hf-176/Hf-177 determination on 50 ng of hafnium separate. Samples down to 15 ng can be analyzed at a lower precision level.