Application of the Zr/Hf Ratio in the Determination of Hafnium in Geochemical Samples by High-Resolution Inductively Coupled Plasma Mass Spectrometry

Abstract

Hafnium content and its change are of significance in geochemistry and cosmochemistry; however, the determination of hafnium has always been problematic in analytical chemistry. In this paper, a new idea is proposed for the determination of hafnium in geochemical samples, including rocks, soils, and stream sediments. Through the comparison of two conventional open-type acid digestion methods (HF-HNO3-HClO4 and HF-HNO3-H2SO4), it was found that although neither of these methods could fully digest the zirconium and hafnium in a sample, the zirconium and hafnium digestion behaviors in one sample were consistent in the 60 experimental geochemical reference materials with different properties, so the experimentally determined Zr/Hf ratio in solution could be used to calculate the hafnium content in a sample. In addition, possible mass spectral interferences during the determination of zirconium and hafnium by high resolution inductively coupled plasma mass spectrometry (HR-ICPMS) were studied, and it was found that the mass spectral interferences of the selected isotopes (90)Zr and (178)Hf could be neglected. The mass spectral behaviors of (90)Zr and (178)Hf were also very consistent during the determination by HR-ICPMS. Since the hafnium content was calculated using the ratio value, all of the errors (including the errors in weighing process, the accidental errors during operation and the instrument fluctuation in the determination) of the Zr/Hf ratio could be effectively reduced or even eliminated. The relative standard deviation of the actual samples was lower than 3.2%, and the detection limit of the method (considering the dilution effect and matrix effect during measurement of the Zr/Hf ratio and zirconium content) was 0.04 μg/g. The proposed method could satisfy the requirement for the determination of hafnium in geochemical samples.