Vaporization and removal of silica for the direct analysis of geological materials by slurry sampling electrothermal vaporization-inductively coupled plasma-mass spectrometry

Abstract

Reported is a method for the removal of silica for the direct analysis of solid geological samples high in silica content using slurry sampling electrothermal vaporization-inductively coupled plasma-mass spectrometry (ETV-ICP-MS). The ETV-ICP-MS vaporization curve for SiO₂, sampled as a slurry, is reported for temperatures ranging from 810–2600 °C. This curve showed that silica was completely vaporized at a temperature of 2200 °C. The effect of using HF as a chemical modifier to remove silica as the tetrafluoride was studied. It was found that HF could completely remove any Si attributed to silica if sufficient modifier were added and an adequate reaction time allowed. At reaction hold times that were less than optimal, two Si signals were observed. The first signal, which appears at an earlier time and at a temperature less than 480 °C, is attributed to the volatilization of silicon tetrafluoride. The second signal, which appears at a later time and at a temperature of about 2500 °C, is attributed to the vaporization of residual unreacted SiO₂ remaining in the graphite tube, or to the vaporization of a mixture of SiO₂ and SiC. 20 µl of 50% HF is effective in completely removing 0.125 µg of SiO₂. No adverse effects including corrosive degradation of the graphite tube were observed over the lifetime of the tube, which exceeded 200 firings. Hydrofluoric acid chemical modifier was successful in removing virtually all the silica content in natural silicate standard reference materials.