Mufida E. Ben Younes, D. Conrad Grégoire and Chuni L. Chakrabarti
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.