Hydrofluoric Acid-Assisted Dissolution of Biological Samples for Silicon Determination by ICP-MS: Examining Silicon Volatility Under HF Digestions

Metals Analysis and Remediation
Oral Presentation

Prepared by Z. Arslan1, E. Sarver2, C. Keles2, C. Rose3, L. Zell-Baran3, J. Hua3, H. Lowers1
1 - US Geological Survey, Geology, Geophysics, and Geochemistry Science Center, Denver Federal Center, 1 Denver Federal Center, Denver, CO, 80225, United States
2 - Virginia Tech, Department of Mining and Minerals Engineering, , Blacksburg, VA, 24061, United States
3 - National Jewish Health-Denver, 1400 Jackson St, Denver, CO, 80206, United States


Contact Information: [email protected]; 303-236-1856

ABSTRACT

Volatility of silicon during hydrofluoric (HF) digestions was investigated to determine trace levels of silicon in biological samples. A set of 50 µg/mL silicon solutions prepared in HNO3, HNO3-HCl and HNO3-HCl-HF from commercial silicon solutions in trace HF (hexafluorosilicic acid, H2SiF6) and water (sodium metasilicate, Na2SiO3) were heated at 120 oC on a hot-block to near dryness. Loss of silicon from H2SiF6 solutions was around 98-99% regardless of the medium. No significant loss occurred for metasilicate solutions prepared in HNO3 and HNO3-HCl media, whereas more than 95% of silicon was lost in HNO3-HCl-HF medium. Effects of aluminum, calcium, iron, magnesium, potassium and sodium matrices, isolated and in combination were examined to stabilize silicon in HF solutions. No significant improvement occurred. NaCl matrix improved stability by about 80% in pure SiO2 and soil matrix but losses could not be fully avoided. These results revealed that silicon species in HF digests were highly volatile that necessitate a judicious sample preparation approach to avoid losses. A two-step digestion scheme was developed using HNO3, HCl, HClO4 and HF for preparation of biological samples for silicon analysis by ICP-MS. A closed-vessel digestion was performed first in 4 mL HNO3, 1 mL HCl and 1 mL HClO4 on a hot plate at 140 oC. Digests were then evaporated to near dryness at 120 oC to remove the acids. A second closed-vessel digestion was performed in 0.5 mL HNO3 and 0.5 mL HF, and the contents were completed to 10 mL. The solutions containing about 5% (v/v) HF directly analyzed by ICP-MS equipped with a completely HF-inert sample introduction system. Limit of detection was about 0.1 µg/mL. The method was validated by analysis of various plant, and tissue and plankton standard reference materials.