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Trace Silicon Determination in Biological Samples by Inductively Coupled Plasma Mass Spectrometry (ICP-MS): Insight into the Volatility of Silicon Species in Hydrofluoric Acid Digests for Optimal Sample Preparation and Introduction to ICP-MS

Minerals, ISSN: 2075-163X, Vol: 14, Issue: 3
2024
  • 1
    Citations
  • 0
    Usage
  • 10
    Captures
  • 2
    Mentions
  • 0
    Social Media
Metric Options:   Counts1 Year3 Year

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  • Citations
    1
  • Captures
    10
  • Mentions
    2
    • Blog Mentions
      1
      • Blog
        1
    • News Mentions
      1
      • News
        1

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Findings from U.S. Geological Survey (USGS) Provide New Insights into Fluorine Compounds [Trace Silicon Determination in Biological Samples by Inductively Coupled Plasma Mass Spectrometry (ICP-MS): Insight into the Volatility of Silicon Species ...]

2024 APR 04 (NewsRx) -- By a News Reporter-Staff News Editor at Chemicals & Chemistry Daily Daily -- A new study on fluorine compounds is

Article Description

A method for the determination of trace levels of silicon from biological materials by inductively coupled plasma mass spectrometry (ICP-MS) has been developed. The volatility of water-soluble silicon species, hexafluorosilicic acid (HSiF), and sodium metasilicate (NaSiO) was investigated by evaporating respective solutions (50 µg/mL silicon) in nitric acid (HNO), nitric acid + hydrochloric acid (HNO + HCl), and nitric acid + hydrochloric acid + hydrofluoric acid (HNO + HCl + HF) at 120 °C on a hot-block to near dryness. The loss of silicon from HSiF solutions was substantial (>99%) regardless of the digestion medium. Losses were also substantial (>98%) for metasilicate solutions heated in HNO + HCl + HF, while no significant loss occurred in HNO or HNO + HCl. These results show that HSiF species were highly volatile and potential losses could confound accuracy at trace level determinations by ICP-MS if digestates prepared in HF are heated to eliminate HF. Among the various matrices comprising major elements, sodium appeared to be effective in reducing silicon loss from HSiF solutions. Excess sodium chloride (NaCl) matrix provided better stability, improving silicon recoveries by up to about 80% in evaporated HF digestates of soil and mine waste samples, but losses could not be fully prevented. To safely remove excess acids and circumvent the adverse effects of excess HF (e.g., risk of high Si background signals), a two-step digestion scheme was adopted for the preparation of biological samples containing trace silicon levels. A closed-vessel digestion was performed either in 4 mL of concentrated HNO and 1 mL of concentrated HCl or 4 mL of concentrated HNO, 1 mL of concentrated HCl and 1 mL of concentrated HClO on a hot plate at 140 °C. Digestates were then evaporated to incipient dryness at 120 °C to remove the acids. A second closed-vessel digestion was carried out to dissolve silicates in 0.5 mL of concentrated HNO and 0.5 mL of concentrated HF at 130 °C. After digestion, digestates were diluted to 10 mL. The solution containing about 5% HNO and 5% HF was directly analyzed by ICP-MS equipped with an HF-inert sample introduction system. The limit of detection was about 110 µg/L for Si when using the Kinetic Energy Discrimination (KED) mode. The method was used to determine silicon in various plant and tissue certified reference materials. Data were acquired for Si using KED and standard (STD) modes, and Ge and Rh as internal standard elements. There was not any significant difference between the accuracy and precision of the results obtained with Ge and Rh within the same measurement mode. Precision, calculated as relative standard deviation for four replicate analyses, varied from 5.3 (tomato leaves) to 21% (peach leaves) for plant and from 2.2 (oyster tissue) to 33% (bovine liver) for tissue SRM/CRMs. Poor precision was attributed to material heterogeneity and the large particle size distribution. An analysis of lung tissue samples from those with occupational exposure to silica dust revealed that tissues possessed substantial levels of water-soluble silicates, but the most silicon was present in the particulate matter fraction.

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