Questions: Sample Dissolution and Digestion Procedures

Question 1

You need to dissolve a silicate rock sample to measure trace metals. Which acid combination is the appropriate choice?

• A. Aqua regia (3:1 HCl/HNO₃), because it dissolves all geological materials
• B. HNO₃/HF, because HF converts silicon to volatile SiF₄, breaking down the silicate matrix
• C. HNO₃/H₂O₂, because silicates contain organic binders that must be oxidized
• D. Concentrated HCl alone, because silicates are essentially metal chloride salts

Answer: (B)

Silicates require HF because it attacks the Si-O backbone by forming volatile SiF₄, releasing the bound metal ions into solution. Aqua regia is specifically for noble metals (gold, platinum), not silicate matrices. HNO₃/H₂O₂ is used for biological and organic samples. HCl alone cannot dissolve the silicate framework.

Question 2

A laboratory analyzes mercury (Hg) in fish tissue. The analyst uses open-vessel hot-plate digestion with HNO₃ instead of microwave-assisted digestion. What is the most critical risk specific to this analyte?

• A. Mercury may precipitate as a chloride salt if any HCl is present
• B. Volatile mercury species will escape the open vessel during heating, causing a biased-low result
• C. The organic matrix of fish tissue is too dense for HNO₃ to penetrate without a sealed vessel
• D. Open-vessel digestion cannot reach temperatures high enough to oxidize mercury compounds

Answer: (B)

Mercury is a notoriously volatile analyte — mercury vapor and organo-mercury species escape from heated open vessels. The sealed environment of microwave digestion prevents this loss, ensuring all mercury is retained in solution. The other options are secondary concerns or incorrect; the fundamental issue is analyte volatility. This is a direct illustration of why digestion method must be matched to analyte properties, not just sample matrix.

Question 3

Running reagent blanks through the full digestion procedure — including the same acid volumes, heating, and vessel — provides a more accurate correction for contamination than simply measuring the acid blank without heating.

• T. True
• F. False

Answer: True

Contamination during digestion comes from multiple sources: impurities in the acids, leaching from the digestion vessel walls during heating, and airborne particles during handling. A blank taken through the full procedure captures all of these contributions. An unheated acid blank only accounts for dissolved impurities in the starting reagents, missing the contamination introduced by heat and vessel contact — which are often the largest sources for trace-level analyses.

Question 4

Choosing the strongest available acid and the highest achievable temperature generally improves digestion completeness and should be the default approach when dissolving a difficult sample.

• T. True
• F. False

Answer: False

More aggressive conditions introduce specific risks. Perchloric acid requires explosion-proof fume hoods; excessive temperatures in microwave vessels risk pressure buildup and unsafe venting. More importantly, volatile analytes (As, Se, Hg, Os) are lost at elevated temperatures in open systems, producing biased-low results — the opposite of complete recovery. Digestion method selection is a matrix- and analyte-specific decision that balances dissolution efficiency against analyte retention and safety.

Question 5

Why is it insufficient to declare a digestion complete simply because the digest solution appears clear with no visible solid particles? What additional quality control step is required, and what would it reveal?

Model answer: A clear digest indicates that the bulk matrix has dissolved, but colloidal particles or co-precipitated analytes may still be present in suspension, and some analytes may have been lost (as volatiles or by adsorption to vessel walls) during the process. Comparison with a certified reference material (CRM) of similar matrix run through the same digestion procedure provides an accuracy check — if the measured value matches the certified value within tolerance, the procedure is validated for recovery. A clear solution alone proves the matrix dissolved; it does not prove the analyte is fully recovered at its true concentration.

Digestion quality has two independent failure modes: incomplete matrix dissolution (analyte trapped in residue → biased low) and analyte loss during the procedure (volatile loss, precipitation, adsorption → also biased low). Visual clarity addresses only the first. CRMs and spiked recoveries test the full analytical process end-to-end, including both dissolution and retention.