Questions: Sample Preparation and Dissolution Techniques

Question 1

A laboratory needs to dissolve a granite rock sample to analyze trace metals. The sample is primarily silicate minerals (SiO2 framework). Which digestion approach is required?

• A. Dilute HCl digestion at room temperature
• B. Aqua regia (HCl/HNO3) digestion
• C. HF-containing acid digestion
• D. Water dissolution followed by filtration

Answer: (C)

Silicate minerals resist even concentrated HCl and aqua regia because the Si-O framework is chemically inert to most acids. HF dissolves silicates by reacting with silicon to form volatile SiF4, breaking the matrix. Dilute HCl, aqua regia, and water all leave the silicate backbone intact.

Question 2

Using a more aggressive digestion procedure (higher acid concentration, higher temperature, longer time) typically improves analytical accuracy by ensuring complete dissolution.

• T. True
• F. False

Answer: False

More aggressive digestion can introduce more contaminants from reagents, volatilize analytes with low boiling points (e.g., mercury, arsenic species, selenium compounds), or create matrix conditions that interfere with the final measurement. The goal is complete dissolution with minimal contamination — the mildest effective procedure is preferred.

Question 3

What is the purpose of running a blank sample through an entire sample preparation procedure, and what would it fail to detect?

Model answer: A blank tracks contamination introduced by the reagents, vessels, and preparation steps. It would fail to detect contamination already present in the sample matrix itself, or contamination introduced after the preparation is complete (e.g., during the instrumental measurement step).

A procedural blank — all reagents, no sample — measures the background signal contributed by the preparation process. This allows analysts to subtract it from sample results. However, contamination intrinsic to the sample matrix or introduced after sample preparation (e.g., from the instrument or post-preparation handling) is not captured by the blank.