9.15 : Sample Preparation for Analysis: Advanced Techniques

Accurate analysis of complex samples often requires advanced preparation techniques to achieve reliable and reproducible results. Samples containing inorganic or organic materials can be challenging to dissolve or decompose effectively. Standard sample preparation methods include acid digestion, fusion, dry ashing, and wet digestion.

Acid digestion with strong acids is commonly used to dissolve inorganic materials that are insoluble (do not dissolve) in water. This method can be useful for analyzing soil or mineral samples. During acid digestion, the insoluble solid sample is finely ground or crushed to increase its surface area, which allows for a more effective reaction with the acids. The typically used acids include hydrochloric acid, nitric acid, sulfuric acid, and aqua regia – a mixture of nitric and hydrochloric acid. Each acid has specific advantages for different sample types:

1. Hydrochloric acid (HCl) is often used to dissolve chlorides, oxides, and carbonates and is also effective on materials such as metals and ores.
2. Nitric acid (HNO₃) is excellent for breaking down organic matter and oxidizing metals, making it a common choice for biological tissues or metal analysis.
3. Sulfuric acid (H₂SO₄) is used when a strong dehydrating agent is needed, for instance, in dissolving more resistant samples like sulfides or metals.
4. Aqua regia, a mixture of nitric and hydrochloric acids, is particularly effective for dissolving noble metals like gold and platinum, which are resistant to individual acids.

The sample is treated with the chosen high-strength acid in a suitable container, where it is broken down into soluble forms or ions. Heat may also be applied to speed up the digestion process. After digestion, the resulting mixture diluted with water is suitable for further analysis.

The fusion technique is an alternative method to dissolve inorganic materials that cannot be broken down to a soluble form by acid digestion. Here, the weighted sample is mixed with a flux material in a specific ratio, typically about 1 to 10 or 20. The sample-flux mixture is placed in a platinum crucible that is resistant to high temperatures and chemical reactions. This crucible is heated in a high-temperature furnace to obtain a molten mixture. After heating, the molten mixture is allowed to cool till it yields a new material soluble in water or dilute acid.

Several examples of flux usage in sample preparation for dissolving solid and liquid compounds are:

1. Na₂CO₃ and Na₂O₂ as Fluxes for Iron and Chromium Alloys: Sodium carbonate (Na₂CO₃) is a commonly used flux, but sodium peroxide (Na₂O₂) is employed due to its stronger oxidizing ability in certain cases. This combination is particularly useful for dissolving iron and chromium alloys in a nickel crucible.
2. K₂S₂O₇ for Refractory Oxides: When the target is refractory oxides (but not silicates), potassium disulfate (K₂S₂O₇) serves as an effective flux. It can be prepared by heating potassium hydrogen sulfate (KHSO₄) to decompose any foaming byproducts. K₂S₂O₇ is effective in dissolving oxides and silicates with the application of heat.
3. Li₂B₄O₇ and Li₂SO₄ Mixture for Rapid Dissolution: A mixture of lithium tetraborate (Li₂B₄O₇) and lithium sulfate (Li₂SO₄) in a 2:1 ratio by weight is highlighted for its rapid ability to dissolve refractory silicates and oxides.

Dry ashing and wet digestion methods are popularly used to decompose organic materials. In dry ashing, the organic sample is heated in a muffle furnace at a high temperature. The heat causes the combustion of the organic material in the presence of atmospheric oxygen, leaving behind an inorganic residue, or 'ash.' This ash can then be dissolved in an appropriate solvent for further analysis. Wet digestion involves the use of acids to break down the organic material.