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Study Notes: More on Sample Preparation for Spectroscopic Analysis

The following briefly outlines some sample preparation procedures commonly used with spectroscopic techniques.

Acid (or wet) digestion
Acid digestion can be used to break down an organic matrix (eg a food) or to dissolve an inorganic matrix (eg geological deposit) by reacting the sample with acid at temperatures normally up to about 100°C. One or a combination of different acids may be used to accomplish digestion. The acids may be oxidising or non-oxidising types such as HNO3 and HCl respectively.

Digestion releases the analyte from its matrix, thereby making it available for further work-up if necessary, followed by analysis in solution.

Acid digestion is commonly used for elemental analysis.

Filtration and Centrifugation

Spectroscopic techniques involve passing light through a sample to measure the amount that has been absorbed by the sample. The presence of a precipitate or undissolved matter in the test solution scatters the light and so interferes with the absorbence reading. Removal of the undissolved material by filtering or centrifuging the sample can overcome the interference. Simple illustrations of a spinning centrifuge containing two tubes and a filtration system using a filter funnel fitted with filter paper through which clear solution is exiting.

Liquid-liquid Extraction
A liquid sample is intimately mixed with an immiscible solvent to selectively transfer the analyte from the sample into the solvent. The transfer occurs as a result of the greater solubility of the analyte in the solvent. After extraction, the solvent is separated from the sample-phase in readiness for analysis or for further processing.

This procedure facilitates purification and/or concentration of the sample.

To prepare beer for bitterness analysis, a sample is extracted with an organic solvent (iso-octane) resulting in the transfer of the bitter compounds to the organic phase. This is a purification process since the bitter compounds are selectively removed from the other (interfering) light-absorbing compounds in the aqueous beer matrix.

Ashing
This procedure can be broken down into wet and dry types and is used to accomplish destruction of organic matrices to enable elemental analysis.

Dry ashing involves heating the sample, usually in a muffle furnace (500 to 600°C), until all the carbonaceous material is oxidised leaving a residue of inorganic solid (ash). This is then normally dissolved in acid for further processing or analysis.The residue contains minerals converted to oxides, sulfates, phosphates, chlorides and silicates. A muffle furnace.

Wet ashing is similar to acid digestion. The sample is oxidised by using a combination of acids and oxidising agents such as hydrogen peroxide.

Wet ashing is usually the preferred approach as dry ashing can lead to volatile metallic compounds being lost at the high temperatures. This introduces inaccuracies into the analysis.

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