A fused sample is a specialized method used in analytical chemistry, especially for techniques like X-ray fluorescence (XRF), Atomic Absorption Spectroscopy (AAS), and Inductively Coupled Plasma (ICP).
This method involves dissolving a fully oxidized sample at high temperatures in a suitable solvent, known as a flux, within a crucible made of platinum, zirconium, or graphite.
The molten mixture is then poured into a mold to create a glass disk or into a beaker to form a solution, depending on the intended analytical technique.
What is a Fused Sample? 4 Key Points to Understand
1. Process of Fusion
High-Temperature Melting: The sample, fully oxidized to ensure uniform reactivity, is heated to very high temperatures, typically between 900°C to 1000°C.
This high-temperature melting is necessary to dissolve the sample completely in the flux.
Use of Flux: The flux, commonly a mixture of lithium tetraborate or tetraborate/metaborate, acts as a solvent that helps in the complete dissolution of the sample.
The flux/sample ratio is usually between 5:1 to 10:1, ensuring adequate coverage and dissolution.
Crucible Material: Crucibles are made from materials like platinum, zirconium, or graphite due to their high melting points and resistance to the corrosive effects of the molten mixture.
2. Formation of Fused Beads
After the sample is dissolved in the flux, the mixture is poured into a mold to create a glass disk, known as a fused bead.
This bead is a homogeneous representation of the sample, devoid of any mineral structures or particle size effects, which can interfere with analytical measurements.
The fused bead is typically 3mm thick and provides a flat, uniform surface suitable for XRF analysis.
However, the thickness can lead to issues with infinite thickness for heavier elements, affecting the accuracy of trace element analysis.
3. Benefits and Drawbacks
Benefits: The primary advantage of fused samples is the elimination of mineralogical and particle size effects, leading to highly accurate and repeatable results.
It also allows for the analysis of a wide variety of sample types, from polymers to ferroalloys, and simplifies calibration and matrix correction.
Drawbacks: The method can be costly due to the need for specialized equipment (fusion devices and platinumware) and consumables.
Additionally, the high dilution factor can affect the analysis of trace elements.
4. Applications
Fused samples are particularly useful in XRF, where the uniform distribution and flat surface of the fused bead ensure accurate elemental analysis.
They are also used in ICP and AAS, where the sample is prepared as a solution after fusion.
In summary, a fused sample is a sophisticated preparation method that enhances the accuracy and reliability of analytical results by creating a homogeneous sample suitable for various spectroscopic techniques.
Despite the higher initial costs and some analytical limitations, its benefits in terms of accuracy and versatility make it a valuable technique in many analytical laboratories.
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