Choosing the Best Flux Materials for XRF Fusion Sample Prep

Fusion is an effective way of converting complex solids into homogeneous glass beads during XRF fusion sample preparation, yet the success of this step depends heavily on the chemistry of the flux materials involved. The behavior of flux materials like lithium metaborate, lithium tetraborate, and their mixed formulations directly shapes bead clarity, melt viscosity, and the completeness of dissolution. Because sample matrices vary widely in acidity, mineralogy, and oxide composition, recognizing how different fluxes respond in the melt helps analysts avoid issues like crystallization, bubbles, or undissolved grains. Such a connection between flux properties and matrix requirements makes flux selection a more predictable and informed part of producing high-quality fusion beads for XRF analysis.

Lithium Metaborate: A Highly Reactive Flux for Acidic Matrices

Lithium metaborate (LiBO₂) is one of the most reactive flux materials used in XRF fusion sample preparation. It supports the breakdown of strong silicate structures that are common in acidic samples and performs well under high temperature conditions typical of fusion workflows.

Why it works well for acidic materials

• It attacks high silica and aluminosilicate networks effectively by depolymerizing the silicon-oxygen (Si-O) and aluminum-oxygen (AI-O) bonds that hold these minerals together, enabling the melt to break down structures that are normally resistant to chemical dissolution.
• It produces low viscosity melts that dissolve refractory minerals as the lithium-boron-oxygen (Li-B-O) melt system lowers polymerization, increasing diffusion and dissolution rates.
• It provides the high chemical reactivity needed to depolymerize the robust silicate frameworks common in acidic matrices, supporting complete dissolution during fusion.

Common Sample Types Analyzed With Lithium Metaborate

• High silica rocks like granites and rhyolites
• Alumino silicate clays
• Minerals containing robust Si–O or Al–O frameworks

Lithium Tetraborate: A Controlled Flux for Basic Oxide Materials

Lithium tetraborate (Li₂B₄O₇) behaves as a more moderately reactive flux material compared to lithium metaborate, which is beneficial to many XRF fusion sample preparation workflows that require controlled melt behavior when working with basic oxide-rich samples. This behavior promotes a stable melt environment for dissolving calcium oxide (CaO), magnesium oxide (MgO), and iron oxides (FeO, Fe₂O₃).

Why it works well for basic materials

• It prevents over dissolution of basic oxides by interacting more gently with calcium-, magnesium-, and iron-bearing phases, avoiding the rapid depolymerization that can destabilize the melt.
• It maintains melt stability in carbonate rich samples through moderating reaction rates as carbonates decompose, helping control foaming and ensuring a more uniform melt.
• It promotes smooth pouring and reduces crystallization risk because its higher melting point supports a more consistent cooling profile, limiting the formation of crystalline phases as the bead solidifies.

Common Sample Types Analyzed With Lithium Tetraborate

• Limestone and dolomite
• Basic slags and metallurgical materials
• Soils with elevated calcium (Ca) or magnesium (Mg) content

Mixed Lithium Borate Fluxes: Versatility for Complex Matrices

Many commonly analyzed materials, including geological samples and industrial products, contain both acidic and basic oxides, meaning a single-component flux is unsuitable for all matrices encountered in XRF fusion sample preparation. Mixed fluxes allow analysts to fine tune melt behavior to the overall sample composition and meet the stability and reactivity requirements needed for producing consistent fusion beads.

Common mixtures include:

• 66:34 LiT:LiM- for mixed geological materials such as limestones.
• 50:50 LiT:LiM- for general purpose applications with a melting point near 870°C.
• 35:65 LiT:LiM- for alumino silicate materials requiring higher reactivity.

Mixed borate fluxes help laboratories achieve a balanced acidity index, reducing the likelihood of undissolved grains or devitrification and improving the reliability of XRF fusion sample preparation.

Additives and Specialized Flux Options

Some samples require additional support during fusion, and certain additives or specialized flux materials are beneficial for ensuring complete oxidation or enhancing melt behavior.

Releasing agents

Lithium bromide or lithium iodide reduces adhesion between the molten sample and platinumware. This improves bead release and protects the platinum crucibles commonly used in fusion.

Oxidizing agents

Lithium nitrate or sodium nitrate helps fully oxidize sulfides, organics, or partially reduced metals, and is therefore valuable when incomplete oxidation could interfere with melt behavior.

Non borate fluxes

In some cases, lanthanum-based or fluoride-based fluxes are chosen to support matrices that do not behave predictably in lithium borates, including phosphate minerals, titanium-bearing phases, and materials with significant volatile components such as fluorides or chlorides. These specialized fluxes are used if standard lithium borates cannot provide reliable dissolution or melt stability.

How to Choose the Best Flux Materials for XRF Fusion Sample Preparation

Flux material selection becomes more straightforward with a structured decision process:

• Identify whether the matrix is primarily acidic, basic, or mixed.
• Choose lithium metaborate for silicate rich, refractory, or strongly acidic materials.
• Select lithium tetraborate for carbonate rich or basic oxide dominated samples.
• Use mixed lithium borate fluxes if the matrix composition varies or contains both acidic and basic components.
• Consider the trace elements that need to be measured, particularly those at low concentrations, and choose high-purity flux materials to avoid contamination that could mask or mimic these signals.
• Review moisture sensitivity and decide on a flux form that supports accurate dosing and consistent handling.
• Should the samples contain sulfides or organics include oxidizing agents; should the beads stick to platinumware, add a releasing agent.

This structured approach links the properties of each flux material to the specific behavior of the sample in the melt, helping laboratories identify the most suitable flux materials for reliable XRF fusion sample preparation.

Optimizing XRF Fusion With Robust Flux Materials

Flux selection is critical for producing stable fusion beads and delivering dependable XRF results. XRF Scientific provides lithium metaborate, lithium tetraborate, mixed fluxes, high-purity grades, additives, and customized blends suited to diverse sample matrices. With our technical expertise and comprehensive product range, we can help you identify the most effective flux materials for your XRF fusion sample preparation workflow. Contact our team to learn more.