What Is the Flux-to-Sample Ratio?
In borate fusion for XRF analysis, the flux-to-sample ratio is one of the most critical parameters affecting the quality of your glass discs. Get it wrong, and you’ll face cracking, incomplete dissolution, or poor analytical accuracy. Get it right, and you’ll produce clear, homogeneous discs that deliver repeatable results every time.
The flux-to-sample ratio refers to the proportion of lithium borate flux (typically lithium tetraborate, lithium metaborate, or a mixture) to the sample material being analyzed. Common ratios range from 5:1 to 20:1, but the optimal ratio depends on your sample matrix, the elements you’re measuring, and the type of flux you’re using.
If you’re new to borate fusion, our guide on how borate fusion works and why platinum crucibles matter covers the fundamentals.
Common Flux-to-Sample Ratios by Application
Cement and Clinker (10:1)
Cement laboratories typically use a 10:1 ratio with a 50/50 lithium tetraborate/metaborate mix. This provides good dissolution of calcium silicates while keeping the glass disc stable. Some labs push to 5:1 for higher sensitivity on trace elements, but this increases the risk of crystallization.
Mining and Geological Samples (10:1 to 20:1)
Ore and rock samples vary enormously in composition. High-silica samples (granite, quartz) dissolve more slowly and often require higher ratios (15:1 to 20:1) or a flux with more lithium metaborate to improve dissolution. Iron ores and basic rocks can typically use the standard 10:1 ratio.
Soils and Sediments (10:1)
Environmental samples generally work well at 10:1 with a mixed flux. The key challenge here is organic matter — samples with high organic content must undergo loss on ignition (LOI) before fusion to prevent foaming and crucible damage. Our complete guide to LOI for XRF explains the process in detail.
Glass and Ceramics (5:1 to 10:1)
These already-vitrified materials dissolve readily, so lower ratios work well. A 5:1 ratio provides better sensitivity for trace element analysis without sacrificing disc quality.
How the Wrong Ratio Causes Problems
Ratio Too Low (Too Much Sample)
- Incomplete dissolution: Undissolved particles create heterogeneity in the glass disc
- Crystallization: The disc may devitrify during cooling, appearing cloudy or cracked
- Matrix effects: Higher sample concentration increases inter-element interactions
- Crucible damage: Concentrated acidic or basic melts are more aggressive on platinum
Crucible damage from aggressive melts is one of the most common — and expensive — consequences. Understanding when to replace your platinum crucible can help you catch damage early before it affects results.
Ratio Too High (Too Much Flux)
- Reduced sensitivity: Diluting the sample too much lowers elemental concentrations below detection limits
- Higher cost: Flux isn’t cheap, especially high-purity grades
- Longer fusion times: More material takes longer to melt and homogenize
Choosing the Right Flux Composition
The ratio isn’t the only variable — the type of flux matters just as much:
- Lithium tetraborate (Li₂B₄O₇): Best for acidic samples (high silica, alumina). Creates stable glass discs but requires higher temperatures (~1050°C)
- Lithium metaborate (LiBO₂): Better for basic samples (iron ores, limestone). Dissolves resistant minerals faster but produces less stable discs
- Mixed flux (e.g., 66/34 or 50/50 tetra/meta): The most versatile option, suitable for the widest range of sample types. Most multi-purpose labs default to a mixed flux
The Role of Your Crucible in Fusion Quality
Your flux-to-sample ratio interacts directly with your platinum crucible alloy. A more aggressive melt (lower ratio, more metaborate) puts greater stress on the crucible. The standard Pt/Au 95/5 alloy handles most fusion applications well, but labs running high-throughput or particularly aggressive fusions may benefit from specialized alloy compositions.
SIB Fusion manufactures custom alloy crucibles tailored to specific fusion conditions — not just standard compositions. If you’re experiencing premature crucible wear, the alloy may be as important as the ratio.
Proper crucible care and maintenance also plays a role. Residual flux from previous fusions can alter the effective ratio if crucibles aren’t properly cleaned between runs.
Practical Tips for Optimizing Your Ratio
- Start with the manufacturer’s recommendation for your fusion instrument, then adjust based on results
- Weigh precisely. A 0.1g error on a 0.5g sample at 10:1 changes the effective ratio by 20%
- Pre-ignite high-LOI samples to remove organics and water before calculating your sample weight
- Add a non-wetting agent (lithium bromide or lithium iodide, typically 1-3% of flux weight) to ensure clean release from the mold
- Document everything. When you find a ratio that works for a sample type, record it in your method so every analyst gets the same result
- Monitor your crucibles. If you notice accelerated wear after changing ratios, the new conditions may be too aggressive for your current alloy
When to Consider Changing Your Ratio
Don’t change a working method without reason. But if you’re seeing any of these issues, the ratio is worth investigating:
- Cracked or cloudy glass discs
- Poor repeatability between fusions of the same sample
- Undissolved particles visible in the disc
- Systematic bias on certain elements compared to reference materials
- Rapid crucible degradation
Many of these symptoms overlap with common XRF sample preparation errors. If adjusting the ratio doesn’t solve the problem, the root cause may lie elsewhere in your preparation workflow.
Conclusion
The flux-to-sample ratio is a balancing act between dissolution power, disc stability, analytical sensitivity, and crucible longevity. There’s no single “best” ratio — it depends on your sample matrix, your analytical goals, and your equipment. Start with established ratios for your industry, then fine-tune based on results.
Need help selecting the right crucible for your fusion conditions? Contact SIB Fusion to discuss custom alloy options designed for your specific flux chemistry and sample types.