What Is Loss on Ignition and Why Does It Matter for XRF?
Loss on Ignition (LOI) is one of the most important — and most overlooked — steps in XRF sample preparation. It measures the mass lost when a sample is heated to high temperature, typically 950–1050°C. That lost mass includes water, carbonates, sulfides, and organic matter — all things that interfere with accurate XRF analysis if left in the sample.
Understanding LOI isn’t optional for labs that care about data quality. Here’s what you need to know.
How LOI Works
The process is straightforward:
- Weigh the sample in a pre-weighed crucible (accuracy to ±0.001g)
- Heat to 950–1050°C in a muffle furnace for 1–2 hours
- Cool in a desiccator to prevent moisture reabsorption
- Reweigh and calculate the percentage of mass lost
The formula: LOI (%) = [(Weight before – Weight after) / Weight before] × 100
Typical LOI values range from less than 1% for igneous rocks to over 40% for carbonate-rich sediments and organic soils.
Why LOI Matters for Borate Fusion
When you prepare glass beads for XRF using borate fusion, your flux-to-sample ratio is carefully calculated. If your sample contains 15% volatile material that you haven’t accounted for, your actual oxide composition in the bead is off by that same amount. This cascading error affects every element you measure.
There are three specific problems LOI addresses:
1. Matrix Correction Accuracy
XRF matrix correction algorithms assume they know the total composition of the bead. Unaccounted volatiles create a gap between assumed and actual composition, degrading the accuracy of inter-element corrections — especially for light elements like Na, Mg, and Al.
2. Crucible Protection
Organic matter and sulfides that aren’t removed before fusion can attack platinum crucibles at high temperature. Sulfur is particularly aggressive — it alloys with platinum, causing permanent surface damage and pitting. Pre-igniting samples to determine LOI also serves as a crucial step in protecting your platinum labware.
3. Bead Quality
Volatiles released during fusion create gas bubbles in the melt. These bubbles get trapped in the glass bead, creating voids that scatter X-rays and produce noisy, unreliable spectra. Pre-ignition eliminates this problem at the source — which is why it’s one of the key fixes for common sample preparation errors.
When You Can Skip LOI (and When You Can’t)
You can sometimes skip LOI for well-characterized sample types where the volatile content is known and consistent — for example, routine production control of glass or cement where the raw materials don’t change.
You should never skip LOI for:
- Geological samples (rocks, soils, sediments) — volatile content varies widely
- Environmental samples — organic content is unpredictable
- New or unfamiliar sample matrices
- Any analysis where accuracy below 1% relative is required
- Samples with visible organic matter,ite calcium carbonate, or sulfide minerals
Choosing the Right Crucible for LOI
LOI determinations require crucibles that can handle repeated thermal cycling to 1050°C without degrading. The most common options:
- Platinum crucibles — the gold standard for accuracy. Chemically inert, consistent tare weight, virtually unlimited reuse. Essential for high-precision work. See available options.
- Porcelain crucibles — adequate for routine LOI but absorb moisture, chip over time, and introduce contamination risk.
- Nickel crucibles — sometimes used but not recommended for LOI due to oxidation at high temperature, which changes the crucible’s own weight and introduces error.
For labs doing both LOI and borate fusion, using platinum crucibles for both steps eliminates a variable and simplifies the workflow.
Best Practices for Accurate LOI
- Always pre-heat crucibles to 1050°C and cool before weighing (removes adsorbed moisture from the crucible itself)
- Use a desiccator for cooling — never leave hot crucibles on the bench to cool in open air
- Record weights immediately after removing from the desiccator
- Use 1–2g sample sizes for consistent results
- Hold at temperature for a minimum of 1 hour — longer for carbonate-rich samples
- Run duplicates on at least 10% of samples to verify precision
- Include a certified reference material (CRM) in each batch
The Bottom Line
LOI is a simple measurement, but skipping it or doing it carelessly creates problems that ripple through every downstream analysis. It takes 2 hours of furnace time and 5 minutes of weighing — a small investment for the data quality it protects.
If you’re setting up or optimizing an XRF sample prep workflow, reach out to our team — we can help you select the right crucibles and accessories for both LOI and fusion work.