Loss on Ignition (LOI) is a simple analytical test that determines the amount of volatile material lost from a sample when it is heated to a high temperature. This typically includes moisture, organic matter, and carbonates, providing a quick estimate of these components.
Understanding Loss on Ignition (LOI)
Loss on Ignition is a gravimetric technique where a sample is heated at a specific temperature (often between 550°C and 1000°C, depending on the material) until all volatile components are driven off. The difference in weight before and after heating is then used to calculate the percentage of material lost. This method is widely used in industries such as cement, mining, ceramics, and environmental analysis for quality control and material characterization.
The Loss on Ignition Formula
The calculation of Loss on Ignition is straightforward and involves comparing the initial weight of the sample to its final weight after ignition.
The formula for calculating the percentage of Loss on Ignition (% LOI) is:
$$
\text{\% LOI} = \frac{(\text{Initial Weight of Sample} - \text{Final Weight of Sample})}{\text{Initial Weight of Sample}} \times 100
$$
Where:
- Initial Weight of Sample: The weight of the dry sample before heating (after drying at a lower temperature if moisture content needs to be excluded from LOI, or the as-received weight if total volatiles including initial moisture are desired).
- Final Weight of Sample: The weight of the sample after being heated to a specified temperature and cooled to room temperature.
- Initial Loss (implied by the formula): This refers to the difference between the initial weight and the final weight of the sample, representing the mass of material lost during ignition.
Step-by-Step Procedure for Calculating LOI
To accurately calculate LOI, the following general steps are followed:
- Prepare the Sample and Crucible:
- Select a clean, dry, and pre-weighed crucible (e.g., porcelain or platinum) that can withstand high temperatures.
- Dry the sample if initial moisture is to be excluded from the LOI measurement. Otherwise, use the as-received sample.
- Weigh the Initial Sample:
- Carefully place a known amount of the prepared sample into the pre-weighed crucible.
- Weigh the crucible with the sample to determine the Initial Weight of Sample (often calculated by subtracting the empty crucible weight from the crucible + sample weight).
- Ignite the Sample:
- Place the crucible with the sample into a high-temperature muffle furnace.
- Heat the sample at a specific temperature (e.g., 950°C for cement clinker, 550°C for organic matter) for a defined period (e.g., 1-2 hours) to ensure all volatile components are driven off.
- Cool and Re-weigh:
- Once the heating period is complete, remove the crucible from the furnace and place it in a desiccator as quickly as possible. This prevents the ignited sample from reabsorbing moisture from the air, which would lead to an inaccurate final weight.
- Allow the crucible and sample to cool completely to room temperature inside the desiccator.
- Weigh the cooled crucible with the ignited sample. This gives you the weight from which the Final Weight of Sample is derived (by subtracting the empty crucible weight).
- Perform the Calculation:
- Determine the % loss on ignition by dividing the "initial loss" (Initial Weight of Sample - Final Weight of Sample) by the initial weight of the sample and multiply by 100. This yields the percentage of material lost.
Example Calculation
Let's say you have a soil sample, and you want to determine its organic matter content via LOI at 550°C.
- Empty Crucible Weight: 25.000 g
- Crucible + Dry Sample Weight (Initial): 30.000 g
- Crucible + Ignited Sample Weight (Final): 28.500 g
Step 1: Calculate Initial Weight of Sample
Initial Weight of Sample = (Crucible + Dry Sample Weight) - Empty Crucible Weight
Initial Weight of Sample = 30.000 g - 25.000 g = 5.000 g
Step 2: Calculate Final Weight of Sample
Final Weight of Sample = (Crucible + Ignited Sample Weight) - Empty Crucible Weight
Final Weight of Sample = 28.500 g - 25.000 g = 3.500 g
Step 3: Apply the LOI Formula
% LOI = $\frac{(\text{Initial Weight of Sample} - \text{Final Weight of Sample})}{\text{Initial Weight of Sample}} \times 100$
% LOI = $\frac{(5.000 \text{ g} - 3.500 \text{ g})}{5.000 \text{ g}} \times 100$
% LOI = $\frac{1.500 \text{ g}}{5.000 \text{ g}} \times 100$
% LOI = 0.300 $\times$ 100
% LOI = 30.0%
Practical Considerations and Best Practices
- Temperature and Time: The specific temperature and duration for heating depend on the material being analyzed and the components you aim to volatilize. Consult relevant standards (e.g., ASTM, ISO) for specific materials.
- Desiccator Use: Always cool samples in a desiccator to prevent reabsorption of atmospheric moisture, which can significantly affect the final weight.
- Crucible Material: Use crucibles made of materials that can withstand the required ignition temperature without reacting with the sample (e.g., platinum for very high temperatures or corrosive samples, porcelain for common applications).
- Multiple Runs: For critical analyses, perform duplicate or triplicate measurements to ensure reproducibility and accuracy.
- Sample Management: After the LOI measurement, it is good laboratory practice to save the LOI sample powder in a small vial labeled with the sample number. This can be useful for further analysis or as a reference.
Why is LOI Important?
LOI is a crucial parameter in various fields:
- Environmental Science: Estimating organic matter in soils, sediments, and biomass.
- Civil Engineering: Determining the organic content in construction materials like aggregates and cement.
- Geology: Characterizing mineral compositions and assessing diagenetic changes in rocks.
- Waste Management: Analyzing the combustible content in solid waste.
- Industrial Quality Control: Monitoring the consistency of raw materials and finished products in industries such as ceramics, refractories, and catalysts.
