Final mass usually less than initial mass for TGA.
Enter MW (g/mol) to calculate moles lost (stoichiometry).
Weight Loss Percent
0.00%
Percentage of original mass lost during this step
Absolute Mass Loss
0.000 mg
Residual Mass %
100.00%
Moles Lost
0.000 mmol
Mass Composition Visualization
Mass Lost
Residual Mass
How to Calculate Weight Percent from TGA Data: A Complete Guide
Thermogravimetric Analysis (TGA) is a fundamental technique in materials science, chemistry, and pharmaceuticals. It measures the change in the mass of a sample as a function of temperature or time. The ability to accurately calculate weight percent from TGA data is crucial for determining composition, thermal stability, and stoichiometry of decomposition reactions.
What is TGA Weight Percent Calculation?
In a TGA experiment, a sample is heated in a controlled atmosphere. As the temperature rises, components of the material may decompose, evaporate, or react, causing the sample's weight to change. The "Weight Percent" (wt%) represents the proportion of the sample lost or remaining during these transitions relative to the initial mass.
This calculation is widely used by laboratory technicians, chemical engineers, and material scientists to:
Quantify moisture content or volatile solvents.
Determine the purity of a substance.
Analyze the stoichiometry of decomposition (e.g., loss of CO or CO2).
Measure filler content in polymers (e.g., ash content analysis).
While modern thermal analysis software performs these calculations automatically, understanding the manual calculation is essential for verifying results and handling raw data exports.
The Formula and Mathematical Explanation
The core principle of calculating weight percent from TGA data relies on the difference between the mass at the start of a thermal event and the mass at the end.
Table 1: Variables used in TGA Weight Percent Calculation
Variable
Meaning
Unit
Typical Range
minitial
Mass at the start of the step
mg or %
1 – 50 mg
mfinal
Mass at the end of the step
mg or %
0 – minitial
Δm
Mass Change (Loss)
mg
> 0 (for loss)
This table outlines the standard inputs required for manual verification of TGA curves.
Practical Examples (Real-World Use Cases)
Example 1: Moisture Content in a Pharmaceutical Powder
A quality control chemist analyzes a drug sample. The TGA curve shows a weight loss step between 25°C and 150°C, attributed to the loss of absorbed water.
Interpretation: The sample contains 5% moisture by weight, which must be accounted for in dosing calculations.
Example 2: Calcium Oxalate Monohydrate Decomposition
Calcium Oxalate Monohydrate (CaC2O4·H2O) is a standard reference material for TGA. The first step involves the loss of the water of hydration.
Initial Mass: 20.0 mg
Final Mass (after 1st step): 17.53 mg
Mass Loss: 2.47 mg
Weight Percent Loss: 12.35%
Theoretical Check: The theoretical water content is roughly 12.3%, confirming the instrument is calibrated correctly.
How to Use This Calculator
This tool simplifies the process of converting raw mass readings from your thermal analysis software into reported percentages.
Identify the Step: Look at your TGA thermogram (mass vs. temperature curve). Identify the temperature range where the weight drops (the "step").
Enter Initial Mass: Input the mass value (mg) recorded at the onset (start) of the step.
Enter Final Mass: Input the mass value (mg) recorded at the offset (end) of the step.
Optional Stoichiometry: If you suspect the loss is a specific molecule (e.g., CO2), enter its Molecular Weight to see how many millimoles were lost.
Analyze Results: Use the generated "Weight Loss Percent" for your report.
Key Factors That Affect TGA Results
When you calculate weight percent from TGA data, the accuracy depends heavily on the experimental conditions. Consider these six factors:
Sample Mass: Larger samples (e.g., >20mg) may suffer from thermal gradients, while very small samples (<2mg) are more susceptible to weighing errors and buoyancy effects.
Heating Rate: Faster heating rates (e.g., 20°C/min) often shift the weight loss step to higher temperatures due to thermal lag, potentially overlapping distinct decomposition steps.
Atmosphere (Purge Gas): Performing TGA in Nitrogen (inert) vs. Air (oxidative) dramatically changes results. Oxidative environments can cause weight gain (oxidation) or faster combustion.
Buoyancy Effect: As gases in the furnace heat up, their density decreases, causing an apparent weight gain in the sample. A blank run subtraction is often required for high-precision data.
Pan Selection: Aluminum, platinum, or alumina pans have different thermal conductivities and maximum temperatures. A reaction between the sample and the pan can distort weight data.
Volatile Impurities: If a sample is not properly dried or stored, surface adsorption of humidity can create an artificial initial weight loss step that skews the primary calculation.
Frequently Asked Questions (FAQ)
1. Can TGA show weight gain instead of loss?
Yes. Weight gain typically indicates a reaction with the purge gas, such as oxidation (rusting) of a metal in an air atmosphere or adsorption of gas on a catalyst surface.
2. How do I differentiate between solvent loss and decomposition?
Solvent/moisture loss usually occurs at lower temperatures (often <150°C) and looks like a smooth curve. Decomposition involves breaking chemical bonds and often happens at higher, specific temperatures typical to the material.
3. What if my TGA data is already in percent?
If your machine exports data in %, you can simply subtract the Final % from the Initial % to get the step size. Our calculator works with both mg and % units—just be consistent.
4. Why is the derivative (DTG) curve important?
The DTG curve (Derivative Thermogravimetry) plots the rate of mass change (dm/dt). Peaks in the DTG curve clearly identify the exact temperature of maximum weight loss, making it easier to select minitial and mfinal points.
5. Does particle size affect the calculation?
It doesn't change the theoretical weight percent, but fine powders decompose faster than coarse chunks due to higher surface area, potentially sharpening the TGA step.
6. What is "Ash Content"?
Ash content is the residual mass remaining after heating a sample to high temperatures (e.g., 600-900°C) in air. It represents inorganic fillers (like glass fiber or silica) in a polymer matrix.
7. How accurate is TGA?
Modern TGA microbalances are incredibly sensitive, often resolving down to 0.1 µg. However, user error in cursor placement on the analysis software is the biggest source of calculation variance.
8. Can I calculate stoichiometry without Molecular Weight?
No. You can calculate the mass percent lost, but to know how many moles of a gas were evolved, you must know the molecular weight of the leaving group (e.g., MW of CO2 = 44.01 g/mol).