I am investigating the thermal analysis of a clay by TGA using nitrogen gas and at the end the curve is going up. Can anyone give me a plausible explanation of that behavior? Or what do you advice? Thank you in advance
There's a few issues that I can think of:
- Look at the curve around 250 - 300°C, you have mass gain that may be related to the buoyancy effect (increased temperature causes a decrease in gas density causes a small weight gain). This is generally a small effect, if you have sufficient sample mass you should not see this effect your data.
- Did you do a blank subtraction? The mass of your TGA pan drifts as you increase temperature. Again, if you have sufficient sample mass this would not effect your run this much.
- Your nitrogen may have oxidizing species in it. Oxidation can result in an increase in mass after reaching a critical temperature. You can check this relatively easily. Ramp to 600°C (see your graph, that is where mass started rising) then do an isothermal hold. If your observed mass starts increasing then you have a nitrogen gas issue. If this is the case, run a nitrogen purge prior to ramping (Isothermal hold at 100°C for 30-60 minutes prior to continuing the ramp).
I would personally try using more sample and incorporating the isothermal hold. This is based off of your observed mass % being higher than 100% at 1000°C.
Faheem Muhammed thank you for your help. I did not do blank subtraction. the machine is calibrated, and I am using Al2O3 pan for the measurement. we did other measurements on organic material; the temperature was 800 C but we did not get issues on that.
Dear Alassane,
in general, in this type of analysis (of clays) peaks must be observed: associated with the loss of free water (up to approximately 120 ºC and refers to the humidity that all clay has); between 450 ºC and 700 ºC clay mineral decomposition reactions occur (loss of hydroxyls). Above this temperature there should be no mass gain. The mass gain observed in your curve above 600 ºC could be associated with the oxidation of your sample - did you take the measurement with the sample holder open or did you close it before taking the measurement?
Best regards
Carlos
Dear Carlos Ariel Samudio Pérez the sample holder was opened during the measurement. thank you for your help
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