TG measurements on the Hitachi STA7300 equipment started to have different results for the same material after some time of use
Some tests have already been carried out to try to correct it:
- Gas flow check (100 mL/min)
- Oven cleaning (Heating until 1400 °C)
- Changing sample supplier (soybean oil (sample tested))
- Pan cleaning
- TG calibration (TG beam expansion)
None of these attempts resolved the problem.
Here's the problem:
We used samples of soybean oil, whose initial mass loss is 300 °C (correct result and according with literature). After some time of using the equipment, the same material shows an initial mass loss at 200 °C and multiple peaks in the DTG (they should not appear, it should just be a single main peak).
What could be happening to the equipment to provide such results? Have you ever experienced something like this?
Here are the mass loss curves for the same sample after some time of using the TG.
Same material, different format like powder, film, chunk? same ramp rate? It almost looks like someone accidentally bumped the instrument or there was a gas surge on the upper run.
Yes, it is exactly the same sample, without significant changes in the mass (which could affect the degradation mechanisms). Same heating rate (10K/min).
The sample is epoxidized soybean oil (viscous liquid). We tried to change the gas flow to 50 mL/min, it didn't solve it...
I don't understand how the degradation may be occurring at lower temperatures, the only thing I can think about now is that there is contact with the atmospheric air somehow, perhaps the purge isnt being efficient and should we increase the flow of gas?
You should try 2.5°C/min. 10°C/min is typically the "rapid" edge and most people using dynamic TGA dismiss results from rates above 5°C. Reduce your gas flow, there is no reason to blow the sample out of the pan.
This compound does not present variations of this nature in mass loss even at rates of 20K/min... The problem lies in the difference in signal between the two samples, there is equipment that promotes a small vibration on the table where the TG is located, do you think Could this vibration be causing this variation? This vibration is from the air conditioning support in the room itself, could it be "shaking" the sample inside the pan? It's not that strong of a vibration, but it's there.
On the other hand, do you suggest that TG runs be under 10 K/min? The kinetics for standard rates 5, 10, 15, 20 K/min provide very interesting results, but I have found that there are differences for the kinetics for slower rates: 1 / 2 / 4 / 8 K/min
Our flow meter has a lower minimum limit of 50 mL/min, we will standardize this flow and try to correct vibrations, which could be the possible cause of the difference between analyzes
Thank you very much for the advice
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