I have attached herewith TGA, DTG, DSC profiles for my sample. I am okay with the DSC result. But, the TGA is showing a negative percentage value and a total maas loss 132.03%. How can we explain such a TGA?
My other query is I got my DTG with a peak downside. But I have found peaks for the same sample (cellulose) in reported works (attached herewith) upside. I am a bit confused on this part. Please help.
Dear Kashmiri
If the atmosphere was not inert such as N2 the oxidation reactions can be occur. so it can be appear as a negative curve
From the attached plot, it can be seen that the instrument was not accurately calibrated to 100% for the start of the essay, even if the difference looks relatively small. It seems that that the initial mass was around 103% or so. The difference may seem small, but if not explained reasonably, may point to a (seemingly) possible analogous inaccurate calibration in what regards to the 0% (bottom limit) of the weighting percentage scale. While attempting to check this (apparent) possibility, it might help if you could access the alternative plot depicting mass of the sample vs. time.
I agree on both answers, check calibration and also it depends on the atmosphere you are using
From cellulose I would not expect > 100 % weight loss under the atmosphere used. Your weight loss below 300 dC is huge for cellulose but if you have oxygen in your purge line this may account for this - under nitrogen I'd expect < 5 wt-%. It could be that the hangdown or arm on the microbalance was contaminated from a previous run that you may have burnt off during your run and would suggest that you do a burn run under air to at least 800 dC, preferably 1000 dC, hold for 10 mins with a reasonable purge rate 100 ml/min and then run a standard sample such as calcium oxalate under nitrogen to check the TGA response
The fact that your mass starts above 100% (especially extrapolated to room temperature!), your cellulose sample (like most) would have some amount of water adsorbed to it (cellulose is insoluble in water, but water will still get on it - think of cotton shirts in humid vs. dry conditions - weighs different).
As for why you get negative mass% -- oxidation reactions would actually increase mass% if we're talking about for example going from metal to metal oxide; oxidation in the sense of combustion of sample should just take you to zero mass%. What is possibly happening is that something that was measured as the "pan weight" is burning off. Your pan is Al2O3 according to your graph, so that shouldn't be burning off (unless it somehow got wet before taring the mass).
My guess is, a previous user of your equipment may not have done a "burn off" of the Al2O3 crucible (I call it "pan" only because we use platinum pans usually and do not go as high a temperature in my lab, sorry if there's any confusion there), so that there were enough residues that subsequently "burned off" during your own experiment. This could be easily verified by heating your crucible again to 800 degC (just in case), and then after cooling back down to ambient temperature, immediately repeating your pure cellulose experiment. Hopefully that solves your problem.
In view of the excellent answers so far I recommend cleaning up your thermobalance (pans, lever arm counter weights and purge gas), calibrating and run a blank. Expect some minor drift in a clean instrument depending on make. Then run your sample and tell us about it.
Hi
negative weight loss means gain of weight, might be oxidation, but what atmosphere gas you are using during the TGA analysis?
or it might be due to experimental error
Have a very good day
thanks
Ahmed
I also have the same problem. My sample is Chitosan with 10% acetic acid. The graph trends is fine but around 270 C the weight percentage goes down zero. I have repeated it several times and still same result. I calibrated the weight and the instruments works perfectly for other material. Do you have any suggestion?
It may occur due to not maintaining an inert atmosphere or the machine is not calibrated correctly.
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