In the thermogravimetric analysis of solid samples (polymer) , the TG curves of the samples are assumed to start at 100% , but my results give TG curves starting at different percentages of mass for each sample (93 to 97%) , the sample mass in the beginning of the analysis (20°C) is lower than the initial mass of the sample. What is the reason of this shift? Are these results correct or do they require corrections ? (TGA analysis was carried out in order to compare the samples)
Either the instrument is not calibrated properly or the highly volatile solvent present along with the undried sample might have escaped from the polymer matrix.
You should follow this protocol: load the sample into the instrument, close the oven, allow to equilibrate for a few minutes, then set the initial mass. If you set the initial mass with the oven open you may get drift. If you still see changes in mass, this usually means the sample contains volatile solvent as S. Balasubramanian suggests.
Once the pan equilibriates for a sufficient time, then your base sample should measure 100% only, nothing else.
But you may consider the following points: presence of volatiles in your samples, take care that the pan is free from any cleaning solvent, get the equipment calibrated.
If your sample contains volatiles, then the data you are getting with 97% at room temperature, is the desired data which should lead you to your conclusions.
If your sample is devoid of volatiles, then your data is not reliable and you need to check till you receive an initial 100% weight.
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