I have been told that because the DSC peak represents heat capacity (Cp) it should and cannot go below the zero mark as the temperature increases. But I have curves that go below the zero mark - for example the curve below. Can I get an explanation for this?
Dear Caleb Ojo ,
The DSC technique measures the amount of heat released or absorbed during the increase of temperature in a sample. This phenomena can be attributed to different processes. This of course will depend on the sample you are measuring. For example: if we measure a semi-crystal polymer in DSC, we will see an endothermic peak, which is due to the melting of the crystals (melting is a process that requires heat in order to happen). Other change can be for example a polymerization that releases heat when happening. In this case, we will observe an exothermic peak.
From your DSC sample, I observed two endothermic peaks (above zero) and one exothermic peak (below zero). So I suggest you to imagine any exothermic process that may occur to your sample and maybe there is the answer!
Best of luck!
Sharp endothermic peak is usually correlated to the melt of crystals whilst exothermic to re-crystallization of amorphous materials, provided the materials were not degraded.
According to DSC/TGA of your sample, it can be concluded that the material was melted (1st sharp peak) during degradation (2nd broad peak) and the last (can be anything like recrystallization of different materials, or artefacts due to degradation etc. )
The technique of DSC measures heat flows by comparing your sample and a reference. So, it is normal to have results below the zero line. So I guess you do not need to worrt about this. This will be your baseline for your analysis and that's ok. Normally, the temperatures of your interest and the areas under the peaks are the results you are looking for, since they do not depend on the reference you employed.
Baseline is in general correlated to the system configuration. If it is not configured propetly, one could get badeline below zero and also possibly not flat but with gradiant against temperature axis, which can be corrected by baseline correction function of the software. One needs to be careful though to do baseline correction for quantification studies by dsc or tga.
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