I do not know experimental results for both measurements, and thus I have to guess them partially. First of all it is clear that DTA and DSC should give to a great extent similar results. This is because DTA measures just the T difference between sample and reference, and DSC makes this more quantitative by measuring the heat flux, which allows determination of enthalpy. Typically DSC setups are more sophisticated, and work with smaller samples. This makes peaks typically sharper. One other (funny) thing is the direction of exo/endo signals: The German DIN standard requires "exo down", the American ASTM standard requires "exo up". To avoid confusion it is a good idea to have a corresponding arrow within the DTA/DSC diagram, which is not the case here. But it may easily happen that the directions are opposite, if different devices were used. (Some devices allow to change it by software, e.g. NETZSCH). Taking this into account, I can imagine that the sharp DSC peak near 240°C is the same effect like the broader effect in the DTA curve, and DTA is just broader for reasons said above. I cannot comment on the origin, because I do not know your substance so good. The question remains, why the other peak near 180°C appears only in DSC. As far as I know, guanidinium chloride looses HCl there. This could be proven by simultaneous TG, if available. Maybe the DTA curve shows a second heating? Then the sample might have lost already its HCl, and you see nothing again at 180°C. It would be nice to know what is happening at the higher temperature, because the effect has opposite sign. But before discussing this, you must make clear the "exo" direction.

As Detlef Klimm wrote, first find out the nature of the transition reactions (endo, exotherm). From the holistic view, the figures show thermoanalytical responds of two different samples.

Detlef Klim and Dietmar Kobertz, thank you very much in advance for your kind and valuable answers.

I'm attaching in this post a complete TGA / DTA / DSC graph that might set things clearer. The TGA / DTA analysis were run simultenously using a TA Instruments model 2960, while the DSC analysis was made in a Perkin Elmer Sapphire calorimeter.

It is of very much importance that while the TGA / DTA data appear to be consistent within themselves, the DSC signals are in accord with an investigation in thermal decomposition of guanidinium salts performed by Wendlandt et al in 1984 and found in a peer-reviewed article in Thermochimica Acta. They report a sharp and endothermic DSC signal at 178 ºC, which they assigned to the melting of guanidinium chloride. This sharp signal is also observed at around 180 ºC in the DSC analysis I performed. Wendlandt also reports a second (but far less sharp) endothermic signal at around 340 ºC, which is definitively NOT observed in the DSC I performed.

For logistic reasons I couldn't perform more than a single TGA / DTA analysis, but I performed five DSC repetitions and all of them showed the same endothermic / exothermic signals at almost the same temperatures. I'm currently working in performing the TGA / DTA analysis in a different analyzer, but any ideas in order to obtain better results are kindly appreciated.

Is the thermal history of both samples the same? Did you try to study annealing experiments? Your sample is crystalline, nevertheless the crystallinity of the sample may depend on cooling rate. Peaks of rversed sign may appear if you heat up an amorphous sample which first crystallizes, then melts. Did you use the same heating rate in both experiments?

An option is to perform an STD run, this way you'll get weight loss, DTG and heat flow in a single shot, I'm not really sure if CIDIIT has such equipment, CIMAV certainly has one.

Dear Sandoval,

DTA is based on the temperature difference while DSC is based on the heat flow difference. Differential thermal analysis (DTA) is suited for the determination of characteristic temperatures, while differential scanning calorimetry (DSC) additionally allows for the determination of caloric values such as the heat of fusion or heat of crystallisation. This can be done with two different measuring techniques: heat-flux differential scanning calorimetry or power-compensated differential scanning calorimetry.

DSC and DTA  are both  used in measuring a glass transition, phase changes, purity evaporation, melting, purity crystallization, sublimation, polymerization, heat capacity, compatibility, pyrolysis, etc.

Hope this helping 

Have a very good day