Usually it is good to test the same polymer both in N2 and air. However, it depends on the type of polymers or composites. Usually thermal oxidative degradation is higher in air than with the N2. The sample preparation condition (e.g. drying condition, same sample weight, same composition) should be right and at least 3 samples should be tested for each test condition (i.e. air or N2). 

Keep in mind that the atmosphere can completely change the degradation pathway (thermal vs. thermooxidative degradation). Hence, it usually does not make sense to directly compare stability of polymer under inert and oxidative atmosphere.

Are you sure that the temperatures (350 vs. 320°C) correspond to the same "event" in the sample?

As Tibor said, be carreful  when doing TGA experiments in different media. In air you can change the degradation mechanism compared to the experiment in N2 medium. Usually way is under sufficient N2 flow. But you can also use the inert gases, like Ar or He who are more expensive.    

Dear Ba,...

please explain a little bit more which kind of composite are you are investigating with the TGA method.

Which kind of polymer do you use ? Which kind of second material is part of the composite? Is it a reinforcing fiber? What is your sample weight, your heating rate and your maximum temperature at the end of the heating.

In principle the TGA is an excellent method to measure the thermal stability of a material. Heating up the sample in nitrogen delivers Information about the principle strength of the chemical bondings. Outgasing, that means loosing weight, is an example for decomposition of the material. The higher the temperature when the weight loss start,  the stronger the bonding is. There is no difference of  the result when using Ar or He.

When using oygen, you will measure an additonal effect of oxidation of the polymer and of the reinforcing material. Natural fibres might oxidise at lower temperature than carbon fibres, .... glass fibre are relatively stable .. and so on.

This measurement at air delivers you a realistic view of the thermal behaviour under normal conditions of use.

Perhaps I can give you some more information, when knowing the details of the used composite.

With best regards

Michael