Thermal Analysis Trivia

8th : Does the sample burn or oxidize even under nitrogen flow?

Even under nitrogen flow, residual oxygen surrounding the sample within the furnace may have an unexpected effect on the TG-DSC or TG-DTA results. 

During TG-DSC or TG-DTA measurements, does the sample undergo combustion or oxidation despite the nitrogen flow? In reality, even with a high-purity nitrogen flow, the residual oxygen concentration around the sample inside the furnace does not reach completely zero percent. Oxygen remains at the ppm level, and the sample may react with that residual oxygen.

Of the two (three) gas inlets on the TG-DSC (TG-DTA), the residual oxygen concentration remaining inside the furnace tends to decrease more easily when nitrogen is flowed through the balance gas port (the inlet at the rear of the balance chamber). If nitrogen gas is currently being fed through the inlets for the middle gas port, switching to the balance gas port inlet may improve the situation.

Another factor is the purge time. When the sample is loaded, it is exposed to the atmosphere. Therefore, rather than starting the measurement immediately after placing the sample in the sample holder and closing the electric furnace, allowing sufficient time to purge the furnace interior before beginning the measurement will likely reduce the residual oxygen concentration. As a general guideline, a purge time of 10 to 15 minutes is recommended. Extending the purge time can be expected to further reduce residual oxygen levels. By using the optional oxygen reduction set, it will be possible to achieve lower residual oxygen concentrations (5ppm) — even with a purge time of approximately 3 minutes —compared to standard simple flow conditions.

As one final important note: do not use silicone tubing for the piping between the gas cylinder and the equipment! Silicone tubing allows oxygen to permeate through it.