Figure 6

The effects of carrier gas and flow rate on residence time. A) CO₂ signal (black, blue, green, and red) and temperature (orange) vs. time for two different flow rates of N₂ and He carrier gases. All runs were done at 30 mbar with a 35°C/minute ramp rate. Temperature data shown are for a run using. He carrier gas. The high diffusivity of CO₂ in He make it very difficult to pump from our system, and the residual CO₂ is ~10x higher than the N₂ carrier gas runs at the same flow rates. The large dip in CO₂ pressure at ~3300 seconds, most apparent in the 3sccm He data is caused by the back reaction of residual CO₂ with CaO to form CaCO₃ when the sample is cooling. B) The ratio of the mass 44 background to the mass 44 signal at t₁ (marked in A). Levels of CO₂ in the N₂ carrier gas return to baseline levels at ~10sccm as opposed to 20 for He. It is clear that for systems with a large chamber volume, N₂ is much more effective at removing evolved gas products than He at almost any flow rate.