This step converts the sample CO2 into graphite (elemental carbon). The sample CO2 is mixed with a stoichiometric amount of hydrogen gas inside a quartz reaction tube. The reaction tube contains a small amount of iron powder which will act as a catalyst for the reaction. The reaction tube is placed in a 700o C furnace and in about 5 hours the CO2 converts to graphite. Water is trapped out as a by-product of the reaction.