TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: In method (A), the first atmosphere is formed from a first pure gas, where the weighted average molecular weight of gases of the first atmosphere is a molecular weight of the first pure gas. The second atmosphere is formed from a second pure gas, where the weighted average molecular weight of gases of the second atmosphere is a molecular weight of the second pure gas. The third atmosphere is formed from a third pure gas, where the weighted average molecular weight of gases of the third atmosphere is a molecular weight of the third pure gas. The first, the second and/or the third gases comprise a mixture of at least two pure gases. The gases constituting the first, the second and the third atmospheres flow upwards through the reaction chamber when the gases have increasingly higher densities or weighted average molecular weights and flow downwards through the reaction chamber when the gases have increasingly lower densities or weighted average molecular weights. In method (A)), flowing gas involves comprises maintaining the reaction chamber at atmospheric pressure. In method (B), the first purge gas and the second purge gas are inert or non-reactive with the substrate. The first purge gas, the process gas and the second purge gas flow vertically to exit the chamber after entering the chamber. The method (B) further involves selecting the first purge gas, the process gas, or the second purge gas having weighted average molecular weights or densities which bias earlier present gases to exit the reaction chamber, by the force of gravity or buoyancy, before later injected gases. Flowing a first purge gas, replacing the first purge gas and replacing the process gas involves (a) injecting gases into the reaction chamber from a gas entrance at a top of the reaction chamber and exhausting gases out of the reaction chamber from a gas exhaust at a bottom of the reaction chamber or (b) injecting gases into the reaction chamber from a gas entrance at a bottom of the reaction chamber and exhausting gases out of the reaction chamber from a gas exhaust at a top of the reaction chamber. The weighted average molecular weights or densities of the first purge gas, the process gas and the second purge gas progressively decrease with injection of each of the gases according to step (a) and progressively increase with injection of each of the last three gases according to step (b). In method (B), replacing the first purge gas involves (a1) actively mixing the first purge gas in the chamber with the process gas, where actively mixing is performed proximate an entrance of the process gas into the chamber; and (b1) actively mixing the second purge gas with the process gas in the chamber, where actively mixing is performed proximate an entrance of the second purge gas into the chamber. In step (a1), a molecular weight of the process gas is more than a molecular weight of the first purge gas when the entrance is proximate a top of the chamber and the molecular weight of the process gas is less than the molecular weight of the first purge gas when the entrance is proximate a bottom of the chamber. In step (b1), a molecular weight of the second purge gas is more than the molecular weight of the process gas when the entrance is proximate a top of the chamber and the molecular weight of the second purge gas is less than the molecular weight of the process gas when the entrance is proximate a bottom of the chamber. Actively mixing the first purge gas in the chamber with the process gas involves flowing the process gas and some of the first purge gas in the chamber through a static mixer or through a venturi device. Actively mixing the second purge gas with the process gas in the chamber involves flowing the second purge gas and some of the process gas in the chamber through a static mixer or through a venturi device. In method (C), the chamber comprises a gas inlet proximate one end of the chamber and a gas exhaust proximate an opposite end of the chamber. A molecular weight of the second gas is more than the molecular weight of the first gas when the inlet is proximate a top end of the chamber and the molecular weight of the second gas is less than the molecular weight of the first gas when the inlet is proximate a bottom end of the chamber. The gas inlet is proximate the top end of the chamber and the gas exhaust is proximate the bottom end of the chamber. In method (C), flowing the second gas into the chamber involves flowing the second gas and the first gas into the chamber from the same gas inlet. The unit of time is a duration required to completely replace an atmosphere of the chamber. The variation amount per unit of time is less than or equal to 1.6 (preferably less than or equal to 0.8, especially less than or equal to 0.4) per minute. The unit of time is greater than or equal to 30 (preferably greater than or equal to 60) seconds.Preferred Components: The second atmosphere of method (A) comprises an oxidant. The process gas of method (B) is selected from O2 and/or H2O. In method (C), the first gas is N2 and the second gas is O2.