A semiconductor processing reactor comprises a reaction chamber with a gas exhaust and a mechanical seal at one end of the chamber. The seal seals off the chamber from the ambient environment and is purged with gas to prevent diffusion of ambient gases into the reaction chamber. Because the purge gas can diffuse through the seal into the reaction chamber, the purge gas is chosen based upon the process gas and the location of the seal and exhaust so that the molecular weight of the purge gas causes the purge gas, by the force of gravity or buoyancy, to remain in the portion of the reaction chamber containing the seal and the gas exhaust. Advantageously, keeping the purge gas at the same end of the chamber as the gas exhaust minimizes dilution of the process gas with the purge gas, thereby preventing the purge gas from detrimentally effecting process results.