Provided is a gas isolation valve which separates reactive materials, principally gases, contained in a high temperature reactor from the surrounding atmosphere. The valve is of modular construction with each module having a gas providing section and a gas removal section. Any number of modules can be provided in series. A central chamber, open at each end gives unimpeded access to the high temperature reactor. It is through the central chamber that the product of the reactor is removed and harvested. In the case of Ilas invention the product is carbon nanotubes.