Abstract: A barrier mechanism for isolating a drive chain from an active chamber in a Langmuir trough is provided. The chain drives a substrate mounted on a backing plate through the active chamber in order to coat the substrate. The chain passes through a "C" shaped barrier adjacent to the active chamber. The barrier mechanism comprises a resilient strip biased by a leaf spring to seal a side opening in the barrier. A wedge is mounted to the backing plate to open the resilient strip and leaf spring and thereby permit the backing plate to pass through the side opening as the substrate is driven into the active chamber.