The present invention is directed to an apparatus and a method for continuously removing water vapor from a closed loop, re-circulated gas flow in an ion mobility spectrometer using a water permeable membrane, having a first side and a second side opposite the first side. The gas flow is disposed adjacent and carried past the first side to deposit water vapor that passes through the membrane to the second side. An exhaust flow is disposed adjacent and carried passed the second side of the membrane to remove the water vapor from the system. Therefore, the water removal apparatus continuously regenerates in-situ. Heaters, heat sinks and additional treatment systems including charcoal filters can also be included in the system to enhance its performance.