Abstract: This disclosure provides systems, methods, and apparatus related to infrared spectroscopy. In one aspect, a device includes a first assembly, a porous membrane, and a second assembly. The first assembly defines a fluid distributor. The porous membrane overlies the fluid distributor and forms a surface of the fluid distributor. The second assembly is disposed on the first assembly. The second assembly defines a plurality of capillary arrays surrounding a window in the second assembly that exposes the porous membrane. Capillaries of each of the capillary arrays have openings on edges of the window of the second assembly and are operable to direct moisture across a surface of the porous membrane.