A cartridge for analyzing a fluid sample has a body defining at least first and second channels and a cavity separating the channels. An end of the first channel is positioned on a first side of the cavity, and an end of the second channel is positioned on a second side of the cavity. The cavity is defined by a first curved surface positioned adjacent the end of the first channel, a second curved surface positioned adjacent the end of the second channel, and at least a third surface between the first and second curved surfaces. The cartridge also comprises an elastic membrane for establishing a seal with the first and second curved surfaces to prevent the flow of fluid between the channels. The third surface is recessed from the first and second surfaces to provide a gap between the membrane and the third surface when the membrane is pressed against the first and second surfaces. A cartridge having a conical valve seat is also disclosed.