Devices and methods are disclosed herein for separating a supernate from a suspension. The apparatus consists of a sample zone, a controllable gate, and an analysis zone. The sample zone holds the suspension. The analysis zone passively transports a supernate formed from the suspension by capillary transport. A controllable gate prevents the suspension in the sample zone from flowing into the analysis zone. The controllable gate can be triggered after the supernate has separated from the suspension to allow the supernate to flow into the analysis zone.