A rotary valve that includes a stator, a rotor and a plurality of sample channels. The stator includes a stator surface having an inlet port, an outlet port and a plurality of selectable ports. The rotor includes a rotor surface having a first rotor channel and a second rotor channel. The rotor is configurable in a plurality of rotor positions, each of which couples the inlet port to one of the selectable ports through the first rotor channel and couples the outlet port to another one of the selectable ports through the second rotor channel. The two selectable ports are coupled to each other through one of the sample channels. The rotor has a bypass state defined by a rotor position, or angular range of rotor positions, at which the inlet port is coupled to the outlet port through the second rotor channel.