An apparatus for determining a presence, a concentration or a change in concentration of a target material in an environment is disclosed. The apparatus comprises first and second sensors configured to respond to the target material. The apparatus further comprises a fluid inlet in fluid communication with the environment, and a valve assembly having a first and second configuration. In the first configuration, the fluid inlet is in fluid communication with only the first sensor. In the second configuration, the fluid inlet is in fluid communication with the first sensor and the second sensor.