Interferent and baseline drift correcting sensor system

A method of determining a presence, concentration or change in concentration of a target material in a gaseous or liquid environment, the method comprising:

• measuring, in either order, a first response and a second response of a sensor of a sensor system wherein the sensor is configured to respond to the target material and wherein the sensor system comprises:
• the sensor;
• an inlet configured to draw gas from an environment into the apparatus;
• and
• a valve arrangement configured to direct fluid drawn from the environment to a first fluid flow path in fluid communication with the sensor or to a second fluid flow path in fluid communication with the sensor,
• wherein the first response is a response of the sensor to fluid from the first flow path which has been treated to remove the target material from the fluid and the second response is a response of the sensor to fluid from the second flow path which has not been treated to remove the target material from the fluid;
• and
• subtracting the first sensor measurement or a derivative thereof from the second sensor measurement or a derivative thereof.