The instant invention provides apparatuses for measuring the level or concentration of chlorine dioxide gas in a sample and methods of using the same. One aspect of the invention provides an apparatus for measuring a concentration of a chlorine dioxide gas in a sample. The apparatus includes a light emitting diode (LED), a light sensor, and a flow path between the LED and the light sensor, and a filter configured to remove chlorine dioxide from a reference stream. The flow path is capable of containing a sample. The sensor is capable of measuring the level of chlorine dioxide in the sample and the reference stream.