A detector has a sensor responsive to a first wavelength, a sensor responsive to a second wavelength, and a sensor for collecting reference readings. A gas sample is analysed to obtain readings corresponding to the first wavelength, the second wavelength and a reference. A first absorption figure is calculated using the first reading and the reference reading, and a second absorption figure using the second reading and the reference reading. A lineariser function is applied to the first and second absorption figures to calculate first and second concentration figures. The sensor for each wavelength is calibrated for detecting the first gas such that the data collected at each wavelength gives the same reading when only the first gas is present. The ratio of the first concentration figure to the second concentration figure is used to identify whether only the first gas is present.