An optical absorption gas analyzer is provided for determining the concentration of a target gas in a sample, comprising: a chamber for containing the sample in use; an optopair, comprising a light emitting diode (LED) arranged to emit radiation into the chamber and a photovoltaic radiation detector arranged to detect radiation transmitted through the chamber from the LED and to output a corresponding detection signal SS; a temperature sensor arranged in thermal contact with the LED and the photovoltaic radiation detector, and to output a temperature signal T representing the temperature of the optopair; a memory having stored therein data representative of the baseline detection signal ST output by the optopair in the absence of the target gas as a function of the temperature of the optopair across a range of temperatures; and a processor adapted to generate a differential detection signal SA indicative of the concentration of target gas in the sample by retrieving from the memory the baseline detection signal ST corresponding to the temperature signal T and calculating the difference between the detection signal SS and the baseline detection signal ST.