The Spectronus FTIR analyses the concentration of methane, carbon dioxide, nitrous oxide, and carbon monoxide in air samples using absorption spectroscopy. Air is pumped into a 3.5L sample cell. An infrared lamp at one end of the cell provides the radiance source. The emitted thermal radiation is passed through the sample cell multiple times by a set of mirrors (to maximise the absorption path length to 24m), before entering the spectrometer. The measured interferograms are Fourier transformed to produce high resolution (1.0 wavenumber) radiance spectra, from which the gas concentrations are retrieved using optimal estimation software (Multiple Atmospheric Layer Transmission, MALT - Griffiths 1996).