FTIRs are the equivalent of DOAS (Differential Optical Absorption Spectrometer) spectrometers but operating in the infrared spectral region. These instruments are capable of analyzing several volcanic gases simultaneously. However, they require a heat source to operate. This can either be an artificial heat lamp set up behind a volcanic vent, or a natural source such as a lava lake. This requirement makes continuous operational use of FTIR instruments challenging.