A method of detecting gaseous species in a mixture by light-emission spectroscopy, in which use is made of the radiation emitted by a plasma (4) present in the gas mixture under analysis, a measurement system (20) is used to take a raw optical spectrum of said radiation emitted by the plasma (4), and the raw optical spectrum is compared with a library of reference optical spectra, the method comprising a step of generating a pruned optical spectrum, which step consists in making use, in the raw optical spectrum, of only those zones of the spectrum that present a significant shape corresponding to a predefined shape criterion, and subsequently said pruned spectrum is compared with the library of reference optical spectra.