A method of determining a concentration of a gas in a sample and/or the composition of a gas using a spectrometer comprises transmitting of radiation whose wavelength substantially continuously runs through a wavelength range, wherein the continuous running through of the wavelength range is overlaid by a wavelength modulation; measuring of an absorption signal as a function of the wavelength of the radiation; converting of the absorption signal into first and second derivative signals; deriving of a first and a second measured gas concentration value from the first and the second derivative signals, respectively; and determining of the concentration and/or the composition of the gas from at least the first measured gas concentration value, wherein the wavelength modulation is adapted in response to a change of a state variable of the gas such that a ratio between the first and the second measured gas concentration values is kept substantially constant.