A method for determining a concentration of at least one individual gas present in a gas-mixture includes irradiating a first component by incident modulated-light-beams characterized by a determined absorption wavelength range, modulation frequencies and a modulation amplitudes to generate first transmitted-light-beams, irradiating a second component, comprising a determined concentration of the at least one individual gas, by the first incident modulated-light-beams to generate second transmitted-light-beams, generating noise signals representative of a characteristic of the first transmitted-light-beams, generating noise-free signals representative of a characteristic of the second transmitted-light-beams, selecting an optimal modulation frequency and an optimal modulation amplitude from the modulation frequencies and the modulation amplitudes based on the noise signals and the noise-free signals, and determining a concentration of the at least one individual gas in the gas-mixture based on the optimal modulation frequency and the optimal modulation amplitude of the determined absorption wavelength range.