Described is a method for quantifying the amount of optically interfering gas impurities in a gas detection system comprising a sample gas inlet, a reference gas inlet, a gas modulation valve, and an infrared absorption gas detector used for analysis of methane or natural gas, wherein the gas modulation valve alternatingly connects the sample gas inlet to the gas detector during a sample gas time period and the reference gas inlet to the gas detector during a reference gas time period. The method includes measuring an infrared absorption for at least two different sample gas concentrations in the gas detector achieved via respective different ratios from the sample gas time period and the reference gas time period, and comparing amplitudes of different measurement signals of the at least two different sample gas concentrations with calibration functions to assess an actual gas impurity concentration in the sampled gas.