Systems and method for analysing contaminants of a gas sample of natural gas are provided. An interrogation light beam propagates into a chamber of a multipass gas cell receiving the gas sample. The interrogation light beam has a wavelength controlled to alternately correspond to an absorption wavelength of H2S and an absorption wavelength of an additional gas contaminant. The additional gas contaminant may for example be CO2 or H2O. In some implementation, a single laser emitter may be used to generate the interrogation light beam at the H2S and CO2 wavelengths. In some implementations, two different laser emitters may be used to generate the interrogation light beam at the H2S and H2O wavelengths. A WMS detection scheme may be used.