A method for distinguishing a target gas from a group of target gases, the method comprising: contacting a gas stream with a chemically treated paper tape ; emitting at least three wavelengths of light comprising a red wavelength, a green wavelength and one of a blue wavelength and an ultraviolet wavelength, from a light source ; detecting a reflected portion for each of the at least three wavelengths of light which reflects from stains in the paper tape ; sensing one or more of a humidity and a temperature of the gas stream; and using pattern recognition to identify and/or distinguish the target gas from the group of target gases, wherein the pattern recognition includes calculating a characteristic from the reflected portions of light for the at least three wavelengths at the sensed one or more of humidity and temperature and matching the characteristic with a previously recorded characteristic and identifying the characteristic as belonging to the target gas or to the group of gases; wherein the characteristic is the location derived from the reflected portions for the at least three wavelengths on a plot of either: ratio of adjusted counts, AC, for the red wavelength to AC for the green wavelength versus ratio of AC for the blue wavelength to AC for the green wavelength, or ratio of AC for the red wavelength to AC for the green wavelength versus ratio of AC for the UV wavelength to AC for the green wavelength; wherein adjusted counts, AC, are calculated according to: AC = S 0 / S t − 1 , wherein S is the original intensity of the light emitted from the light source at time zero and S(t) is the intensity of the reflected portion of the light at a respective one of the at least three wavelengths which is detected at time, t. contacting a gas stream with a chemically treated paper tape ; emitting at least three wavelengths of light comprising a red wavelength, a green wavelength and one of a blue wavelength and an ultraviolet wavelength, from a light source ; detecting a reflected portion for each of the at least three wavelengths of light which reflects from stains in the paper tape ; sensing one or more of a humidity and a temperature of the gas stream; and using pattern recognition to identify and/or distinguish the target gas from the group of target gases, wherein the pattern recognition includes calculating a characteristic from the reflected portions of light for the at least three wavelengths at the sensed one or more of humidity and temperature and matching the characteristic with a previously recorded characteristic and identifying the characteristic as belonging to the target gas or to the group of gases; wherein the characteristic is the location derived from the reflected portions for the at least three wavelengths on a plot of either: ratio of adjusted counts, AC, for the red wavelength to AC for the green wavelength versus ratio of AC for the blue wavelength to AC for the green wavelength, or ratio of AC for the red wavelength to AC for the green wavelength versus ratio of AC for the UV wavelength to AC for the green wavelength; ratio of adjusted counts, AC, for the red wavelength to AC for the green wavelength versus ratio of AC for the blue wavelength to AC for the green wavelength, or ratio of AC for the red wavelength to AC for the green wavelength versus ratio of AC for the UV wavelength to AC for the green wavelength; wherein adjusted counts, AC, are calculated according to: AC = S 0 / S t − 1 , wherein S is the original intensity of the light emitted from the light source at time zero and S(t) is the intensity of the reflected portion of the light at a respective one of the at least three wavelengths which is detected at time, t. <EOS>