Abstract: The sample gas is divided into two portions, one being subjected to flame ionization detection to measure all hydrocarbons, the other one being passed through a tube having active carbon to remove all higher hydrocarbons by adsorption except methane. The resulting gas is also subjected to flame ionization detection and the difference in detection gives the non-methane impurities, either electrically as difference signal or graphically. The adsorber tube may be exchanged for another one while the former is cleaned and purged. A simple flame ionization detector is used alternatingly or two are operated in parallel to obtain the two readings.