Gas samples containing oxygen, nitrogen, hydrogen, helium, carbon dioxide, methane, and trace amounts of C2 to C2 hydrocarbons are analyzed by a gas chromatographic method that utilizes both thermal conductivity and flame ionization detectors. Porapak and molecular sieve columns are used with argon as the carrier gas. A column selector valve makes it possible to perform a complete analysis either on one 1 ml sample in 40 minutes, or on two 1 ml samples in 20 minutes. Samples are introduced either by on-column injection or through a gas sampling valve. By varying operational parameters, the speed, sensitivity, and accuracy of analysis may be modified.