This method is for determining low concentrations (0.2 to 500 mol-ppm) of carbon monoxide (CO) and carbon dioxide (CO2) in high purity hydrogen, methane and other gas-phase samples. The lower limit of quantitation for CO and CO2 in samples containing high concentrations of hydrogen, nitrogen, and other gases, is 0.2 mol-ppm. For samples containing high methane concentrations, an analysis under a different valve configuration is necessary to resolve carbon dioxide from interfering methane.