This method is for determining low concentrations (0.5 to 500 mol-ppm) of carbon monoxide (CO) and carbon dioxide (CO₂) in high purity hydrogen and other gas-phase samples. The lower limit of detection for either CO or CO₂ in hydrogen is 0.5 mol-ppm. The lower limit of detection for CO in samples containing high concentrations of oxygen, argon or nitrogen is 5 mol-ppm. Methane at concentrations above 5 mol-% will interfere with the determination of carbon dioxide.