I am attempting to use TCD-GC with a 5A mol. sieve column and N2 carrier gas to detect CO produced from carbon dioxide reduction. When calibrating the instrument with either a commercial calibration gas mixture (0.5%CO and CO2 in nitrogen) or by chemically generating CO by dehydrating formic acid I get a linear calibration curve for moles CO vs peak area. If I repeat the formic acid dehydration under a CO2 atmosphere (instead of air or N2) I get about half the peak are for the same amount of CO.