Noun
zero air
atmospheric air specially cleaned so that it contains less than 0.1 ppm of hydrocarbon impurities; it then is usable in laboratory procedures such as versions of gas chromatography in which sufficiently pure air may serve as a carrier gas.
For calibration, first pass a known range of concentrations of the gas through the gas chromatograph, for comparison with zero air that contains effectively none of the gas to be measured.