This can be considered very broadly: Stack gas is anything that comes out of a burner stack in gaseous form. This is, however, not so simple as it sounds! Stack gas will contain water in vapour form when it is hot. This water will liquefy when the stack gas cools down, reducing the volume and producing condensate. Gas analysers are used for many purposes, but the main reason is measurements in stack gas, and most definitions are made for this medium. Extractive methods measure the concentration of pollutants in dry stack gas, whilst in-site equipment tends to measure on a wet basis, including the volume of the water vapour at that temperature.
Stack gas consists mostly of nitrogen and carbon dioxide. When hot, water vapour must also be considered. There will also be some oxygen in there, but this should not be too much. All other components are, roughly speaking, pollutants. Stack gas should be sampled before any dilution has been carried out. It is possible to calculate the concentrations of the components before dilution, using the reference oxygen factor, but this will magnify errors and means that the true oxygen content of the stack gas cannot be ascertained.