substantially moisture and contaminate free air the ratio of air to flue gas will vary depending on the moisture content of the flue gas and the precise ratio needed can easily be determined by one having ordinary skill in the art although the flue gas sample need not be completely dry it is necessary that the temperature of the flue gas at the electrochemical sensors be above the dew point psychrometric charts are readily available and are used to determine the quantity of air that must be added to the flue gas sample to insure that the sample remains in the vapor phase through the sensor area the dilution rate should be minimized because the detection limit is directly and proportionally impacted thereby it is preferred that the sample of flue gas be preconditioned such as in a way that is illustrated in fig2 hereof in fig2 hereof a sample of flue gas is obtained from the stack via a suitable probe and passed via line 10 to a first separator s 1 for removing at least a portion of the particulates that may be present particularly the larger size particulates separator s 1 can be any suitable device for separating solids from a gas non limiting examples of such devices include electrostatic precipitation devices and cyclones cyclones are preferred the resulting effluent sample flue gas from separator s 1 is optionally but preferably passed via line 20 to second separator s 2 wherein at least a portion of any remaining particulates particularly the smaller size particulates are.