Abstract: A method and apparatus for continuously withdrawing a gas sub-sample from a chamber enclosing a source of waste gas for introduction of the sub-sample to a gas monitor. The method includes withdrawing a sample of the waste gas from the chamber by use of a vacuum source located exterior to the chamber. From the latter sample a sub-sample of waste gas is extracted prior to the waste gas entering the vacuum source. The sub-sample is then cooled below the ambient dew point so that condensate contained in the sub-sample is separated from the latter. The cooled sub-sample is then directed to a gas monitor for analysis of the gaseous components contained in the latter.