a method and apparatus is disclosed for flowing a sample gas and a reactant gas past a corona discharge electrode situated at a first location in an ion drift chamber , applying a pulsed voltage waveform comprising a varying pulse component and a dc bias component to the corona discharge electrode to cause a corona which in turn produces ions from the sample gas and the reactant gas , applying a dc bias to the ion drift chamber to cause the ions to drift to a second location in the ion drift chamber , detecting the ions at the second location in the drift chamber , and timing the period for the ions to drift from the corona discharge electrode to the selected location in the drift chamber .